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//===----- X86DynAllocaExpander.cpp - Expand DynAlloca pseudo instruction -===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This file defines a pass that expands DynAlloca pseudo-instructions.
//
// It performs a conservative analysis to determine whether each allocation
// falls within a region of the stack that is safe to use, or whether stack
// probes must be emitted.
//
//===----------------------------------------------------------------------===//
#include "X86.h"
#include "X86InstrBuilder.h"
#include "X86InstrInfo.h"
#include "X86MachineFunctionInfo.h"
#include "X86Subtarget.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
namespace {
class X86DynAllocaExpander : public MachineFunctionPass {
public:
X86DynAllocaExpander() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
private:
/// Strategies for lowering a DynAlloca.
enum Lowering { TouchAndSub, Sub, Probe };
/// Deterministic-order map from DynAlloca instruction to desired lowering.
typedef MapVector<MachineInstr*, Lowering> LoweringMap;
/// Compute which lowering to use for each DynAlloca instruction.
void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
/// Get the appropriate lowering based on current offset and amount.
Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
/// Lower a DynAlloca instruction.
void lower(MachineInstr* MI, Lowering L);
MachineRegisterInfo *MRI = nullptr;
const X86Subtarget *STI = nullptr;
const TargetInstrInfo *TII = nullptr;
const X86RegisterInfo *TRI = nullptr;
unsigned StackPtr = 0;
unsigned SlotSize = 0;
int64_t StackProbeSize = 0;
bool NoStackArgProbe = false;
StringRef getPassName() const override { return "X86 DynAlloca Expander"; }
static char ID;
};
char X86DynAllocaExpander::ID = 0;
} // end anonymous namespace
FunctionPass *llvm::createX86DynAllocaExpander() {
return new X86DynAllocaExpander();
}
/// Return the allocation amount for a DynAlloca instruction, or -1 if unknown.
static int64_t getDynAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
assert(MI->getOpcode() == X86::DYN_ALLOCA_32 ||
MI->getOpcode() == X86::DYN_ALLOCA_64);
assert(MI->getOperand(0).isReg());
Register AmountReg = MI->getOperand(0).getReg();
MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
if (!Def ||
(Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
!Def->getOperand(1).isImm())
return -1;
return Def->getOperand(1).getImm();
}
X86DynAllocaExpander::Lowering
X86DynAllocaExpander::getLowering(int64_t CurrentOffset,
int64_t AllocaAmount) {
// For a non-constant amount or a large amount, we have to probe.
if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
return Probe;
// If it fits within the safe region of the stack, just subtract.
if (CurrentOffset + AllocaAmount <= StackProbeSize)
return Sub;
// Otherwise, touch the current tip of the stack, then subtract.
return TouchAndSub;
}
static bool isPushPop(const MachineInstr &MI) {
switch (MI.getOpcode()) {
case X86::PUSH32i8:
case X86::PUSH32r:
case X86::PUSH32rmm:
case X86::PUSH32rmr:
case X86::PUSHi32:
case X86::PUSH64i8:
case X86::PUSH64r:
case X86::PUSH64rmm:
case X86::PUSH64rmr:
case X86::PUSH64i32:
case X86::POP32r:
case X86::POP64r:
return true;
default:
return false;
}
}
void X86DynAllocaExpander::computeLowerings(MachineFunction &MF,
LoweringMap &Lowerings) {
// Do a one-pass reverse post-order walk of the CFG to conservatively estimate
// the offset between the stack pointer and the lowest touched part of the
// stack, and use that to decide how to lower each DynAlloca instruction.
// Initialize OutOffset[B], the stack offset at exit from B, to something big.
DenseMap<MachineBasicBlock *, int64_t> OutOffset;
for (MachineBasicBlock &MBB : MF)
OutOffset[&MBB] = INT32_MAX;
// Note: we don't know the offset at the start of the entry block since the
// prologue hasn't been inserted yet, and how much that will adjust the stack
// pointer depends on register spills, which have not been computed yet.
// Compute the reverse post-order.
ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
for (MachineBasicBlock *MBB : RPO) {
int64_t Offset = -1;
for (MachineBasicBlock *Pred : MBB->predecessors())
Offset = std::max(Offset, OutOffset[Pred]);
if (Offset == -1) Offset = INT32_MAX;
for (MachineInstr &MI : *MBB) {
if (MI.getOpcode() == X86::DYN_ALLOCA_32 ||
MI.getOpcode() == X86::DYN_ALLOCA_64) {
// A DynAlloca moves StackPtr, and potentially touches it.
int64_t Amount = getDynAllocaAmount(&MI, MRI);
Lowering L = getLowering(Offset, Amount);
Lowerings[&MI] = L;
switch (L) {
case Sub:
Offset += Amount;
break;
case TouchAndSub:
Offset = Amount;
break;
case Probe:
Offset = 0;
break;
}
} else if (MI.isCall() || isPushPop(MI)) {
// Calls, pushes and pops touch the tip of the stack.
Offset = 0;
} else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
MI.getOpcode() == X86::ADJCALLSTACKUP64) {
Offset -= MI.getOperand(0).getImm();
} else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
Offset += MI.getOperand(0).getImm();
} else if (MI.modifiesRegister(StackPtr, TRI)) {
// Any other modification of SP means we've lost track of it.
Offset = INT32_MAX;
}
}
OutOffset[MBB] = Offset;
}
}
static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
if (Is64Bit)
return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
}
void X86DynAllocaExpander::lower(MachineInstr *MI, Lowering L) {
const DebugLoc &DL = MI->getDebugLoc();
MachineBasicBlock *MBB = MI->getParent();
MachineBasicBlock::iterator I = *MI;
int64_t Amount = getDynAllocaAmount(MI, MRI);
if (Amount == 0) {
MI->eraseFromParent();
return;
}
// These two variables differ on x32, which is a 64-bit target with a
// 32-bit alloca.
bool Is64Bit = STI->is64Bit();
bool Is64BitAlloca = MI->getOpcode() == X86::DYN_ALLOCA_64;
assert(SlotSize == 4 || SlotSize == 8);
std::optional<MachineFunction::DebugInstrOperandPair> InstrNum;
if (unsigned Num = MI->peekDebugInstrNum()) {
// Operand 2 of DYN_ALLOCAs contains the stack def.
InstrNum = {Num, 2};
}
switch (L) {
case TouchAndSub: {
assert(Amount >= SlotSize);
// Use a push to touch the top of the stack.
unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
.addReg(RegA, RegState::Undef);
Amount -= SlotSize;
if (!Amount)
break;
// Fall through to make any remaining adjustment.
[[fallthrough]];
}
case Sub:
assert(Amount > 0);
if (Amount == SlotSize) {
// Use push to save size.
unsigned RegA = Is64Bit ? X86::RAX : X86::EAX;
BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
.addReg(RegA, RegState::Undef);
} else {
// Sub.
BuildMI(*MBB, I, DL,
TII->get(getSubOpcode(Is64BitAlloca, Amount)), StackPtr)
.addReg(StackPtr)
.addImm(Amount);
}
break;
case Probe:
if (!NoStackArgProbe) {
// The probe lowering expects the amount in RAX/EAX.
unsigned RegA = Is64BitAlloca ? X86::RAX : X86::EAX;
BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
.addReg(MI->getOperand(0).getReg());
// Do the probe.
STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
/*InProlog=*/false, InstrNum);
} else {
// Sub
BuildMI(*MBB, I, DL,
TII->get(Is64BitAlloca ? X86::SUB64rr : X86::SUB32rr), StackPtr)
.addReg(StackPtr)
.addReg(MI->getOperand(0).getReg());
}
break;
}
Register AmountReg = MI->getOperand(0).getReg();
MI->eraseFromParent();
// Delete the definition of AmountReg.
if (MRI->use_empty(AmountReg))
if (MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg))
AmountDef->eraseFromParent();
}
bool X86DynAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
if (!MF.getInfo<X86MachineFunctionInfo>()->hasDynAlloca())
return false;
MRI = &MF.getRegInfo();
STI = &MF.getSubtarget<X86Subtarget>();
TII = STI->getInstrInfo();
TRI = STI->getRegisterInfo();
StackPtr = TRI->getStackRegister();
SlotSize = TRI->getSlotSize();
StackProbeSize = STI->getTargetLowering()->getStackProbeSize(MF);
NoStackArgProbe = MF.getFunction().hasFnAttribute("no-stack-arg-probe");
if (NoStackArgProbe)
StackProbeSize = INT64_MAX;
LoweringMap Lowerings;
computeLowerings(MF, Lowerings);
for (auto &P : Lowerings)
lower(P.first, P.second);
return true;
}