llvm-project/llvm/lib/CodeGen/TargetSubtargetInfo.cpp

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//===- TargetSubtargetInfo.cpp - General Target Information ----------------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file This file describes the general parts of a Subtarget.
//
//===----------------------------------------------------------------------===//
#include "llvm/CodeGen/TargetSubtargetInfo.h"
using namespace llvm;
TargetSubtargetInfo::TargetSubtargetInfo(
const Triple &TT, StringRef CPU, StringRef FS,
ArrayRef<SubtargetFeatureKV> PF, ArrayRef<SubtargetSubTypeKV> PD,
const MCWriteProcResEntry *WPR,
const MCWriteLatencyEntry *WL, const MCReadAdvanceEntry *RA,
const InstrStage *IS, const unsigned *OC, const unsigned *FP)
: MCSubtargetInfo(TT, CPU, FS, PF, PD, WPR, WL, RA, IS, OC, FP) {
}
TargetSubtargetInfo::~TargetSubtargetInfo() = default;
bool TargetSubtargetInfo::enableAtomicExpand() const {
return true;
}
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
bool TargetSubtargetInfo::enableIndirectBrExpand() const {
return false;
}
bool TargetSubtargetInfo::enableMachineScheduler() const {
return false;
}
bool TargetSubtargetInfo::enableJoinGlobalCopies() const {
return enableMachineScheduler();
}
bool TargetSubtargetInfo::enableRALocalReassignment(
CodeGenOpt::Level OptLevel) const {
return true;
}
Add logic to greedy reg alloc to avoid bad eviction chains This fixes bugzilla 26810 https://bugs.llvm.org/show_bug.cgi?id=26810 This is intended to prevent sequences like: movl %ebp, 8(%esp) # 4-byte Spill movl %ecx, %ebp movl %ebx, %ecx movl %edi, %ebx movl %edx, %edi cltd idivl %esi movl %edi, %edx movl %ebx, %edi movl %ecx, %ebx movl %ebp, %ecx movl 16(%esp), %ebp # 4 - byte Reload Such sequences are created in 2 scenarios: Scenario #1: vreg0 is evicted from physreg0 by vreg1 Evictee vreg0 is intended for region splitting with split candidate physreg0 (the reg vreg0 was evicted from) Region splitting creates a local interval because of interference with the evictor vreg1 (normally region spliiting creates 2 interval, the "by reg" and "by stack" intervals. Local interval created when interference occurs.) one of the split intervals ends up evicting vreg2 from physreg1 Evictee vreg2 is intended for region splitting with split candidate physreg1 one of the split intervals ends up evicting vreg3 from physreg2 etc.. until someone spills Scenario #2 vreg0 is evicted from physreg0 by vreg1 vreg2 is evicted from physreg2 by vreg3 etc Evictee vreg0 is intended for region splitting with split candidate physreg1 Region splitting creates a local interval because of interference with the evictor vreg1 one of the split intervals ends up evicting back original evictor vreg1 from physreg0 (the reg vreg0 was evicted from) Another evictee vreg2 is intended for region splitting with split candidate physreg1 one of the split intervals ends up evicting vreg3 from physreg2 etc.. until someone spills As compile time was a concern, I've added a flag to control weather we do cost calculations for local intervals we expect to be created (it's on by default for X86 target, off for the rest). Differential Revision: https://reviews.llvm.org/D35816 Change-Id: Id9411ff7bbb845463d289ba2ae97737a1ee7cc39 llvm-svn: 316295
2017-10-23 01:59:38 +08:00
bool TargetSubtargetInfo::enableAdvancedRASplitCost() const {
return false;
}
bool TargetSubtargetInfo::enablePostRAScheduler() const {
return getSchedModel().PostRAScheduler;
}
bool TargetSubtargetInfo::enablePostRAMachineScheduler() const {
return enableMachineScheduler() && enablePostRAScheduler();
}
bool TargetSubtargetInfo::useAA() const {
return false;
}
[AsmPrinter] Remove hidden flag -print-schedule. This patch removes hidden codegen flag -print-schedule effectively reverting the logic originally committed as r300311 (https://llvm.org/viewvc/llvm-project?view=revision&revision=300311). Flag -print-schedule was originally introduced by r300311 to address PR32216 (https://bugs.llvm.org/show_bug.cgi?id=32216). That bug was about adding "Better testing of schedule model instruction latencies/throughputs". These days, we can use llvm-mca to test scheduling models. So there is no longer a need for flag -print-schedule in LLVM. The main use case for PR32216 is now addressed by llvm-mca. Flag -print-schedule is mainly used for debugging purposes, and it is only actually used by x86 specific tests. We already have extensive (latency and throughput) tests under "test/tools/llvm-mca" for X86 processor models. That means, most (if not all) existing -print-schedule tests for X86 are redundant. When flag -print-schedule was first added to LLVM, several files had to be modified; a few APIs gained new arguments (see for example method MCAsmStreamer::EmitInstruction), and MCSubtargetInfo/TargetSubtargetInfo gained a couple of getSchedInfoStr() methods. Method getSchedInfoStr() had to originally work for both MCInst and MachineInstr. The original implmentation of getSchedInfoStr() introduced a subtle layering violation (reported as PR37160 and then fixed/worked-around by r330615). In retrospect, that new API could have been designed more optimally. We can always query MCSchedModel to get the latency and throughput. More importantly, the "sched-info" string should not have been generated by the subtarget. Note, r317782 fixed an issue where "print-schedule" didn't work very well in the presence of inline assembly. That commit is also reverted by this change. Differential Revision: https://reviews.llvm.org/D57244 llvm-svn: 353043
2019-02-04 20:51:26 +08:00
void TargetSubtargetInfo::mirFileLoaded(MachineFunction &MF) const { }