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llvm-project/llvm/lib/Target/X86/X86CallFrameOptimization.cpp

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//===----- X86CallFrameOptimization.cpp - Optimize x86 call sequences -----===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines a pass that optimizes call sequences on x86.
// Currently, it converts movs of function parameters onto the stack into
// pushes. This is beneficial for two main reasons:
// 1) The push instruction encoding is much smaller than an esp-relative mov
// 2) It is possible to push memory arguments directly. So, if the
// the transformation is preformed pre-reg-alloc, it can help relieve
// register pressure.
//
//===----------------------------------------------------------------------===//
#include <algorithm>
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "X86MachineFunctionInfo.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetInstrInfo.h"
using namespace llvm;
#define DEBUG_TYPE "x86-cf-opt"
static cl::opt<bool>
NoX86CFOpt("no-x86-call-frame-opt",
cl::desc("Avoid optimizing x86 call frames for size"),
cl::init(false), cl::Hidden);
namespace {
class X86CallFrameOptimization : public MachineFunctionPass {
public:
X86CallFrameOptimization() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
// Information we know about a particular call site
struct CallContext {
CallContext()
: FrameSetup(nullptr), Call(nullptr), SPCopy(nullptr), ExpectedDist(0),
MovVector(4, nullptr), NoStackParams(false), UsePush(false){}
// Iterator referring to the frame setup instruction
MachineBasicBlock::iterator FrameSetup;
// Actual call instruction
MachineInstr *Call;
// A copy of the stack pointer
MachineInstr *SPCopy;
// The total displacement of all passed parameters
int64_t ExpectedDist;
// The sequence of movs used to pass the parameters
SmallVector<MachineInstr *, 4> MovVector;
// True if this call site has no stack parameters
bool NoStackParams;
// True of this callsite can use push instructions
bool UsePush;
};
typedef SmallVector<CallContext, 8> ContextVector;
bool isLegal(MachineFunction &MF);
bool isProfitable(MachineFunction &MF, ContextVector &CallSeqMap);
void collectCallInfo(MachineFunction &MF, MachineBasicBlock &MBB,
MachineBasicBlock::iterator I, CallContext &Context);
bool adjustCallSequence(MachineFunction &MF, const CallContext &Context);
MachineInstr *canFoldIntoRegPush(MachineBasicBlock::iterator FrameSetup,
unsigned Reg);
enum InstClassification { Convert, Skip, Exit };
InstClassification classifyInstruction(MachineBasicBlock &MBB,
MachineBasicBlock::iterator MI,
const X86RegisterInfo &RegInfo,
DenseSet<unsigned int> &UsedRegs);
const char *getPassName() const override { return "X86 Optimize Call Frame"; }
const TargetInstrInfo *TII;
const X86FrameLowering *TFL;
const X86Subtarget *STI;
const MachineRegisterInfo *MRI;
static char ID;
};
char X86CallFrameOptimization::ID = 0;
} // end anonymous namespace
FunctionPass *llvm::createX86CallFrameOptimization() {
return new X86CallFrameOptimization();
}
// This checks whether the transformation is legal.
// Also returns false in cases where it's potentially legal, but
// we don't even want to try.
bool X86CallFrameOptimization::isLegal(MachineFunction &MF) {
if (NoX86CFOpt.getValue())
return false;
// We currently only support call sequences where *all* parameters.
// are passed on the stack.
// No point in running this in 64-bit mode, since some arguments are
// passed in-register in all common calling conventions, so the pattern
// we're looking for will never match.
if (STI->is64Bit())
return false;
// We can't encode multiple DW_CFA_GNU_args_size or DW_CFA_def_cfa_offset
// in the compact unwind encoding that Darwin uses. So, bail if there
// is a danger of that being generated.
if (STI->isTargetDarwin() &&
(!MF.getMMI().getLandingPads().empty() ||
(MF.getFunction()->needsUnwindTableEntry() && !TFL->hasFP(MF))))
return false;
// You would expect straight-line code between call-frame setup and
// call-frame destroy. You would be wrong. There are circumstances (e.g.
// CMOV_GR8 expansion of a select that feeds a function call!) where we can
// end up with the setup and the destroy in different basic blocks.
// This is bad, and breaks SP adjustment.
// So, check that all of the frames in the function are closed inside
// the same block, and, for good measure, that there are no nested frames.
unsigned FrameSetupOpcode = TII->getCallFrameSetupOpcode();
unsigned FrameDestroyOpcode = TII->getCallFrameDestroyOpcode();
for (MachineBasicBlock &BB : MF) {
bool InsideFrameSequence = false;
for (MachineInstr &MI : BB) {
if (MI.getOpcode() == FrameSetupOpcode) {
if (InsideFrameSequence)
return false;
InsideFrameSequence = true;
} else if (MI.getOpcode() == FrameDestroyOpcode) {
if (!InsideFrameSequence)
return false;
InsideFrameSequence = false;
}
}
if (InsideFrameSequence)
return false;
}
return true;
}
// Check whether this trasnformation is profitable for a particular
// function - in terms of code size.
bool X86CallFrameOptimization::isProfitable(MachineFunction &MF,
ContextVector &CallSeqVector) {
// This transformation is always a win when we do not expect to have
// a reserved call frame. Under other circumstances, it may be either
// a win or a loss, and requires a heuristic.
bool CannotReserveFrame = MF.getFrameInfo()->hasVarSizedObjects();
if (CannotReserveFrame)
return true;
// Don't do this when not optimizing for size.
if (!MF.getFunction()->optForSize())
return false;
unsigned StackAlign = TFL->getStackAlignment();
int64_t Advantage = 0;
for (auto CC : CallSeqVector) {
// Call sites where no parameters are passed on the stack
// do not affect the cost, since there needs to be no
// stack adjustment.
if (CC.NoStackParams)
continue;
if (!CC.UsePush) {
// If we don't use pushes for a particular call site,
// we pay for not having a reserved call frame with an
// additional sub/add esp pair. The cost is ~3 bytes per instruction,
// depending on the size of the constant.
// TODO: Callee-pop functions should have a smaller penalty, because
// an add is needed even with a reserved call frame.
Advantage -= 6;
} else {
// We can use pushes. First, account for the fixed costs.
// We'll need a add after the call.
Advantage -= 3;
// If we have to realign the stack, we'll also need and sub before
if (CC.ExpectedDist % StackAlign)
Advantage -= 3;
// Now, for each push, we save ~3 bytes. For small constants, we actually,
// save more (up to 5 bytes), but 3 should be a good approximation.
Advantage += (CC.ExpectedDist / 4) * 3;
}
}
return (Advantage >= 0);
}
bool X86CallFrameOptimization::runOnMachineFunction(MachineFunction &MF) {
STI = &MF.getSubtarget<X86Subtarget>();
TII = STI->getInstrInfo();
TFL = STI->getFrameLowering();
MRI = &MF.getRegInfo();
if (!isLegal(MF))
return false;
unsigned FrameSetupOpcode = TII->getCallFrameSetupOpcode();
bool Changed = false;
ContextVector CallSeqVector;
for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); BB != E; ++BB)
for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
if (I->getOpcode() == FrameSetupOpcode) {
CallContext Context;
collectCallInfo(MF, *BB, I, Context);
CallSeqVector.push_back(Context);
}
if (!isProfitable(MF, CallSeqVector))
return false;
for (auto CC : CallSeqVector)
if (CC.UsePush)
Changed |= adjustCallSequence(MF, CC);
return Changed;
}
X86CallFrameOptimization::InstClassification
X86CallFrameOptimization::classifyInstruction(
MachineBasicBlock &MBB, MachineBasicBlock::iterator MI,
const X86RegisterInfo &RegInfo, DenseSet<unsigned int> &UsedRegs) {
if (MI == MBB.end())
return Exit;
// The instructions we actually care about are movs onto the stack
int Opcode = MI->getOpcode();
if (Opcode == X86::MOV32mi || Opcode == X86::MOV32mr)
return Convert;
// Not all calling conventions have only stack MOVs between the stack
// adjust and the call.
// We want to tolerate other instructions, to cover more cases.
// In particular:
// a) PCrel calls, where we expect an additional COPY of the basereg.
// b) Passing frame-index addresses.
// c) Calling conventions that have inreg parameters. These generate
// both copies and movs into registers.
// To avoid creating lots of special cases, allow any instruction
// that does not write into memory, does not def or use the stack
// pointer, and does not def any register that was used by a preceding
// push.
// (Reading from memory is allowed, even if referenced through a
// frame index, since these will get adjusted properly in PEI)
// The reason for the last condition is that the pushes can't replace
// the movs in place, because the order must be reversed.
// So if we have a MOV32mr that uses EDX, then an instruction that defs
// EDX, and then the call, after the transformation the push will use
// the modified version of EDX, and not the original one.
// Since we are still in SSA form at this point, we only need to
// make sure we don't clobber any *physical* registers that were
// used by an earlier mov that will become a push.
if (MI->isCall() || MI->mayStore())
return Exit;
for (const MachineOperand &MO : MI->operands()) {
if (!MO.isReg())
continue;
unsigned int Reg = MO.getReg();
if (!RegInfo.isPhysicalRegister(Reg))
continue;
if (RegInfo.regsOverlap(Reg, RegInfo.getStackRegister()))
return Exit;
if (MO.isDef()) {
for (unsigned int U : UsedRegs)
if (RegInfo.regsOverlap(Reg, U))
return Exit;
}
}
return Skip;
}
void X86CallFrameOptimization::collectCallInfo(MachineFunction &MF,
MachineBasicBlock &MBB,
MachineBasicBlock::iterator I,
CallContext &Context) {
// Check that this particular call sequence is amenable to the
// transformation.
const X86RegisterInfo &RegInfo = *static_cast<const X86RegisterInfo *>(
STI->getRegisterInfo());
unsigned FrameDestroyOpcode = TII->getCallFrameDestroyOpcode();
// We expect to enter this at the beginning of a call sequence
assert(I->getOpcode() == TII->getCallFrameSetupOpcode());
MachineBasicBlock::iterator FrameSetup = I++;
Context.FrameSetup = FrameSetup;
// How much do we adjust the stack? This puts an upper bound on
// the number of parameters actually passed on it.
unsigned int MaxAdjust = FrameSetup->getOperand(0).getImm() / 4;
// A zero adjustment means no stack parameters
if (!MaxAdjust) {
Context.NoStackParams = true;
return;
}
// For globals in PIC mode, we can have some LEAs here.
// Ignore them, they don't bother us.
// TODO: Extend this to something that covers more cases.
while (I->getOpcode() == X86::LEA32r)
++I;
// We expect a copy instruction here.
// TODO: The copy instruction is a lowering artifact.
// We should also support a copy-less version, where the stack
// pointer is used directly.
if (!I->isCopy() || !I->getOperand(0).isReg())
return;
Context.SPCopy = I++;
unsigned StackPtr = Context.SPCopy->getOperand(0).getReg();
// Scan the call setup sequence for the pattern we're looking for.
// We only handle a simple case - a sequence of MOV32mi or MOV32mr
// instructions, that push a sequence of 32-bit values onto the stack, with
// no gaps between them.
if (MaxAdjust > 4)
Context.MovVector.resize(MaxAdjust, nullptr);
InstClassification Classification;
DenseSet<unsigned int> UsedRegs;
while ((Classification = classifyInstruction(MBB, I, RegInfo, UsedRegs)) !=
Exit) {
if (Classification == Skip) {
++I;
continue;
}
// We know the instruction is a MOV32mi/MOV32mr.
// We only want movs of the form:
// movl imm/r32, k(%esp)
// If we run into something else, bail.
// Note that AddrBaseReg may, counter to its name, not be a register,
// but rather a frame index.
// TODO: Support the fi case. This should probably work now that we
// have the infrastructure to track the stack pointer within a call
// sequence.
if (!I->getOperand(X86::AddrBaseReg).isReg() ||
(I->getOperand(X86::AddrBaseReg).getReg() != StackPtr) ||
!I->getOperand(X86::AddrScaleAmt).isImm() ||
(I->getOperand(X86::AddrScaleAmt).getImm() != 1) ||
(I->getOperand(X86::AddrIndexReg).getReg() != X86::NoRegister) ||
(I->getOperand(X86::AddrSegmentReg).getReg() != X86::NoRegister) ||
!I->getOperand(X86::AddrDisp).isImm())
return;
int64_t StackDisp = I->getOperand(X86::AddrDisp).getImm();
assert(StackDisp >= 0 &&
"Negative stack displacement when passing parameters");
// We really don't want to consider the unaligned case.
if (StackDisp % 4)
return;
StackDisp /= 4;
assert((size_t)StackDisp < Context.MovVector.size() &&
"Function call has more parameters than the stack is adjusted for.");
// If the same stack slot is being filled twice, something's fishy.
if (Context.MovVector[StackDisp] != nullptr)
return;
Context.MovVector[StackDisp] = I;
for (const MachineOperand &MO : I->uses()) {
if (!MO.isReg())
continue;
unsigned int Reg = MO.getReg();
if (RegInfo.isPhysicalRegister(Reg))
UsedRegs.insert(Reg);
}
++I;
}
// We now expect the end of the sequence. If we stopped early,
// or reached the end of the block without finding a call, bail.
if (I == MBB.end() || !I->isCall())
return;
Context.Call = I;
if ((++I)->getOpcode() != FrameDestroyOpcode)
return;
// Now, go through the vector, and see that we don't have any gaps,
// but only a series of 32-bit MOVs.
auto MMI = Context.MovVector.begin(), MME = Context.MovVector.end();
for (; MMI != MME; ++MMI, Context.ExpectedDist += 4)
if (*MMI == nullptr)
break;
// If the call had no parameters, do nothing
if (MMI == Context.MovVector.begin())
return;
// We are either at the last parameter, or a gap.
// Make sure it's not a gap
for (; MMI != MME; ++MMI)
if (*MMI != nullptr)
return;
Context.UsePush = true;
}
bool X86CallFrameOptimization::adjustCallSequence(MachineFunction &MF,
const CallContext &Context) {
// Ok, we can in fact do the transformation for this call.
// Do not remove the FrameSetup instruction, but adjust the parameters.
// PEI will end up finalizing the handling of this.
MachineBasicBlock::iterator FrameSetup = Context.FrameSetup;
MachineBasicBlock &MBB = *(FrameSetup->getParent());
FrameSetup->getOperand(1).setImm(Context.ExpectedDist);
DebugLoc DL = FrameSetup->getDebugLoc();
// Now, iterate through the vector in reverse order, and replace the movs
// with pushes. MOVmi/MOVmr doesn't have any defs, so no need to
// replace uses.
for (int Idx = (Context.ExpectedDist / 4) - 1; Idx >= 0; --Idx) {
MachineBasicBlock::iterator MOV = *Context.MovVector[Idx];
MachineOperand PushOp = MOV->getOperand(X86::AddrNumOperands);
MachineBasicBlock::iterator Push = nullptr;
if (MOV->getOpcode() == X86::MOV32mi) {
unsigned PushOpcode = X86::PUSHi32;
// If the operand is a small (8-bit) immediate, we can use a
// PUSH instruction with a shorter encoding.
// Note that isImm() may fail even though this is a MOVmi, because
// the operand can also be a symbol.
if (PushOp.isImm()) {
int64_t Val = PushOp.getImm();
if (isInt<8>(Val))
PushOpcode = X86::PUSH32i8;
}
Push = BuildMI(MBB, Context.Call, DL, TII->get(PushOpcode))
.addOperand(PushOp);
} else {
unsigned int Reg = PushOp.getReg();
// If PUSHrmm is not slow on this target, try to fold the source of the
// push into the instruction.
bool SlowPUSHrmm = STI->isAtom() || STI->isSLM();
// Check that this is legal to fold. Right now, we're extremely
// conservative about that.
MachineInstr *DefMov = nullptr;
if (!SlowPUSHrmm && (DefMov = canFoldIntoRegPush(FrameSetup, Reg))) {
Push = BuildMI(MBB, Context.Call, DL, TII->get(X86::PUSH32rmm));
unsigned NumOps = DefMov->getDesc().getNumOperands();
for (unsigned i = NumOps - X86::AddrNumOperands; i != NumOps; ++i)
Push->addOperand(DefMov->getOperand(i));
DefMov->eraseFromParent();
} else {
Push = BuildMI(MBB, Context.Call, DL, TII->get(X86::PUSH32r))
.addReg(Reg)
.getInstr();
}
}
// For debugging, when using SP-based CFA, we need to adjust the CFA
// offset after each push.
// TODO: This is needed only if we require precise CFA.
if (!TFL->hasFP(MF))
TFL->BuildCFI(MBB, std::next(Push), DL,
MCCFIInstruction::createAdjustCfaOffset(nullptr, 4));
MBB.erase(MOV);
}
// The stack-pointer copy is no longer used in the call sequences.
// There should not be any other users, but we can't commit to that, so:
if (MRI->use_empty(Context.SPCopy->getOperand(0).getReg()))
Context.SPCopy->eraseFromParent();
// Once we've done this, we need to make sure PEI doesn't assume a reserved
// frame.
X86MachineFunctionInfo *FuncInfo = MF.getInfo<X86MachineFunctionInfo>();
FuncInfo->setHasPushSequences(true);
return true;
}
MachineInstr *X86CallFrameOptimization::canFoldIntoRegPush(
MachineBasicBlock::iterator FrameSetup, unsigned Reg) {
// Do an extremely restricted form of load folding.
// ISel will often create patterns like:
// movl 4(%edi), %eax
// movl 8(%edi), %ecx
// movl 12(%edi), %edx
// movl %edx, 8(%esp)
// movl %ecx, 4(%esp)
// movl %eax, (%esp)
// call
// Get rid of those with prejudice.
if (!TargetRegisterInfo::isVirtualRegister(Reg))
return nullptr;
// Make sure this is the only use of Reg.
if (!MRI->hasOneNonDBGUse(Reg))
return nullptr;
MachineBasicBlock::iterator DefMI = MRI->getVRegDef(Reg);
// Make sure the def is a MOV from memory.
// If the def is an another block, give up.
if (DefMI->getOpcode() != X86::MOV32rm ||
DefMI->getParent() != FrameSetup->getParent())
return nullptr;
// Make sure we don't have any instructions between DefMI and the
// push that make folding the load illegal.
for (auto I = DefMI; I != FrameSetup; ++I)
if (I->isLoadFoldBarrier())
return nullptr;
return DefMI;
}
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