llvm-project/llvm/lib/Target/X86/X86TargetMachine.cpp

492 lines
16 KiB
C++
Raw Normal View History

//===-- X86TargetMachine.cpp - Define TargetMachine for the X86 -----------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the X86 specific subclass of TargetMachine.
//
//===----------------------------------------------------------------------===//
#include "X86TargetMachine.h"
#include "MCTargetDesc/X86MCTargetDesc.h"
#include "X86.h"
#include "X86CallLowering.h"
#include "X86LegalizerInfo.h"
#include "X86MacroFusion.h"
#include "X86Subtarget.h"
#include "X86TargetObjectFile.h"
#include "X86TargetTransformInfo.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Triple.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/CodeGen/ExecutionDomainFix.h"
#include "llvm/CodeGen/GlobalISel/CallLowering.h"
#include "llvm/CodeGen/GlobalISel/IRTranslator.h"
#include "llvm/CodeGen/GlobalISel/InstructionSelect.h"
#include "llvm/CodeGen/GlobalISel/Legalizer.h"
#include "llvm/CodeGen/GlobalISel/RegBankSelect.h"
#include "llvm/CodeGen/MachineScheduler.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetPassConfig.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Function.h"
#include "llvm/Pass.h"
#include "llvm/Support/CodeGen.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/TargetRegistry.h"
#include "llvm/Target/TargetLoweringObjectFile.h"
#include "llvm/Target/TargetOptions.h"
#include <memory>
#include <string>
using namespace llvm;
static cl::opt<bool> EnableMachineCombinerPass("x86-machine-combiner",
cl::desc("Enable the machine combiner pass"),
cl::init(true), cl::Hidden);
namespace llvm {
void initializeWinEHStatePassPass(PassRegistry &);
void initializeFixupLEAPassPass(PassRegistry &);
void initializeX86CallFrameOptimizationPass(PassRegistry &);
void initializeX86CmovConverterPassPass(PassRegistry &);
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
void initializeX86ExecutionDomainFixPass(PassRegistry &);
void initializeX86DomainReassignmentPass(PassRegistry &);
void initializeX86AvoidSFBPassPass(PassRegistry &);
} // end namespace llvm
2011-02-17 20:23:50 +08:00
extern "C" void LLVMInitializeX86Target() {
// Register the target.
RegisterTargetMachine<X86TargetMachine> X(getTheX86_32Target());
RegisterTargetMachine<X86TargetMachine> Y(getTheX86_64Target());
PassRegistry &PR = *PassRegistry::getPassRegistry();
initializeGlobalISel(PR);
initializeWinEHStatePassPass(PR);
initializeFixupBWInstPassPass(PR);
initializeEvexToVexInstPassPass(PR);
initializeFixupLEAPassPass(PR);
initializeX86CallFrameOptimizationPass(PR);
initializeX86CmovConverterPassPass(PR);
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
initializeX86ExecutionDomainFixPass(PR);
initializeX86DomainReassignmentPass(PR);
initializeX86AvoidSFBPassPass(PR);
}
static std::unique_ptr<TargetLoweringObjectFile> createTLOF(const Triple &TT) {
if (TT.isOSBinFormatMachO()) {
if (TT.getArch() == Triple::x86_64)
return llvm::make_unique<X86_64MachoTargetObjectFile>();
return llvm::make_unique<TargetLoweringObjectFileMachO>();
}
if (TT.isOSFreeBSD())
return llvm::make_unique<X86FreeBSDTargetObjectFile>();
if (TT.isOSLinux() || TT.isOSNaCl() || TT.isOSIAMCU())
return llvm::make_unique<X86LinuxNaClTargetObjectFile>();
if (TT.isOSSolaris())
return llvm::make_unique<X86SolarisTargetObjectFile>();
if (TT.isOSFuchsia())
return llvm::make_unique<X86FuchsiaTargetObjectFile>();
if (TT.isOSBinFormatELF())
return llvm::make_unique<X86ELFTargetObjectFile>();
if (TT.isKnownWindowsMSVCEnvironment() || TT.isWindowsCoreCLREnvironment())
return llvm::make_unique<X86WindowsTargetObjectFile>();
if (TT.isOSBinFormatCOFF())
return llvm::make_unique<TargetLoweringObjectFileCOFF>();
llvm_unreachable("unknown subtarget type");
}
static std::string computeDataLayout(const Triple &TT) {
// X86 is little endian
std::string Ret = "e";
Ret += DataLayout::getManglingComponent(TT);
// X86 and x32 have 32 bit pointers.
if ((TT.isArch64Bit() &&
(TT.getEnvironment() == Triple::GNUX32 || TT.isOSNaCl())) ||
!TT.isArch64Bit())
Ret += "-p:32:32";
// Some ABIs align 64 bit integers and doubles to 64 bits, others to 32.
if (TT.isArch64Bit() || TT.isOSWindows() || TT.isOSNaCl())
Ret += "-i64:64";
else if (TT.isOSIAMCU())
Ret += "-i64:32-f64:32";
else
Ret += "-f64:32:64";
// Some ABIs align long double to 128 bits, others to 32.
if (TT.isOSNaCl() || TT.isOSIAMCU())
; // No f80
else if (TT.isArch64Bit() || TT.isOSDarwin())
Ret += "-f80:128";
else
Ret += "-f80:32";
if (TT.isOSIAMCU())
Ret += "-f128:32";
// The registers can hold 8, 16, 32 or, in x86-64, 64 bits.
if (TT.isArch64Bit())
Ret += "-n8:16:32:64";
else
Ret += "-n8:16:32";
// The stack is aligned to 32 bits on some ABIs and 128 bits on others.
if ((!TT.isArch64Bit() && TT.isOSWindows()) || TT.isOSIAMCU())
Ret += "-a:0:32-S32";
else
Ret += "-S128";
return Ret;
}
static Reloc::Model getEffectiveRelocModel(const Triple &TT,
Optional<Reloc::Model> RM) {
bool is64Bit = TT.getArch() == Triple::x86_64;
if (!RM.hasValue()) {
// Darwin defaults to PIC in 64 bit mode and dynamic-no-pic in 32 bit mode.
// Win64 requires rip-rel addressing, thus we force it to PIC. Otherwise we
// use static relocation model by default.
if (TT.isOSDarwin()) {
if (is64Bit)
return Reloc::PIC_;
return Reloc::DynamicNoPIC;
}
if (TT.isOSWindows() && is64Bit)
return Reloc::PIC_;
return Reloc::Static;
}
// ELF and X86-64 don't have a distinct DynamicNoPIC model. DynamicNoPIC
// is defined as a model for code which may be used in static or dynamic
// executables but not necessarily a shared library. On X86-32 we just
// compile in -static mode, in x86-64 we use PIC.
if (*RM == Reloc::DynamicNoPIC) {
if (is64Bit)
return Reloc::PIC_;
if (!TT.isOSDarwin())
return Reloc::Static;
}
// If we are on Darwin, disallow static relocation model in X86-64 mode, since
// the Mach-O file format doesn't support it.
if (*RM == Reloc::Static && TT.isOSDarwin() && is64Bit)
return Reloc::PIC_;
return *RM;
}
static CodeModel::Model getEffectiveCodeModel(Optional<CodeModel::Model> CM,
bool JIT, bool Is64Bit) {
if (CM)
return *CM;
if (JIT)
return Is64Bit ? CodeModel::Large : CodeModel::Small;
return CodeModel::Small;
}
/// Create an X86 target.
///
X86TargetMachine::X86TargetMachine(const Target &T, const Triple &TT,
StringRef CPU, StringRef FS,
const TargetOptions &Options,
Optional<Reloc::Model> RM,
Optional<CodeModel::Model> CM,
CodeGenOpt::Level OL, bool JIT)
: LLVMTargetMachine(
T, computeDataLayout(TT), TT, CPU, FS, Options,
getEffectiveRelocModel(TT, RM),
getEffectiveCodeModel(CM, JIT, TT.getArch() == Triple::x86_64), OL),
TLOF(createTLOF(getTargetTriple())) {
// Windows stack unwinder gets confused when execution flow "falls through"
// after a call to 'noreturn' function.
// To prevent that, we emit a trap for 'unreachable' IR instructions.
// (which on X86, happens to be the 'ud2' instruction)
// On PS4, the "return address" of a 'noreturn' call must still be within
// the calling function, and TrapUnreachable is an easy way to get that.
// The check here for 64-bit windows is a bit icky, but as we're unlikely
// to ever want to mix 32 and 64-bit windows code in a single module
// this should be fine.
if ((TT.isOSWindows() && TT.getArch() == Triple::x86_64) || TT.isPS4())
this->Options.TrapUnreachable = true;
initAsmInfo();
2006-02-04 02:59:39 +08:00
}
X86TargetMachine::~X86TargetMachine() = default;
const X86Subtarget *
X86TargetMachine::getSubtargetImpl(const Function &F) const {
Attribute CPUAttr = F.getFnAttribute("target-cpu");
Attribute FSAttr = F.getFnAttribute("target-features");
StringRef CPU = !CPUAttr.hasAttribute(Attribute::None)
? CPUAttr.getValueAsString()
: (StringRef)TargetCPU;
StringRef FS = !FSAttr.hasAttribute(Attribute::None)
? FSAttr.getValueAsString()
: (StringRef)TargetFS;
SmallString<512> Key;
Key.reserve(CPU.size() + FS.size());
Key += CPU;
Key += FS;
// FIXME: This is related to the code below to reset the target options,
// we need to know whether or not the soft float flag is set on the
// function before we can generate a subtarget. We also need to use
// it as a key for the subtarget since that can be the only difference
// between two functions.
bool SoftFloat =
F.getFnAttribute("use-soft-float").getValueAsString() == "true";
// If the soft float attribute is set on the function turn on the soft float
// subtarget feature.
if (SoftFloat)
Key += FS.empty() ? "+soft-float" : ",+soft-float";
// Keep track of the key width after all features are added so we can extract
// the feature string out later.
unsigned CPUFSWidth = Key.size();
[X86] Don't make 512-bit vectors legal when preferred vector width is 256 bits and 512 bits aren't required This patch adds a new function attribute "required-vector-width" that can be set by the frontend to indicate the maximum vector width present in the original source code. The idea is that this would be set based on ABI requirements, intrinsics or explicit vector types being used, maybe simd pragmas, etc. The backend will then use this information to determine if its save to make 512-bit vectors illegal when the preference is for 256-bit vectors. For code that has no vectors in it originally and only get vectors through the loop and slp vectorizers this allows us to generate code largely similar to our AVX2 only output while still enabling AVX512 features like mask registers and gather/scatter. The loop vectorizer doesn't always obey TTI and will create oversized vectors with the expectation the backend will legalize it. In order to avoid changing the vectorizer and potentially harm our AVX2 codegen this patch tries to make the legalizer behavior similar. This is restricted to CPUs that support AVX512F and AVX512VL so that we have good fallback options to use 128 and 256-bit vectors and still get masking. I've qualified every place I could find in X86ISelLowering.cpp and added tests cases for many of them with 2 different values for the attribute to see the codegen differences. We still need to do frontend work for the attribute and teach the inliner how to merge it, etc. But this gets the codegen layer ready for it. Differential Revision: https://reviews.llvm.org/D42724 llvm-svn: 324834
2018-02-11 16:06:27 +08:00
// Extract prefer-vector-width attribute.
unsigned PreferVectorWidthOverride = 0;
if (F.hasFnAttribute("prefer-vector-width")) {
StringRef Val = F.getFnAttribute("prefer-vector-width").getValueAsString();
unsigned Width;
if (!Val.getAsInteger(0, Width)) {
Key += ",prefer-vector-width=";
Key += Val;
PreferVectorWidthOverride = Width;
}
}
[X86] Don't make 512-bit vectors legal when preferred vector width is 256 bits and 512 bits aren't required This patch adds a new function attribute "required-vector-width" that can be set by the frontend to indicate the maximum vector width present in the original source code. The idea is that this would be set based on ABI requirements, intrinsics or explicit vector types being used, maybe simd pragmas, etc. The backend will then use this information to determine if its save to make 512-bit vectors illegal when the preference is for 256-bit vectors. For code that has no vectors in it originally and only get vectors through the loop and slp vectorizers this allows us to generate code largely similar to our AVX2 only output while still enabling AVX512 features like mask registers and gather/scatter. The loop vectorizer doesn't always obey TTI and will create oversized vectors with the expectation the backend will legalize it. In order to avoid changing the vectorizer and potentially harm our AVX2 codegen this patch tries to make the legalizer behavior similar. This is restricted to CPUs that support AVX512F and AVX512VL so that we have good fallback options to use 128 and 256-bit vectors and still get masking. I've qualified every place I could find in X86ISelLowering.cpp and added tests cases for many of them with 2 different values for the attribute to see the codegen differences. We still need to do frontend work for the attribute and teach the inliner how to merge it, etc. But this gets the codegen layer ready for it. Differential Revision: https://reviews.llvm.org/D42724 llvm-svn: 324834
2018-02-11 16:06:27 +08:00
// Extract required-vector-width attribute.
unsigned RequiredVectorWidth = UINT32_MAX;
if (F.hasFnAttribute("required-vector-width")) {
StringRef Val = F.getFnAttribute("required-vector-width").getValueAsString();
unsigned Width;
if (!Val.getAsInteger(0, Width)) {
Key += ",required-vector-width=";
Key += Val;
RequiredVectorWidth = Width;
}
}
// Extracted here so that we make sure there is backing for the StringRef. If
// we assigned earlier, its possible the SmallString reallocated leaving a
// dangling StringRef.
FS = Key.slice(CPU.size(), CPUFSWidth);
auto &I = SubtargetMap[Key];
if (!I) {
// This needs to be done before we create a new subtarget since any
// creation will depend on the TM and the code generation flags on the
// function that reside in TargetOptions.
resetTargetOptions(F);
I = llvm::make_unique<X86Subtarget>(TargetTriple, CPU, FS, *this,
Options.StackAlignmentOverride,
[X86] Don't make 512-bit vectors legal when preferred vector width is 256 bits and 512 bits aren't required This patch adds a new function attribute "required-vector-width" that can be set by the frontend to indicate the maximum vector width present in the original source code. The idea is that this would be set based on ABI requirements, intrinsics or explicit vector types being used, maybe simd pragmas, etc. The backend will then use this information to determine if its save to make 512-bit vectors illegal when the preference is for 256-bit vectors. For code that has no vectors in it originally and only get vectors through the loop and slp vectorizers this allows us to generate code largely similar to our AVX2 only output while still enabling AVX512 features like mask registers and gather/scatter. The loop vectorizer doesn't always obey TTI and will create oversized vectors with the expectation the backend will legalize it. In order to avoid changing the vectorizer and potentially harm our AVX2 codegen this patch tries to make the legalizer behavior similar. This is restricted to CPUs that support AVX512F and AVX512VL so that we have good fallback options to use 128 and 256-bit vectors and still get masking. I've qualified every place I could find in X86ISelLowering.cpp and added tests cases for many of them with 2 different values for the attribute to see the codegen differences. We still need to do frontend work for the attribute and teach the inliner how to merge it, etc. But this gets the codegen layer ready for it. Differential Revision: https://reviews.llvm.org/D42724 llvm-svn: 324834
2018-02-11 16:06:27 +08:00
PreferVectorWidthOverride,
RequiredVectorWidth);
}
return I.get();
}
//===----------------------------------------------------------------------===//
// Command line options for x86
//===----------------------------------------------------------------------===//
static cl::opt<bool>
UseVZeroUpper("x86-use-vzeroupper", cl::Hidden,
cl::desc("Minimize AVX to SSE transition penalty"),
cl::init(true));
Switch TargetTransformInfo from an immutable analysis pass that requires a TargetMachine to construct (and thus isn't always available), to an analysis group that supports layered implementations much like AliasAnalysis does. This is a pretty massive change, with a few parts that I was unable to easily separate (sorry), so I'll walk through it. The first step of this conversion was to make TargetTransformInfo an analysis group, and to sink the nonce implementations in ScalarTargetTransformInfo and VectorTargetTranformInfo into a NoTargetTransformInfo pass. This allows other passes to add a hard requirement on TTI, and assume they will always get at least on implementation. The TargetTransformInfo analysis group leverages the delegation chaining trick that AliasAnalysis uses, where the base class for the analysis group delegates to the previous analysis *pass*, allowing all but tho NoFoo analysis passes to only implement the parts of the interfaces they support. It also introduces a new trick where each pass in the group retains a pointer to the top-most pass that has been initialized. This allows passes to implement one API in terms of another API and benefit when some other pass above them in the stack has more precise results for the second API. The second step of this conversion is to create a pass that implements the TargetTransformInfo analysis using the target-independent abstractions in the code generator. This replaces the ScalarTargetTransformImpl and VectorTargetTransformImpl classes in lib/Target with a single pass in lib/CodeGen called BasicTargetTransformInfo. This class actually provides most of the TTI functionality, basing it upon the TargetLowering abstraction and other information in the target independent code generator. The third step of the conversion adds support to all TargetMachines to register custom analysis passes. This allows building those passes with access to TargetLowering or other target-specific classes, and it also allows each target to customize the set of analysis passes desired in the pass manager. The baseline LLVMTargetMachine implements this interface to add the BasicTTI pass to the pass manager, and all of the tools that want to support target-aware TTI passes call this routine on whatever target machine they end up with to add the appropriate passes. The fourth step of the conversion created target-specific TTI analysis passes for the X86 and ARM backends. These passes contain the custom logic that was previously in their extensions of the ScalarTargetTransformInfo and VectorTargetTransformInfo interfaces. I separated them into their own file, as now all of the interface bits are private and they just expose a function to create the pass itself. Then I extended these target machines to set up a custom set of analysis passes, first adding BasicTTI as a fallback, and then adding their customized TTI implementations. The fourth step required logic that was shared between the target independent layer and the specific targets to move to a different interface, as they no longer derive from each other. As a consequence, a helper functions were added to TargetLowering representing the common logic needed both in the target implementation and the codegen implementation of the TTI pass. While technically this is the only change that could have been committed separately, it would have been a nightmare to extract. The final step of the conversion was just to delete all the old boilerplate. This got rid of the ScalarTargetTransformInfo and VectorTargetTransformInfo classes, all of the support in all of the targets for producing instances of them, and all of the support in the tools for manually constructing a pass based around them. Now that TTI is a relatively normal analysis group, two things become straightforward. First, we can sink it into lib/Analysis which is a more natural layer for it to live. Second, clients of this interface can depend on it *always* being available which will simplify their code and behavior. These (and other) simplifications will follow in subsequent commits, this one is clearly big enough. Finally, I'm very aware that much of the comments and documentation needs to be updated. As soon as I had this working, and plausibly well commented, I wanted to get it committed and in front of the build bots. I'll be doing a few passes over documentation later if it sticks. Commits to update DragonEgg and Clang will be made presently. llvm-svn: 171681
2013-01-07 09:37:14 +08:00
//===----------------------------------------------------------------------===//
// X86 TTI query.
Switch TargetTransformInfo from an immutable analysis pass that requires a TargetMachine to construct (and thus isn't always available), to an analysis group that supports layered implementations much like AliasAnalysis does. This is a pretty massive change, with a few parts that I was unable to easily separate (sorry), so I'll walk through it. The first step of this conversion was to make TargetTransformInfo an analysis group, and to sink the nonce implementations in ScalarTargetTransformInfo and VectorTargetTranformInfo into a NoTargetTransformInfo pass. This allows other passes to add a hard requirement on TTI, and assume they will always get at least on implementation. The TargetTransformInfo analysis group leverages the delegation chaining trick that AliasAnalysis uses, where the base class for the analysis group delegates to the previous analysis *pass*, allowing all but tho NoFoo analysis passes to only implement the parts of the interfaces they support. It also introduces a new trick where each pass in the group retains a pointer to the top-most pass that has been initialized. This allows passes to implement one API in terms of another API and benefit when some other pass above them in the stack has more precise results for the second API. The second step of this conversion is to create a pass that implements the TargetTransformInfo analysis using the target-independent abstractions in the code generator. This replaces the ScalarTargetTransformImpl and VectorTargetTransformImpl classes in lib/Target with a single pass in lib/CodeGen called BasicTargetTransformInfo. This class actually provides most of the TTI functionality, basing it upon the TargetLowering abstraction and other information in the target independent code generator. The third step of the conversion adds support to all TargetMachines to register custom analysis passes. This allows building those passes with access to TargetLowering or other target-specific classes, and it also allows each target to customize the set of analysis passes desired in the pass manager. The baseline LLVMTargetMachine implements this interface to add the BasicTTI pass to the pass manager, and all of the tools that want to support target-aware TTI passes call this routine on whatever target machine they end up with to add the appropriate passes. The fourth step of the conversion created target-specific TTI analysis passes for the X86 and ARM backends. These passes contain the custom logic that was previously in their extensions of the ScalarTargetTransformInfo and VectorTargetTransformInfo interfaces. I separated them into their own file, as now all of the interface bits are private and they just expose a function to create the pass itself. Then I extended these target machines to set up a custom set of analysis passes, first adding BasicTTI as a fallback, and then adding their customized TTI implementations. The fourth step required logic that was shared between the target independent layer and the specific targets to move to a different interface, as they no longer derive from each other. As a consequence, a helper functions were added to TargetLowering representing the common logic needed both in the target implementation and the codegen implementation of the TTI pass. While technically this is the only change that could have been committed separately, it would have been a nightmare to extract. The final step of the conversion was just to delete all the old boilerplate. This got rid of the ScalarTargetTransformInfo and VectorTargetTransformInfo classes, all of the support in all of the targets for producing instances of them, and all of the support in the tools for manually constructing a pass based around them. Now that TTI is a relatively normal analysis group, two things become straightforward. First, we can sink it into lib/Analysis which is a more natural layer for it to live. Second, clients of this interface can depend on it *always* being available which will simplify their code and behavior. These (and other) simplifications will follow in subsequent commits, this one is clearly big enough. Finally, I'm very aware that much of the comments and documentation needs to be updated. As soon as I had this working, and plausibly well commented, I wanted to get it committed and in front of the build bots. I'll be doing a few passes over documentation later if it sticks. Commits to update DragonEgg and Clang will be made presently. llvm-svn: 171681
2013-01-07 09:37:14 +08:00
//===----------------------------------------------------------------------===//
TargetTransformInfo
X86TargetMachine::getTargetTransformInfo(const Function &F) {
return TargetTransformInfo(X86TTIImpl(this, F));
Switch TargetTransformInfo from an immutable analysis pass that requires a TargetMachine to construct (and thus isn't always available), to an analysis group that supports layered implementations much like AliasAnalysis does. This is a pretty massive change, with a few parts that I was unable to easily separate (sorry), so I'll walk through it. The first step of this conversion was to make TargetTransformInfo an analysis group, and to sink the nonce implementations in ScalarTargetTransformInfo and VectorTargetTranformInfo into a NoTargetTransformInfo pass. This allows other passes to add a hard requirement on TTI, and assume they will always get at least on implementation. The TargetTransformInfo analysis group leverages the delegation chaining trick that AliasAnalysis uses, where the base class for the analysis group delegates to the previous analysis *pass*, allowing all but tho NoFoo analysis passes to only implement the parts of the interfaces they support. It also introduces a new trick where each pass in the group retains a pointer to the top-most pass that has been initialized. This allows passes to implement one API in terms of another API and benefit when some other pass above them in the stack has more precise results for the second API. The second step of this conversion is to create a pass that implements the TargetTransformInfo analysis using the target-independent abstractions in the code generator. This replaces the ScalarTargetTransformImpl and VectorTargetTransformImpl classes in lib/Target with a single pass in lib/CodeGen called BasicTargetTransformInfo. This class actually provides most of the TTI functionality, basing it upon the TargetLowering abstraction and other information in the target independent code generator. The third step of the conversion adds support to all TargetMachines to register custom analysis passes. This allows building those passes with access to TargetLowering or other target-specific classes, and it also allows each target to customize the set of analysis passes desired in the pass manager. The baseline LLVMTargetMachine implements this interface to add the BasicTTI pass to the pass manager, and all of the tools that want to support target-aware TTI passes call this routine on whatever target machine they end up with to add the appropriate passes. The fourth step of the conversion created target-specific TTI analysis passes for the X86 and ARM backends. These passes contain the custom logic that was previously in their extensions of the ScalarTargetTransformInfo and VectorTargetTransformInfo interfaces. I separated them into their own file, as now all of the interface bits are private and they just expose a function to create the pass itself. Then I extended these target machines to set up a custom set of analysis passes, first adding BasicTTI as a fallback, and then adding their customized TTI implementations. The fourth step required logic that was shared between the target independent layer and the specific targets to move to a different interface, as they no longer derive from each other. As a consequence, a helper functions were added to TargetLowering representing the common logic needed both in the target implementation and the codegen implementation of the TTI pass. While technically this is the only change that could have been committed separately, it would have been a nightmare to extract. The final step of the conversion was just to delete all the old boilerplate. This got rid of the ScalarTargetTransformInfo and VectorTargetTransformInfo classes, all of the support in all of the targets for producing instances of them, and all of the support in the tools for manually constructing a pass based around them. Now that TTI is a relatively normal analysis group, two things become straightforward. First, we can sink it into lib/Analysis which is a more natural layer for it to live. Second, clients of this interface can depend on it *always* being available which will simplify their code and behavior. These (and other) simplifications will follow in subsequent commits, this one is clearly big enough. Finally, I'm very aware that much of the comments and documentation needs to be updated. As soon as I had this working, and plausibly well commented, I wanted to get it committed and in front of the build bots. I'll be doing a few passes over documentation later if it sticks. Commits to update DragonEgg and Clang will be made presently. llvm-svn: 171681
2013-01-07 09:37:14 +08:00
}
//===----------------------------------------------------------------------===//
// Pass Pipeline Configuration
//===----------------------------------------------------------------------===//
2003-08-06 00:34:44 +08:00
namespace {
/// X86 Code Generator Pass Configuration Options.
class X86PassConfig : public TargetPassConfig {
public:
X86PassConfig(X86TargetMachine &TM, PassManagerBase &PM)
: TargetPassConfig(TM, PM) {}
X86TargetMachine &getX86TargetMachine() const {
return getTM<X86TargetMachine>();
}
ScheduleDAGInstrs *
createMachineScheduler(MachineSchedContext *C) const override {
ScheduleDAGMILive *DAG = createGenericSchedLive(C);
DAG->addMutation(createX86MacroFusionDAGMutation());
return DAG;
}
void addIRPasses() override;
bool addInstSelector() override;
bool addIRTranslator() override;
bool addLegalizeMachineIR() override;
bool addRegBankSelect() override;
bool addGlobalInstructionSelect() override;
bool addILPOpts() override;
bool addPreISel() override;
void addMachineSSAOptimization() override;
void addPreRegAlloc() override;
void addPostRegAlloc() override;
void addPreEmitPass() override;
Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre.. Summary: First, we need to explain the core of the vulnerability. Note that this is a very incomplete description, please see the Project Zero blog post for details: https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html The basis for branch target injection is to direct speculative execution of the processor to some "gadget" of executable code by poisoning the prediction of indirect branches with the address of that gadget. The gadget in turn contains an operation that provides a side channel for reading data. Most commonly, this will look like a load of secret data followed by a branch on the loaded value and then a load of some predictable cache line. The attacker then uses timing of the processors cache to determine which direction the branch took *in the speculative execution*, and in turn what one bit of the loaded value was. Due to the nature of these timing side channels and the branch predictor on Intel processors, this allows an attacker to leak data only accessible to a privileged domain (like the kernel) back into an unprivileged domain. The goal is simple: avoid generating code which contains an indirect branch that could have its prediction poisoned by an attacker. In many cases, the compiler can simply use directed conditional branches and a small search tree. LLVM already has support for lowering switches in this way and the first step of this patch is to disable jump-table lowering of switches and introduce a pass to rewrite explicit indirectbr sequences into a switch over integers. However, there is no fully general alternative to indirect calls. We introduce a new construct we call a "retpoline" to implement indirect calls in a non-speculatable way. It can be thought of loosely as a trampoline for indirect calls which uses the RET instruction on x86. Further, we arrange for a specific call->ret sequence which ensures the processor predicts the return to go to a controlled, known location. The retpoline then "smashes" the return address pushed onto the stack by the call with the desired target of the original indirect call. The result is a predicted return to the next instruction after a call (which can be used to trap speculative execution within an infinite loop) and an actual indirect branch to an arbitrary address. On 64-bit x86 ABIs, this is especially easily done in the compiler by using a guaranteed scratch register to pass the target into this device. For 32-bit ABIs there isn't a guaranteed scratch register and so several different retpoline variants are introduced to use a scratch register if one is available in the calling convention and to otherwise use direct stack push/pop sequences to pass the target address. This "retpoline" mitigation is fully described in the following blog post: https://support.google.com/faqs/answer/7625886 We also support a target feature that disables emission of the retpoline thunk by the compiler to allow for custom thunks if users want them. These are particularly useful in environments like kernels that routinely do hot-patching on boot and want to hot-patch their thunk to different code sequences. They can write this custom thunk and use `-mretpoline-external-thunk` *in addition* to `-mretpoline`. In this case, on x86-64 thu thunk names must be: ``` __llvm_external_retpoline_r11 ``` or on 32-bit: ``` __llvm_external_retpoline_eax __llvm_external_retpoline_ecx __llvm_external_retpoline_edx __llvm_external_retpoline_push ``` And the target of the retpoline is passed in the named register, or in the case of the `push` suffix on the top of the stack via a `pushl` instruction. There is one other important source of indirect branches in x86 ELF binaries: the PLT. These patches also include support for LLD to generate PLT entries that perform a retpoline-style indirection. The only other indirect branches remaining that we are aware of are from precompiled runtimes (such as crt0.o and similar). The ones we have found are not really attackable, and so we have not focused on them here, but eventually these runtimes should also be replicated for retpoline-ed configurations for completeness. For kernels or other freestanding or fully static executables, the compiler switch `-mretpoline` is sufficient to fully mitigate this particular attack. For dynamic executables, you must compile *all* libraries with `-mretpoline` and additionally link the dynamic executable and all shared libraries with LLD and pass `-z retpolineplt` (or use similar functionality from some other linker). We strongly recommend also using `-z now` as non-lazy binding allows the retpoline-mitigated PLT to be substantially smaller. When manually apply similar transformations to `-mretpoline` to the Linux kernel we observed very small performance hits to applications running typical workloads, and relatively minor hits (approximately 2%) even for extremely syscall-heavy applications. This is largely due to the small number of indirect branches that occur in performance sensitive paths of the kernel. When using these patches on statically linked applications, especially C++ applications, you should expect to see a much more dramatic performance hit. For microbenchmarks that are switch, indirect-, or virtual-call heavy we have seen overheads ranging from 10% to 50%. However, real-world workloads exhibit substantially lower performance impact. Notably, techniques such as PGO and ThinLTO dramatically reduce the impact of hot indirect calls (by speculatively promoting them to direct calls) and allow optimized search trees to be used to lower switches. If you need to deploy these techniques in C++ applications, we *strongly* recommend that you ensure all hot call targets are statically linked (avoiding PLT indirection) and use both PGO and ThinLTO. Well tuned servers using all of these techniques saw 5% - 10% overhead from the use of retpoline. We will add detailed documentation covering these components in subsequent patches, but wanted to make the core functionality available as soon as possible. Happy for more code review, but we'd really like to get these patches landed and backported ASAP for obvious reasons. We're planning to backport this to both 6.0 and 5.0 release streams and get a 5.0 release with just this cherry picked ASAP for distros and vendors. This patch is the work of a number of people over the past month: Eric, Reid, Rui, and myself. I'm mailing it out as a single commit due to the time sensitive nature of landing this and the need to backport it. Huge thanks to everyone who helped out here, and everyone at Intel who helped out in discussions about how to craft this. Also, credit goes to Paul Turner (at Google, but not an LLVM contributor) for much of the underlying retpoline design. Reviewers: echristo, rnk, ruiu, craig.topper, DavidKreitzer Subscribers: sanjoy, emaste, mcrosier, mgorny, mehdi_amini, hiraditya, llvm-commits Differential Revision: https://reviews.llvm.org/D41723 llvm-svn: 323155
2018-01-23 06:05:25 +08:00
void addPreEmitPass2() override;
void addPreSched2() override;
};
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
class X86ExecutionDomainFix : public ExecutionDomainFix {
public:
static char ID;
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
X86ExecutionDomainFix() : ExecutionDomainFix(ID, X86::VR128XRegClass) {}
StringRef getPassName() const override {
return "X86 Execution Dependency Fix";
}
};
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
char X86ExecutionDomainFix::ID;
} // end anonymous namespace
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
INITIALIZE_PASS_BEGIN(X86ExecutionDomainFix, "x86-execution-domain-fix",
"X86 Execution Domain Fix", false, false)
INITIALIZE_PASS_DEPENDENCY(ReachingDefAnalysis)
INITIALIZE_PASS_END(X86ExecutionDomainFix, "x86-execution-domain-fix",
"X86 Execution Domain Fix", false, false)
TargetPassConfig *X86TargetMachine::createPassConfig(PassManagerBase &PM) {
return new X86PassConfig(*this, PM);
}
void X86PassConfig::addIRPasses() {
addPass(createAtomicExpandPass());
TargetPassConfig::addIRPasses();
if (TM->getOptLevel() != CodeGenOpt::None)
addPass(createInterleavedAccessPass());
Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre.. Summary: First, we need to explain the core of the vulnerability. Note that this is a very incomplete description, please see the Project Zero blog post for details: https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html The basis for branch target injection is to direct speculative execution of the processor to some "gadget" of executable code by poisoning the prediction of indirect branches with the address of that gadget. The gadget in turn contains an operation that provides a side channel for reading data. Most commonly, this will look like a load of secret data followed by a branch on the loaded value and then a load of some predictable cache line. The attacker then uses timing of the processors cache to determine which direction the branch took *in the speculative execution*, and in turn what one bit of the loaded value was. Due to the nature of these timing side channels and the branch predictor on Intel processors, this allows an attacker to leak data only accessible to a privileged domain (like the kernel) back into an unprivileged domain. The goal is simple: avoid generating code which contains an indirect branch that could have its prediction poisoned by an attacker. In many cases, the compiler can simply use directed conditional branches and a small search tree. LLVM already has support for lowering switches in this way and the first step of this patch is to disable jump-table lowering of switches and introduce a pass to rewrite explicit indirectbr sequences into a switch over integers. However, there is no fully general alternative to indirect calls. We introduce a new construct we call a "retpoline" to implement indirect calls in a non-speculatable way. It can be thought of loosely as a trampoline for indirect calls which uses the RET instruction on x86. Further, we arrange for a specific call->ret sequence which ensures the processor predicts the return to go to a controlled, known location. The retpoline then "smashes" the return address pushed onto the stack by the call with the desired target of the original indirect call. The result is a predicted return to the next instruction after a call (which can be used to trap speculative execution within an infinite loop) and an actual indirect branch to an arbitrary address. On 64-bit x86 ABIs, this is especially easily done in the compiler by using a guaranteed scratch register to pass the target into this device. For 32-bit ABIs there isn't a guaranteed scratch register and so several different retpoline variants are introduced to use a scratch register if one is available in the calling convention and to otherwise use direct stack push/pop sequences to pass the target address. This "retpoline" mitigation is fully described in the following blog post: https://support.google.com/faqs/answer/7625886 We also support a target feature that disables emission of the retpoline thunk by the compiler to allow for custom thunks if users want them. These are particularly useful in environments like kernels that routinely do hot-patching on boot and want to hot-patch their thunk to different code sequences. They can write this custom thunk and use `-mretpoline-external-thunk` *in addition* to `-mretpoline`. In this case, on x86-64 thu thunk names must be: ``` __llvm_external_retpoline_r11 ``` or on 32-bit: ``` __llvm_external_retpoline_eax __llvm_external_retpoline_ecx __llvm_external_retpoline_edx __llvm_external_retpoline_push ``` And the target of the retpoline is passed in the named register, or in the case of the `push` suffix on the top of the stack via a `pushl` instruction. There is one other important source of indirect branches in x86 ELF binaries: the PLT. These patches also include support for LLD to generate PLT entries that perform a retpoline-style indirection. The only other indirect branches remaining that we are aware of are from precompiled runtimes (such as crt0.o and similar). The ones we have found are not really attackable, and so we have not focused on them here, but eventually these runtimes should also be replicated for retpoline-ed configurations for completeness. For kernels or other freestanding or fully static executables, the compiler switch `-mretpoline` is sufficient to fully mitigate this particular attack. For dynamic executables, you must compile *all* libraries with `-mretpoline` and additionally link the dynamic executable and all shared libraries with LLD and pass `-z retpolineplt` (or use similar functionality from some other linker). We strongly recommend also using `-z now` as non-lazy binding allows the retpoline-mitigated PLT to be substantially smaller. When manually apply similar transformations to `-mretpoline` to the Linux kernel we observed very small performance hits to applications running typical workloads, and relatively minor hits (approximately 2%) even for extremely syscall-heavy applications. This is largely due to the small number of indirect branches that occur in performance sensitive paths of the kernel. When using these patches on statically linked applications, especially C++ applications, you should expect to see a much more dramatic performance hit. For microbenchmarks that are switch, indirect-, or virtual-call heavy we have seen overheads ranging from 10% to 50%. However, real-world workloads exhibit substantially lower performance impact. Notably, techniques such as PGO and ThinLTO dramatically reduce the impact of hot indirect calls (by speculatively promoting them to direct calls) and allow optimized search trees to be used to lower switches. If you need to deploy these techniques in C++ applications, we *strongly* recommend that you ensure all hot call targets are statically linked (avoiding PLT indirection) and use both PGO and ThinLTO. Well tuned servers using all of these techniques saw 5% - 10% overhead from the use of retpoline. We will add detailed documentation covering these components in subsequent patches, but wanted to make the core functionality available as soon as possible. Happy for more code review, but we'd really like to get these patches landed and backported ASAP for obvious reasons. We're planning to backport this to both 6.0 and 5.0 release streams and get a 5.0 release with just this cherry picked ASAP for distros and vendors. This patch is the work of a number of people over the past month: Eric, Reid, Rui, and myself. I'm mailing it out as a single commit due to the time sensitive nature of landing this and the need to backport it. Huge thanks to everyone who helped out here, and everyone at Intel who helped out in discussions about how to craft this. Also, credit goes to Paul Turner (at Google, but not an LLVM contributor) for much of the underlying retpoline design. Reviewers: echristo, rnk, ruiu, craig.topper, DavidKreitzer Subscribers: sanjoy, emaste, mcrosier, mgorny, mehdi_amini, hiraditya, llvm-commits Differential Revision: https://reviews.llvm.org/D41723 llvm-svn: 323155
2018-01-23 06:05:25 +08:00
// Add passes that handle indirect branch removal and insertion of a retpoline
// thunk. These will be a no-op unless a function subtarget has the retpoline
// feature enabled.
addPass(createIndirectBrExpandPass());
}
bool X86PassConfig::addInstSelector() {
// Install an instruction selector.
addPass(createX86ISelDag(getX86TargetMachine(), getOptLevel()));
// For ELF, cleanup any local-dynamic TLS accesses.
if (TM->getTargetTriple().isOSBinFormatELF() &&
getOptLevel() != CodeGenOpt::None)
addPass(createCleanupLocalDynamicTLSPass());
addPass(createX86GlobalBaseRegPass());
return false;
}
bool X86PassConfig::addIRTranslator() {
addPass(new IRTranslator());
return false;
}
bool X86PassConfig::addLegalizeMachineIR() {
addPass(new Legalizer());
return false;
}
bool X86PassConfig::addRegBankSelect() {
addPass(new RegBankSelect());
return false;
}
bool X86PassConfig::addGlobalInstructionSelect() {
addPass(new InstructionSelect());
return false;
}
bool X86PassConfig::addILPOpts() {
addPass(&EarlyIfConverterID);
if (EnableMachineCombinerPass)
addPass(&MachineCombinerID);
addPass(createX86CmovConverterPass());
return true;
}
bool X86PassConfig::addPreISel() {
// Only add this pass for 32-bit x86 Windows.
const Triple &TT = TM->getTargetTriple();
if (TT.isOSWindows() && TT.getArch() == Triple::x86)
addPass(createX86WinEHStatePass());
return true;
}
void X86PassConfig::addPreRegAlloc() {
if (getOptLevel() != CodeGenOpt::None) {
addPass(&LiveRangeShrinkID);
addPass(createX86FixupSetCC());
addPass(createX86OptimizeLEAs());
addPass(createX86CallFrameOptimization());
addPass(createX86AvoidStoreForwardingBlocks());
}
addPass(createX86WinAllocaExpander());
}
void X86PassConfig::addMachineSSAOptimization() {
addPass(createX86DomainReassignmentPass());
TargetPassConfig::addMachineSSAOptimization();
}
void X86PassConfig::addPostRegAlloc() {
addPass(createX86FloatingPointStackifierPass());
}
void X86PassConfig::addPreSched2() { addPass(createX86ExpandPseudoPass()); }
void X86PassConfig::addPreEmitPass() {
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
if (getOptLevel() != CodeGenOpt::None) {
addPass(new X86ExecutionDomainFix());
addPass(createBreakFalseDeps());
Separate ExecutionDepsFix into 4 parts: 1. ReachingDefsAnalysis - Allows to identify for each instruction what is the “closest” reaching def of a certain register. Used by BreakFalseDeps (for clearance calculation) and ExecutionDomainFix (for arbitrating conflicting domains). 2. ExecutionDomainFix - Changes the variant of the instructions in order to minimize domain crossings. 3. BreakFalseDeps - Breaks false dependencies. 4. LoopTraversal - Creatws a traversal order of the basic blocks that is optimal for loops (introduced in revision L293571). Both ExecutionDomainFix and ReachingDefsAnalysis use this to determine the order they will traverse the basic blocks. This also included the following changes to ExcecutionDepsFix original logic: 1. BreakFalseDeps and ReachingDefsAnalysis logic no longer restricted by a register class. 2. ReachingDefsAnalysis tracks liveness of reg units instead of reg indices into a given reg class. Additional changes in affected files: 1. X86 and ARM targets now inherit from ExecutionDomainFix instead of ExecutionDepsFix. BreakFalseDeps also was added to the passes they activate. 2. Comments and references to ExecutionDepsFix replaced with ExecutionDomainFix and BreakFalseDeps, as appropriate. Additional refactoring changes will follow. This commit is (almost) NFC. The only functional change is that now BreakFalseDeps will break dependency for all register classes. Since no additional instructions were added to the list of instructions that have false dependencies, there is no actual change yet. In a future commit several instructions (and tests) will be added. This is the first of multiple patches that fix bugzilla https://bugs.llvm.org/show_bug.cgi?id=33869 Most of the patches are intended at refactoring the existent code. Additional relevant reviews: https://reviews.llvm.org/D40331 https://reviews.llvm.org/D40332 https://reviews.llvm.org/D40333 https://reviews.llvm.org/D40334 Differential Revision: https://reviews.llvm.org/D40330 Change-Id: Icaeb75e014eff96a8f721377783f9a3e6c679275 llvm-svn: 323087
2018-01-22 18:05:23 +08:00
}
addPass(createX86IndirectBranchTrackingPass());
if (UseVZeroUpper)
addPass(createX86IssueVZeroUpperPass());
if (getOptLevel() != CodeGenOpt::None) {
addPass(createX86FixupBWInsts());
addPass(createX86PadShortFunctions());
addPass(createX86FixupLEAs());
addPass(createX86EvexToVexInsts());
}
}
Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715, "Branch Target Injection", and is one of the two halves to Spectre.. Summary: First, we need to explain the core of the vulnerability. Note that this is a very incomplete description, please see the Project Zero blog post for details: https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html The basis for branch target injection is to direct speculative execution of the processor to some "gadget" of executable code by poisoning the prediction of indirect branches with the address of that gadget. The gadget in turn contains an operation that provides a side channel for reading data. Most commonly, this will look like a load of secret data followed by a branch on the loaded value and then a load of some predictable cache line. The attacker then uses timing of the processors cache to determine which direction the branch took *in the speculative execution*, and in turn what one bit of the loaded value was. Due to the nature of these timing side channels and the branch predictor on Intel processors, this allows an attacker to leak data only accessible to a privileged domain (like the kernel) back into an unprivileged domain. The goal is simple: avoid generating code which contains an indirect branch that could have its prediction poisoned by an attacker. In many cases, the compiler can simply use directed conditional branches and a small search tree. LLVM already has support for lowering switches in this way and the first step of this patch is to disable jump-table lowering of switches and introduce a pass to rewrite explicit indirectbr sequences into a switch over integers. However, there is no fully general alternative to indirect calls. We introduce a new construct we call a "retpoline" to implement indirect calls in a non-speculatable way. It can be thought of loosely as a trampoline for indirect calls which uses the RET instruction on x86. Further, we arrange for a specific call->ret sequence which ensures the processor predicts the return to go to a controlled, known location. The retpoline then "smashes" the return address pushed onto the stack by the call with the desired target of the original indirect call. The result is a predicted return to the next instruction after a call (which can be used to trap speculative execution within an infinite loop) and an actual indirect branch to an arbitrary address. On 64-bit x86 ABIs, this is especially easily done in the compiler by using a guaranteed scratch register to pass the target into this device. For 32-bit ABIs there isn't a guaranteed scratch register and so several different retpoline variants are introduced to use a scratch register if one is available in the calling convention and to otherwise use direct stack push/pop sequences to pass the target address. This "retpoline" mitigation is fully described in the following blog post: https://support.google.com/faqs/answer/7625886 We also support a target feature that disables emission of the retpoline thunk by the compiler to allow for custom thunks if users want them. These are particularly useful in environments like kernels that routinely do hot-patching on boot and want to hot-patch their thunk to different code sequences. They can write this custom thunk and use `-mretpoline-external-thunk` *in addition* to `-mretpoline`. In this case, on x86-64 thu thunk names must be: ``` __llvm_external_retpoline_r11 ``` or on 32-bit: ``` __llvm_external_retpoline_eax __llvm_external_retpoline_ecx __llvm_external_retpoline_edx __llvm_external_retpoline_push ``` And the target of the retpoline is passed in the named register, or in the case of the `push` suffix on the top of the stack via a `pushl` instruction. There is one other important source of indirect branches in x86 ELF binaries: the PLT. These patches also include support for LLD to generate PLT entries that perform a retpoline-style indirection. The only other indirect branches remaining that we are aware of are from precompiled runtimes (such as crt0.o and similar). The ones we have found are not really attackable, and so we have not focused on them here, but eventually these runtimes should also be replicated for retpoline-ed configurations for completeness. For kernels or other freestanding or fully static executables, the compiler switch `-mretpoline` is sufficient to fully mitigate this particular attack. For dynamic executables, you must compile *all* libraries with `-mretpoline` and additionally link the dynamic executable and all shared libraries with LLD and pass `-z retpolineplt` (or use similar functionality from some other linker). We strongly recommend also using `-z now` as non-lazy binding allows the retpoline-mitigated PLT to be substantially smaller. When manually apply similar transformations to `-mretpoline` to the Linux kernel we observed very small performance hits to applications running typical workloads, and relatively minor hits (approximately 2%) even for extremely syscall-heavy applications. This is largely due to the small number of indirect branches that occur in performance sensitive paths of the kernel. When using these patches on statically linked applications, especially C++ applications, you should expect to see a much more dramatic performance hit. For microbenchmarks that are switch, indirect-, or virtual-call heavy we have seen overheads ranging from 10% to 50%. However, real-world workloads exhibit substantially lower performance impact. Notably, techniques such as PGO and ThinLTO dramatically reduce the impact of hot indirect calls (by speculatively promoting them to direct calls) and allow optimized search trees to be used to lower switches. If you need to deploy these techniques in C++ applications, we *strongly* recommend that you ensure all hot call targets are statically linked (avoiding PLT indirection) and use both PGO and ThinLTO. Well tuned servers using all of these techniques saw 5% - 10% overhead from the use of retpoline. We will add detailed documentation covering these components in subsequent patches, but wanted to make the core functionality available as soon as possible. Happy for more code review, but we'd really like to get these patches landed and backported ASAP for obvious reasons. We're planning to backport this to both 6.0 and 5.0 release streams and get a 5.0 release with just this cherry picked ASAP for distros and vendors. This patch is the work of a number of people over the past month: Eric, Reid, Rui, and myself. I'm mailing it out as a single commit due to the time sensitive nature of landing this and the need to backport it. Huge thanks to everyone who helped out here, and everyone at Intel who helped out in discussions about how to craft this. Also, credit goes to Paul Turner (at Google, but not an LLVM contributor) for much of the underlying retpoline design. Reviewers: echristo, rnk, ruiu, craig.topper, DavidKreitzer Subscribers: sanjoy, emaste, mcrosier, mgorny, mehdi_amini, hiraditya, llvm-commits Differential Revision: https://reviews.llvm.org/D41723 llvm-svn: 323155
2018-01-23 06:05:25 +08:00
void X86PassConfig::addPreEmitPass2() {
addPass(createX86RetpolineThunksPass());
}