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llvm-project/llvm/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp

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//===-- WebAssemblyRegStackify.cpp - Register Stackification --------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// \brief This file implements a register stacking pass.
///
/// This pass reorders instructions to put register uses and defs in an order
/// such that they form single-use expression trees. Registers fitting this form
/// are then marked as "stackified", meaning references to them are replaced by
/// "push" and "pop" from the value stack.
///
/// This is primarily a code size optimization, since temporary values on the
/// value stack don't need to be named.
///
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h" // for WebAssembly::ARGUMENT_*
#include "WebAssembly.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyUtilities.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/LiveIntervals.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfoImpls.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-reg-stackify"
namespace {
class WebAssemblyRegStackify final : public MachineFunctionPass {
StringRef getPassName() const override {
return "WebAssembly Register Stackify";
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<MachineDominatorTree>();
AU.addRequired<LiveIntervals>();
AU.addPreserved<MachineBlockFrequencyInfo>();
AU.addPreserved<SlotIndexes>();
AU.addPreserved<LiveIntervals>();
AU.addPreservedID(LiveVariablesID);
AU.addPreserved<MachineDominatorTree>();
MachineFunctionPass::getAnalysisUsage(AU);
}
bool runOnMachineFunction(MachineFunction &MF) override;
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyRegStackify() : MachineFunctionPass(ID) {}
};
} // end anonymous namespace
char WebAssemblyRegStackify::ID = 0;
FunctionPass *llvm::createWebAssemblyRegStackify() {
return new WebAssemblyRegStackify();
}
// Decorate the given instruction with implicit operands that enforce the
// expression stack ordering constraints for an instruction which is on
// the expression stack.
static void ImposeStackOrdering(MachineInstr *MI) {
// Write the opaque VALUE_STACK register.
if (!MI->definesRegister(WebAssembly::VALUE_STACK))
MI->addOperand(MachineOperand::CreateReg(WebAssembly::VALUE_STACK,
/*isDef=*/true,
/*isImp=*/true));
// Also read the opaque VALUE_STACK register.
if (!MI->readsRegister(WebAssembly::VALUE_STACK))
MI->addOperand(MachineOperand::CreateReg(WebAssembly::VALUE_STACK,
/*isDef=*/false,
/*isImp=*/true));
}
// Convert an IMPLICIT_DEF instruction into an instruction which defines
// a constant zero value.
static void ConvertImplicitDefToConstZero(MachineInstr *MI,
MachineRegisterInfo &MRI,
const TargetInstrInfo *TII,
MachineFunction &MF) {
assert(MI->getOpcode() == TargetOpcode::IMPLICIT_DEF);
const auto *RegClass =
MRI.getRegClass(MI->getOperand(0).getReg());
if (RegClass == &WebAssembly::I32RegClass) {
MI->setDesc(TII->get(WebAssembly::CONST_I32));
MI->addOperand(MachineOperand::CreateImm(0));
} else if (RegClass == &WebAssembly::I64RegClass) {
MI->setDesc(TII->get(WebAssembly::CONST_I64));
MI->addOperand(MachineOperand::CreateImm(0));
} else if (RegClass == &WebAssembly::F32RegClass) {
MI->setDesc(TII->get(WebAssembly::CONST_F32));
ConstantFP *Val = cast<ConstantFP>(Constant::getNullValue(
Type::getFloatTy(MF.getFunction().getContext())));
MI->addOperand(MachineOperand::CreateFPImm(Val));
} else if (RegClass == &WebAssembly::F64RegClass) {
MI->setDesc(TII->get(WebAssembly::CONST_F64));
ConstantFP *Val = cast<ConstantFP>(Constant::getNullValue(
Type::getDoubleTy(MF.getFunction().getContext())));
MI->addOperand(MachineOperand::CreateFPImm(Val));
} else {
llvm_unreachable("Unexpected reg class");
}
}
// Determine whether a call to the callee referenced by
// MI->getOperand(CalleeOpNo) reads memory, writes memory, and/or has side
// effects.
static void QueryCallee(const MachineInstr &MI, unsigned CalleeOpNo, bool &Read,
bool &Write, bool &Effects, bool &StackPointer) {
// All calls can use the stack pointer.
StackPointer = true;
const MachineOperand &MO = MI.getOperand(CalleeOpNo);
if (MO.isGlobal()) {
const Constant *GV = MO.getGlobal();
if (const GlobalAlias *GA = dyn_cast<GlobalAlias>(GV))
if (!GA->isInterposable())
GV = GA->getAliasee();
if (const Function *F = dyn_cast<Function>(GV)) {
if (!F->doesNotThrow())
Effects = true;
if (F->doesNotAccessMemory())
return;
if (F->onlyReadsMemory()) {
Read = true;
return;
}
}
}
// Assume the worst.
Write = true;
Read = true;
Effects = true;
}
// Determine whether MI reads memory, writes memory, has side effects,
// and/or uses the stack pointer value.
static void Query(const MachineInstr &MI, AliasAnalysis &AA, bool &Read,
bool &Write, bool &Effects, bool &StackPointer) {
assert(!MI.isPosition());
assert(!MI.isTerminator());
if (MI.isDebugValue())
return;
// Check for loads.
if (MI.mayLoad() && !MI.isDereferenceableInvariantLoad(&AA))
Read = true;
// Check for stores.
if (MI.mayStore()) {
Write = true;
// Check for stores to __stack_pointer.
for (auto MMO : MI.memoperands()) {
const MachinePointerInfo &MPI = MMO->getPointerInfo();
if (MPI.V.is<const PseudoSourceValue *>()) {
auto PSV = MPI.V.get<const PseudoSourceValue *>();
if (const ExternalSymbolPseudoSourceValue *EPSV =
dyn_cast<ExternalSymbolPseudoSourceValue>(PSV))
if (StringRef(EPSV->getSymbol()) == "__stack_pointer") {
StackPointer = true;
}
}
}
} else if (MI.hasOrderedMemoryRef()) {
switch (MI.getOpcode()) {
case WebAssembly::DIV_S_I32: case WebAssembly::DIV_S_I64:
case WebAssembly::REM_S_I32: case WebAssembly::REM_S_I64:
case WebAssembly::DIV_U_I32: case WebAssembly::DIV_U_I64:
case WebAssembly::REM_U_I32: case WebAssembly::REM_U_I64:
case WebAssembly::I32_TRUNC_S_F32: case WebAssembly::I64_TRUNC_S_F32:
case WebAssembly::I32_TRUNC_S_F64: case WebAssembly::I64_TRUNC_S_F64:
case WebAssembly::I32_TRUNC_U_F32: case WebAssembly::I64_TRUNC_U_F32:
case WebAssembly::I32_TRUNC_U_F64: case WebAssembly::I64_TRUNC_U_F64:
// These instruction have hasUnmodeledSideEffects() returning true
// because they trap on overflow and invalid so they can't be arbitrarily
// moved, however hasOrderedMemoryRef() interprets this plus their lack
// of memoperands as having a potential unknown memory reference.
break;
default:
// Record volatile accesses, unless it's a call, as calls are handled
// specially below.
if (!MI.isCall()) {
Write = true;
Effects = true;
}
break;
}
}
// Check for side effects.
if (MI.hasUnmodeledSideEffects()) {
switch (MI.getOpcode()) {
case WebAssembly::DIV_S_I32: case WebAssembly::DIV_S_I64:
case WebAssembly::REM_S_I32: case WebAssembly::REM_S_I64:
case WebAssembly::DIV_U_I32: case WebAssembly::DIV_U_I64:
case WebAssembly::REM_U_I32: case WebAssembly::REM_U_I64:
case WebAssembly::I32_TRUNC_S_F32: case WebAssembly::I64_TRUNC_S_F32:
case WebAssembly::I32_TRUNC_S_F64: case WebAssembly::I64_TRUNC_S_F64:
case WebAssembly::I32_TRUNC_U_F32: case WebAssembly::I64_TRUNC_U_F32:
case WebAssembly::I32_TRUNC_U_F64: case WebAssembly::I64_TRUNC_U_F64:
// These instructions have hasUnmodeledSideEffects() returning true
// because they trap on overflow and invalid so they can't be arbitrarily
// moved, however in the specific case of register stackifying, it is safe
// to move them because overflow and invalid are Undefined Behavior.
break;
default:
Effects = true;
break;
}
}
// Analyze calls.
if (MI.isCall()) {
switch (MI.getOpcode()) {
case WebAssembly::CALL_VOID:
case WebAssembly::CALL_INDIRECT_VOID:
QueryCallee(MI, 0, Read, Write, Effects, StackPointer);
break;
case WebAssembly::CALL_I32: case WebAssembly::CALL_I64:
case WebAssembly::CALL_F32: case WebAssembly::CALL_F64:
case WebAssembly::CALL_INDIRECT_I32: case WebAssembly::CALL_INDIRECT_I64:
case WebAssembly::CALL_INDIRECT_F32: case WebAssembly::CALL_INDIRECT_F64:
QueryCallee(MI, 1, Read, Write, Effects, StackPointer);
break;
default:
llvm_unreachable("unexpected call opcode");
}
}
}
// Test whether Def is safe and profitable to rematerialize.
static bool ShouldRematerialize(const MachineInstr &Def, AliasAnalysis &AA,
const WebAssemblyInstrInfo *TII) {
return Def.isAsCheapAsAMove() && TII->isTriviallyReMaterializable(Def, &AA);
}
// Identify the definition for this register at this point. This is a
// generalization of MachineRegisterInfo::getUniqueVRegDef that uses
// LiveIntervals to handle complex cases.
static MachineInstr *GetVRegDef(unsigned Reg, const MachineInstr *Insert,
const MachineRegisterInfo &MRI,
const LiveIntervals &LIS)
{
// Most registers are in SSA form here so we try a quick MRI query first.
if (MachineInstr *Def = MRI.getUniqueVRegDef(Reg))
return Def;
// MRI doesn't know what the Def is. Try asking LIS.
if (const VNInfo *ValNo = LIS.getInterval(Reg).getVNInfoBefore(
LIS.getInstructionIndex(*Insert)))
return LIS.getInstructionFromIndex(ValNo->def);
return nullptr;
}
// Test whether Reg, as defined at Def, has exactly one use. This is a
// generalization of MachineRegisterInfo::hasOneUse that uses LiveIntervals
// to handle complex cases.
static bool HasOneUse(unsigned Reg, MachineInstr *Def,
MachineRegisterInfo &MRI, MachineDominatorTree &MDT,
LiveIntervals &LIS) {
// Most registers are in SSA form here so we try a quick MRI query first.
if (MRI.hasOneUse(Reg))
return true;
bool HasOne = false;
const LiveInterval &LI = LIS.getInterval(Reg);
const VNInfo *DefVNI = LI.getVNInfoAt(
LIS.getInstructionIndex(*Def).getRegSlot());
assert(DefVNI);
for (auto &I : MRI.use_nodbg_operands(Reg)) {
const auto &Result = LI.Query(LIS.getInstructionIndex(*I.getParent()));
if (Result.valueIn() == DefVNI) {
if (!Result.isKill())
return false;
if (HasOne)
return false;
HasOne = true;
}
}
return HasOne;
}
// Test whether it's safe to move Def to just before Insert.
// TODO: Compute memory dependencies in a way that doesn't require always
// walking the block.
// TODO: Compute memory dependencies in a way that uses AliasAnalysis to be
// more precise.
static bool IsSafeToMove(const MachineInstr *Def, const MachineInstr *Insert,
AliasAnalysis &AA, const MachineRegisterInfo &MRI) {
assert(Def->getParent() == Insert->getParent());
// Check for register dependencies.
SmallVector<unsigned, 4> MutableRegisters;
for (const MachineOperand &MO : Def->operands()) {
if (!MO.isReg() || MO.isUndef())
continue;
unsigned Reg = MO.getReg();
// If the register is dead here and at Insert, ignore it.
if (MO.isDead() && Insert->definesRegister(Reg) &&
!Insert->readsRegister(Reg))
continue;
if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
// Ignore ARGUMENTS; it's just used to keep the ARGUMENT_* instructions
// from moving down, and we've already checked for that.
if (Reg == WebAssembly::ARGUMENTS)
continue;
// If the physical register is never modified, ignore it.
if (!MRI.isPhysRegModified(Reg))
continue;
// Otherwise, it's a physical register with unknown liveness.
return false;
}
// If one of the operands isn't in SSA form, it has different values at
// different times, and we need to make sure we don't move our use across
// a different def.
if (!MO.isDef() && !MRI.hasOneDef(Reg))
MutableRegisters.push_back(Reg);
}
bool Read = false, Write = false, Effects = false, StackPointer = false;
Query(*Def, AA, Read, Write, Effects, StackPointer);
// If the instruction does not access memory and has no side effects, it has
// no additional dependencies.
bool HasMutableRegisters = !MutableRegisters.empty();
if (!Read && !Write && !Effects && !StackPointer && !HasMutableRegisters)
return true;
// Scan through the intervening instructions between Def and Insert.
MachineBasicBlock::const_iterator D(Def), I(Insert);
for (--I; I != D; --I) {
bool InterveningRead = false;
bool InterveningWrite = false;
bool InterveningEffects = false;
bool InterveningStackPointer = false;
Query(*I, AA, InterveningRead, InterveningWrite, InterveningEffects,
InterveningStackPointer);
if (Effects && InterveningEffects)
return false;
if (Read && InterveningWrite)
return false;
if (Write && (InterveningRead || InterveningWrite))
return false;
if (StackPointer && InterveningStackPointer)
return false;
for (unsigned Reg : MutableRegisters)
for (const MachineOperand &MO : I->operands())
if (MO.isReg() && MO.isDef() && MO.getReg() == Reg)
return false;
}
return true;
}
/// Test whether OneUse, a use of Reg, dominates all of Reg's other uses.
static bool OneUseDominatesOtherUses(unsigned Reg, const MachineOperand &OneUse,
const MachineBasicBlock &MBB,
const MachineRegisterInfo &MRI,
const MachineDominatorTree &MDT,
LiveIntervals &LIS,
WebAssemblyFunctionInfo &MFI) {
const LiveInterval &LI = LIS.getInterval(Reg);
const MachineInstr *OneUseInst = OneUse.getParent();
VNInfo *OneUseVNI = LI.getVNInfoBefore(LIS.getInstructionIndex(*OneUseInst));
for (const MachineOperand &Use : MRI.use_nodbg_operands(Reg)) {
if (&Use == &OneUse)
continue;
const MachineInstr *UseInst = Use.getParent();
VNInfo *UseVNI = LI.getVNInfoBefore(LIS.getInstructionIndex(*UseInst));
if (UseVNI != OneUseVNI)
continue;
const MachineInstr *OneUseInst = OneUse.getParent();
if (UseInst == OneUseInst) {
// Another use in the same instruction. We need to ensure that the one
// selected use happens "before" it.
if (&OneUse > &Use)
return false;
} else {
// Test that the use is dominated by the one selected use.
while (!MDT.dominates(OneUseInst, UseInst)) {
// Actually, dominating is over-conservative. Test that the use would
// happen after the one selected use in the stack evaluation order.
//
// This is needed as a consequence of using implicit get_locals for
// uses and implicit set_locals for defs.
if (UseInst->getDesc().getNumDefs() == 0)
return false;
const MachineOperand &MO = UseInst->getOperand(0);
if (!MO.isReg())
return false;
unsigned DefReg = MO.getReg();
if (!TargetRegisterInfo::isVirtualRegister(DefReg) ||
!MFI.isVRegStackified(DefReg))
return false;
assert(MRI.hasOneUse(DefReg));
const MachineOperand &NewUse = *MRI.use_begin(DefReg);
const MachineInstr *NewUseInst = NewUse.getParent();
if (NewUseInst == OneUseInst) {
if (&OneUse > &NewUse)
return false;
break;
}
UseInst = NewUseInst;
}
}
}
return true;
}
/// Get the appropriate tee opcode for the given register class.
static unsigned GetTeeOpcode(const TargetRegisterClass *RC) {
if (RC == &WebAssembly::I32RegClass)
return WebAssembly::TEE_I32;
if (RC == &WebAssembly::I64RegClass)
return WebAssembly::TEE_I64;
if (RC == &WebAssembly::F32RegClass)
return WebAssembly::TEE_F32;
if (RC == &WebAssembly::F64RegClass)
return WebAssembly::TEE_F64;
if (RC == &WebAssembly::V128RegClass)
return WebAssembly::TEE_V128;
llvm_unreachable("Unexpected register class");
}
// Shrink LI to its uses, cleaning up LI.
static void ShrinkToUses(LiveInterval &LI, LiveIntervals &LIS) {
if (LIS.shrinkToUses(&LI)) {
SmallVector<LiveInterval*, 4> SplitLIs;
LIS.splitSeparateComponents(LI, SplitLIs);
}
}
/// A single-use def in the same block with no intervening memory or register
/// dependencies; move the def down and nest it with the current instruction.
static MachineInstr *MoveForSingleUse(unsigned Reg, MachineOperand& Op,
MachineInstr *Def,
MachineBasicBlock &MBB,
MachineInstr *Insert, LiveIntervals &LIS,
WebAssemblyFunctionInfo &MFI,
MachineRegisterInfo &MRI) {
DEBUG(dbgs() << "Move for single use: "; Def->dump());
MBB.splice(Insert, &MBB, Def);
LIS.handleMove(*Def);
if (MRI.hasOneDef(Reg) && MRI.hasOneUse(Reg)) {
// No one else is using this register for anything so we can just stackify
// it in place.
MFI.stackifyVReg(Reg);
} else {
// The register may have unrelated uses or defs; create a new register for
// just our one def and use so that we can stackify it.
unsigned NewReg = MRI.createVirtualRegister(MRI.getRegClass(Reg));
Def->getOperand(0).setReg(NewReg);
Op.setReg(NewReg);
// Tell LiveIntervals about the new register.
LIS.createAndComputeVirtRegInterval(NewReg);
// Tell LiveIntervals about the changes to the old register.
LiveInterval &LI = LIS.getInterval(Reg);
LI.removeSegment(LIS.getInstructionIndex(*Def).getRegSlot(),
LIS.getInstructionIndex(*Op.getParent()).getRegSlot(),
/*RemoveDeadValNo=*/true);
MFI.stackifyVReg(NewReg);
DEBUG(dbgs() << " - Replaced register: "; Def->dump());
}
ImposeStackOrdering(Def);
return Def;
}
/// A trivially cloneable instruction; clone it and nest the new copy with the
/// current instruction.
static MachineInstr *RematerializeCheapDef(
unsigned Reg, MachineOperand &Op, MachineInstr &Def, MachineBasicBlock &MBB,
MachineBasicBlock::instr_iterator Insert, LiveIntervals &LIS,
WebAssemblyFunctionInfo &MFI, MachineRegisterInfo &MRI,
const WebAssemblyInstrInfo *TII, const WebAssemblyRegisterInfo *TRI) {
DEBUG(dbgs() << "Rematerializing cheap def: "; Def.dump());
DEBUG(dbgs() << " - for use in "; Op.getParent()->dump());
unsigned NewReg = MRI.createVirtualRegister(MRI.getRegClass(Reg));
TII->reMaterialize(MBB, Insert, NewReg, 0, Def, *TRI);
Op.setReg(NewReg);
MachineInstr *Clone = &*std::prev(Insert);
LIS.InsertMachineInstrInMaps(*Clone);
LIS.createAndComputeVirtRegInterval(NewReg);
MFI.stackifyVReg(NewReg);
ImposeStackOrdering(Clone);
DEBUG(dbgs() << " - Cloned to "; Clone->dump());
// Shrink the interval.
bool IsDead = MRI.use_empty(Reg);
if (!IsDead) {
LiveInterval &LI = LIS.getInterval(Reg);
ShrinkToUses(LI, LIS);
IsDead = !LI.liveAt(LIS.getInstructionIndex(Def).getDeadSlot());
}
// If that was the last use of the original, delete the original.
if (IsDead) {
DEBUG(dbgs() << " - Deleting original\n");
SlotIndex Idx = LIS.getInstructionIndex(Def).getRegSlot();
LIS.removePhysRegDefAt(WebAssembly::ARGUMENTS, Idx);
LIS.removeInterval(Reg);
LIS.RemoveMachineInstrFromMaps(Def);
Def.eraseFromParent();
}
return Clone;
}
/// A multiple-use def in the same block with no intervening memory or register
/// dependencies; move the def down, nest it with the current instruction, and
/// insert a tee to satisfy the rest of the uses. As an illustration, rewrite
/// this:
///
/// Reg = INST ... // Def
/// INST ..., Reg, ... // Insert
/// INST ..., Reg, ...
/// INST ..., Reg, ...
///
/// to this:
///
/// DefReg = INST ... // Def (to become the new Insert)
/// TeeReg, Reg = TEE_... DefReg
/// INST ..., TeeReg, ... // Insert
/// INST ..., Reg, ...
/// INST ..., Reg, ...
///
/// with DefReg and TeeReg stackified. This eliminates a get_local from the
/// resulting code.
static MachineInstr *MoveAndTeeForMultiUse(
unsigned Reg, MachineOperand &Op, MachineInstr *Def, MachineBasicBlock &MBB,
MachineInstr *Insert, LiveIntervals &LIS, WebAssemblyFunctionInfo &MFI,
MachineRegisterInfo &MRI, const WebAssemblyInstrInfo *TII) {
DEBUG(dbgs() << "Move and tee for multi-use:"; Def->dump());
// Move Def into place.
MBB.splice(Insert, &MBB, Def);
LIS.handleMove(*Def);
// Create the Tee and attach the registers.
const auto *RegClass = MRI.getRegClass(Reg);
unsigned TeeReg = MRI.createVirtualRegister(RegClass);
unsigned DefReg = MRI.createVirtualRegister(RegClass);
MachineOperand &DefMO = Def->getOperand(0);
MachineInstr *Tee = BuildMI(MBB, Insert, Insert->getDebugLoc(),
TII->get(GetTeeOpcode(RegClass)), TeeReg)
.addReg(Reg, RegState::Define)
.addReg(DefReg, getUndefRegState(DefMO.isDead()));
Op.setReg(TeeReg);
DefMO.setReg(DefReg);
SlotIndex TeeIdx = LIS.InsertMachineInstrInMaps(*Tee).getRegSlot();
SlotIndex DefIdx = LIS.getInstructionIndex(*Def).getRegSlot();
// Tell LiveIntervals we moved the original vreg def from Def to Tee.
LiveInterval &LI = LIS.getInterval(Reg);
LiveInterval::iterator I = LI.FindSegmentContaining(DefIdx);
VNInfo *ValNo = LI.getVNInfoAt(DefIdx);
I->start = TeeIdx;
ValNo->def = TeeIdx;
ShrinkToUses(LI, LIS);
// Finish stackifying the new regs.
LIS.createAndComputeVirtRegInterval(TeeReg);
LIS.createAndComputeVirtRegInterval(DefReg);
MFI.stackifyVReg(DefReg);
MFI.stackifyVReg(TeeReg);
ImposeStackOrdering(Def);
ImposeStackOrdering(Tee);
DEBUG(dbgs() << " - Replaced register: "; Def->dump());
DEBUG(dbgs() << " - Tee instruction: "; Tee->dump());
return Def;
}
namespace {
/// A stack for walking the tree of instructions being built, visiting the
/// MachineOperands in DFS order.
class TreeWalkerState {
typedef MachineInstr::mop_iterator mop_iterator;
typedef std::reverse_iterator<mop_iterator> mop_reverse_iterator;
typedef iterator_range<mop_reverse_iterator> RangeTy;
SmallVector<RangeTy, 4> Worklist;
public:
explicit TreeWalkerState(MachineInstr *Insert) {
const iterator_range<mop_iterator> &Range = Insert->explicit_uses();
if (Range.begin() != Range.end())
Worklist.push_back(reverse(Range));
}
bool Done() const { return Worklist.empty(); }
MachineOperand &Pop() {
RangeTy &Range = Worklist.back();
MachineOperand &Op = *Range.begin();
Range = drop_begin(Range, 1);
if (Range.begin() == Range.end())
Worklist.pop_back();
assert((Worklist.empty() ||
Worklist.back().begin() != Worklist.back().end()) &&
"Empty ranges shouldn't remain in the worklist");
return Op;
}
/// Push Instr's operands onto the stack to be visited.
void PushOperands(MachineInstr *Instr) {
const iterator_range<mop_iterator> &Range(Instr->explicit_uses());
if (Range.begin() != Range.end())
Worklist.push_back(reverse(Range));
}
/// Some of Instr's operands are on the top of the stack; remove them and
/// re-insert them starting from the beginning (because we've commuted them).
void ResetTopOperands(MachineInstr *Instr) {
assert(HasRemainingOperands(Instr) &&
"Reseting operands should only be done when the instruction has "
"an operand still on the stack");
Worklist.back() = reverse(Instr->explicit_uses());
}
/// Test whether Instr has operands remaining to be visited at the top of
/// the stack.
bool HasRemainingOperands(const MachineInstr *Instr) const {
if (Worklist.empty())
return false;
const RangeTy &Range = Worklist.back();
return Range.begin() != Range.end() && Range.begin()->getParent() == Instr;
}
/// Test whether the given register is present on the stack, indicating an
/// operand in the tree that we haven't visited yet. Moving a definition of
/// Reg to a point in the tree after that would change its value.
///
/// This is needed as a consequence of using implicit get_locals for
/// uses and implicit set_locals for defs.
bool IsOnStack(unsigned Reg) const {
for (const RangeTy &Range : Worklist)
for (const MachineOperand &MO : Range)
if (MO.isReg() && MO.getReg() == Reg)
return true;
return false;
}
};
/// State to keep track of whether commuting is in flight or whether it's been
/// tried for the current instruction and didn't work.
class CommutingState {
/// There are effectively three states: the initial state where we haven't
/// started commuting anything and we don't know anything yet, the tenative
/// state where we've commuted the operands of the current instruction and are
/// revisting it, and the declined state where we've reverted the operands
/// back to their original order and will no longer commute it further.
bool TentativelyCommuting;
bool Declined;
/// During the tentative state, these hold the operand indices of the commuted
/// operands.
unsigned Operand0, Operand1;
public:
CommutingState() : TentativelyCommuting(false), Declined(false) {}
/// Stackification for an operand was not successful due to ordering
/// constraints. If possible, and if we haven't already tried it and declined
/// it, commute Insert's operands and prepare to revisit it.
void MaybeCommute(MachineInstr *Insert, TreeWalkerState &TreeWalker,
const WebAssemblyInstrInfo *TII) {
if (TentativelyCommuting) {
assert(!Declined &&
"Don't decline commuting until you've finished trying it");
// Commuting didn't help. Revert it.
TII->commuteInstruction(*Insert, /*NewMI=*/false, Operand0, Operand1);
TentativelyCommuting = false;
Declined = true;
} else if (!Declined && TreeWalker.HasRemainingOperands(Insert)) {
Operand0 = TargetInstrInfo::CommuteAnyOperandIndex;
Operand1 = TargetInstrInfo::CommuteAnyOperandIndex;
if (TII->findCommutedOpIndices(*Insert, Operand0, Operand1)) {
// Tentatively commute the operands and try again.
TII->commuteInstruction(*Insert, /*NewMI=*/false, Operand0, Operand1);
TreeWalker.ResetTopOperands(Insert);
TentativelyCommuting = true;
Declined = false;
}
}
}
/// Stackification for some operand was successful. Reset to the default
/// state.
void Reset() {
TentativelyCommuting = false;
Declined = false;
}
};
} // end anonymous namespace
bool WebAssemblyRegStackify::runOnMachineFunction(MachineFunction &MF) {
DEBUG(dbgs() << "********** Register Stackifying **********\n"
"********** Function: "
<< MF.getName() << '\n');
bool Changed = false;
MachineRegisterInfo &MRI = MF.getRegInfo();
WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
const auto *TRI = MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
MachineDominatorTree &MDT = getAnalysis<MachineDominatorTree>();
LiveIntervals &LIS = getAnalysis<LiveIntervals>();
// Disable the TEE optimization if we aren't doing direct wasm object
// emission, because lowering TEE to TEE_LOCAL is done in the ExplicitLocals
// pass, which is also disabled.
bool UseTee = true;
if (MF.getSubtarget<WebAssemblySubtarget>()
.getTargetTriple().isOSBinFormatELF())
UseTee = false;
// Walk the instructions from the bottom up. Currently we don't look past
// block boundaries, and the blocks aren't ordered so the block visitation
// order isn't significant, but we may want to change this in the future.
for (MachineBasicBlock &MBB : MF) {
// Don't use a range-based for loop, because we modify the list as we're
// iterating over it and the end iterator may change.
for (auto MII = MBB.rbegin(); MII != MBB.rend(); ++MII) {
MachineInstr *Insert = &*MII;
// Don't nest anything inside an inline asm, because we don't have
// constraints for $push inputs.
if (Insert->getOpcode() == TargetOpcode::INLINEASM)
continue;
// Ignore debugging intrinsics.
if (Insert->getOpcode() == TargetOpcode::DBG_VALUE)
continue;
// Iterate through the inputs in reverse order, since we'll be pulling
// operands off the stack in LIFO order.
CommutingState Commuting;
TreeWalkerState TreeWalker(Insert);
while (!TreeWalker.Done()) {
MachineOperand &Op = TreeWalker.Pop();
// We're only interested in explicit virtual register operands.
if (!Op.isReg())
continue;
unsigned Reg = Op.getReg();
assert(Op.isUse() && "explicit_uses() should only iterate over uses");
assert(!Op.isImplicit() &&
"explicit_uses() should only iterate over explicit operands");
if (TargetRegisterInfo::isPhysicalRegister(Reg))
continue;
// Identify the definition for this register at this point.
MachineInstr *Def = GetVRegDef(Reg, Insert, MRI, LIS);
if (!Def)
continue;
// Don't nest an INLINE_ASM def into anything, because we don't have
// constraints for $pop outputs.
if (Def->getOpcode() == TargetOpcode::INLINEASM)
continue;
// Argument instructions represent live-in registers and not real
// instructions.
if (WebAssembly::isArgument(*Def))
continue;
// Decide which strategy to take. Prefer to move a single-use value
// over cloning it, and prefer cloning over introducing a tee.
// For moving, we require the def to be in the same block as the use;
// this makes things simpler (LiveIntervals' handleMove function only
// supports intra-block moves) and it's MachineSink's job to catch all
// the sinking opportunities anyway.
bool SameBlock = Def->getParent() == &MBB;
bool CanMove = SameBlock && IsSafeToMove(Def, Insert, AA, MRI) &&
!TreeWalker.IsOnStack(Reg);
if (CanMove && HasOneUse(Reg, Def, MRI, MDT, LIS)) {
Insert = MoveForSingleUse(Reg, Op, Def, MBB, Insert, LIS, MFI, MRI);
} else if (ShouldRematerialize(*Def, AA, TII)) {
Insert =
RematerializeCheapDef(Reg, Op, *Def, MBB, Insert->getIterator(),
LIS, MFI, MRI, TII, TRI);
} else if (UseTee && CanMove &&
OneUseDominatesOtherUses(Reg, Op, MBB, MRI, MDT, LIS, MFI)) {
Insert = MoveAndTeeForMultiUse(Reg, Op, Def, MBB, Insert, LIS, MFI,
MRI, TII);
} else {
// We failed to stackify the operand. If the problem was ordering
// constraints, Commuting may be able to help.
if (!CanMove && SameBlock)
Commuting.MaybeCommute(Insert, TreeWalker, TII);
// Proceed to the next operand.
continue;
}
// If the instruction we just stackified is an IMPLICIT_DEF, convert it
// to a constant 0 so that the def is explicit, and the push/pop
// correspondence is maintained.
if (Insert->getOpcode() == TargetOpcode::IMPLICIT_DEF)
ConvertImplicitDefToConstZero(Insert, MRI, TII, MF);
// We stackified an operand. Add the defining instruction's operands to
// the worklist stack now to continue to build an ever deeper tree.
Commuting.Reset();
TreeWalker.PushOperands(Insert);
}
// If we stackified any operands, skip over the tree to start looking for
// the next instruction we can build a tree on.
if (Insert != &*MII) {
ImposeStackOrdering(&*MII);
MII = MachineBasicBlock::iterator(Insert).getReverse();
Changed = true;
}
}
}
// If we used VALUE_STACK anywhere, add it to the live-in sets everywhere so
// that it never looks like a use-before-def.
if (Changed) {
MF.getRegInfo().addLiveIn(WebAssembly::VALUE_STACK);
for (MachineBasicBlock &MBB : MF)
MBB.addLiveIn(WebAssembly::VALUE_STACK);
}
#ifndef NDEBUG
// Verify that pushes and pops are performed in LIFO order.
SmallVector<unsigned, 0> Stack;
for (MachineBasicBlock &MBB : MF) {
for (MachineInstr &MI : MBB) {
if (MI.isDebugValue())
continue;
for (MachineOperand &MO : reverse(MI.explicit_operands())) {
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
if (MFI.isVRegStackified(Reg)) {
if (MO.isDef())
Stack.push_back(Reg);
else
assert(Stack.pop_back_val() == Reg &&
"Register stack pop should be paired with a push");
}
}
}
// TODO: Generalize this code to support keeping values on the stack across
// basic block boundaries.
assert(Stack.empty() &&
"Register stack pushes and pops should be balanced");
}
#endif
return Changed;
}
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