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llvm-mirror/lib/Target/WebAssembly/WebAssemblyRegStackify.cpp
Mircea Trofin 074f58db6d [NFC][MC] MCRegister API typing. 
Mostly LiveIntervals, with their effects (users).

Differential Revision: https://reviews.llvm.org/D89018
2020-10-08 15:08:34 -07:00

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38 KiB
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//===-- WebAssemblyRegStackify.cpp - Register Stackification --------------===//
//
// 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 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 "WebAssemblyDebugValueManager.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyUtilities.h"
#include "llvm/ADT/SmallPtrSet.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"
#include <iterator>
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;
INITIALIZE_PASS(WebAssemblyRegStackify, DEBUG_TYPE,
"Reorder instructions to use the WebAssembly value stack",
false, false)
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,
LiveIntervals &LIS) {
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));
auto *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));
auto *Val = cast<ConstantFP>(Constant::getNullValue(
Type::getDoubleTy(MF.getFunction().getContext())));
MI->addOperand(MachineOperand::CreateFPImm(Val));
} else if (RegClass == &WebAssembly::V128RegClass) {
// TODO: Replace this with v128.const 0 once that is supported in V8
Register TempReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
MI->setDesc(TII->get(WebAssembly::SPLAT_v4i32));
MI->addOperand(MachineOperand::CreateReg(TempReg, false));
MachineInstr *Const = BuildMI(*MI->getParent(), MI, MI->getDebugLoc(),
TII->get(WebAssembly::CONST_I32), TempReg)
.addImm(0);
LIS.InsertMachineInstrInMaps(*Const);
} 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, bool &Read, bool &Write,
bool &Effects, bool &StackPointer) {
// All calls can use the stack pointer.
StackPointer = true;
const MachineOperand &MO = WebAssembly::getCalleeOp(MI);
if (MO.isGlobal()) {
const Constant *GV = MO.getGlobal();
if (const auto *GA = dyn_cast<GlobalAlias>(GV))
if (!GA->isInterposable())
GV = GA->getAliasee();
if (const auto *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.isTerminator());
if (MI.isDebugInstr() || MI.isPosition())
return;
// Check for loads.
if (MI.mayLoad() && !MI.isDereferenceableInvariantLoad(&AA))
Read = true;
// Check for stores.
if (MI.mayStore()) {
Write = 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;
}
}
// Check for writes to __stack_pointer global.
if ((MI.getOpcode() == WebAssembly::GLOBAL_SET_I32 ||
MI.getOpcode() == WebAssembly::GLOBAL_SET_I64) &&
strcmp(MI.getOperand(0).getSymbolName(), "__stack_pointer") == 0)
StackPointer = true;
// Analyze calls.
if (MI.isCall()) {
queryCallee(MI, Read, Write, Effects, StackPointer);
}
}
// 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 MachineOperand *Def, const MachineOperand *Use,
const MachineInstr *Insert, AliasAnalysis &AA,
const WebAssemblyFunctionInfo &MFI,
const MachineRegisterInfo &MRI) {
const MachineInstr *DefI = Def->getParent();
const MachineInstr *UseI = Use->getParent();
assert(DefI->getParent() == Insert->getParent());
assert(UseI->getParent() == Insert->getParent());
// The first def of a multivalue instruction can be stackified by moving,
// since the later defs can always be placed into locals if necessary. Later
// defs can only be stackified if all previous defs are already stackified
// since ExplicitLocals will not know how to place a def in a local if a
// subsequent def is stackified. But only one def can be stackified by moving
// the instruction, so it must be the first one.
//
// TODO: This could be loosened to be the first *live* def, but care would
// have to be taken to ensure the drops of the initial dead defs can be
// placed. This would require checking that no previous defs are used in the
// same instruction as subsequent defs.
if (Def != DefI->defs().begin())
return false;
// If any subsequent def is used prior to the current value by the same
// instruction in which the current value is used, we cannot
// stackify. Stackifying in this case would require that def moving below the
// current def in the stack, which cannot be achieved, even with locals.
for (const auto &SubsequentDef : drop_begin(DefI->defs(), 1)) {
for (const auto &PriorUse : UseI->uses()) {
if (&PriorUse == Use)
break;
if (PriorUse.isReg() && SubsequentDef.getReg() == PriorUse.getReg())
return false;
}
}
// If moving is a semantic nop, it is always allowed
const MachineBasicBlock *MBB = DefI->getParent();
auto NextI = std::next(MachineBasicBlock::const_iterator(DefI));
for (auto E = MBB->end(); NextI != E && NextI->isDebugInstr(); ++NextI)
;
if (NextI == Insert)
return true;
// 'catch' and 'extract_exception' should be the first instruction of a BB and
// cannot move.
if (DefI->getOpcode() == WebAssembly::CATCH ||
DefI->getOpcode() == WebAssembly::EXTRACT_EXCEPTION_I32)
return false;
// Check for register dependencies.
SmallVector<unsigned, 4> MutableRegisters;
for (const MachineOperand &MO : DefI->operands()) {
if (!MO.isReg() || MO.isUndef())
continue;
Register 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 (Register::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(*DefI, 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 DefI and Insert.
MachineBasicBlock::const_iterator D(DefI), 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;
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 local.gets for
// uses and implicit local.sets for defs.
if (UseInst->getDesc().getNumDefs() == 0)
return false;
const MachineOperand &MO = UseInst->getOperand(0);
if (!MO.isReg())
return false;
Register DefReg = MO.getReg();
if (!Register::isVirtualRegister(DefReg) ||
!MFI.isVRegStackified(DefReg))
return false;
assert(MRI.hasOneNonDBGUse(DefReg));
const MachineOperand &NewUse = *MRI.use_nodbg_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) {
LLVM_DEBUG(dbgs() << "Move for single use: "; Def->dump());
WebAssemblyDebugValueManager DefDIs(Def);
MBB.splice(Insert, &MBB, Def);
DefDIs.move(Insert);
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(MRI, 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.
Register 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(MRI, NewReg);
DefDIs.updateReg(NewReg);
LLVM_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) {
LLVM_DEBUG(dbgs() << "Rematerializing cheap def: "; Def.dump());
LLVM_DEBUG(dbgs() << " - for use in "; Op.getParent()->dump());
WebAssemblyDebugValueManager DefDIs(&Def);
Register 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(MRI, NewReg);
imposeStackOrdering(Clone);
LLVM_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.
// Move or clone corresponding DBG_VALUEs to the 'Insert' location.
if (IsDead) {
LLVM_DEBUG(dbgs() << " - Deleting original\n");
SlotIndex Idx = LIS.getInstructionIndex(Def).getRegSlot();
LIS.removePhysRegDefAt(MCRegister::from(WebAssembly::ARGUMENTS), Idx);
LIS.removeInterval(Reg);
LIS.RemoveMachineInstrFromMaps(Def);
Def.eraseFromParent();
DefDIs.move(&*Insert);
DefDIs.updateReg(NewReg);
} else {
DefDIs.clone(&*Insert, NewReg);
}
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 local.get 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) {
LLVM_DEBUG(dbgs() << "Move and tee for multi-use:"; Def->dump());
WebAssemblyDebugValueManager DefDIs(Def);
// 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);
Register TeeReg = MRI.createVirtualRegister(RegClass);
Register 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();
DefDIs.move(Insert);
// 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(MRI, DefReg);
MFI.stackifyVReg(MRI, TeeReg);
imposeStackOrdering(Def);
imposeStackOrdering(Tee);
DefDIs.clone(Tee, DefReg);
DefDIs.clone(Insert, TeeReg);
LLVM_DEBUG(dbgs() << " - Replaced register: "; Def->dump());
LLVM_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 {
using mop_iterator = MachineInstr::mop_iterator;
using mop_reverse_iterator = std::reverse_iterator<mop_iterator>;
using RangeTy = iterator_range<mop_reverse_iterator>;
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 local.gets for
/// uses and implicit local.sets 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 tentative
/// state where we've commuted the operands of the current instruction and are
/// revisiting 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 = false;
bool Declined = false;
/// During the tentative state, these hold the operand indices of the commuted
/// operands.
unsigned Operand0, Operand1;
public:
/// 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) {
LLVM_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();
auto &MDT = getAnalysis<MachineDominatorTree>();
auto &LIS = getAnalysis<LiveIntervals>();
// 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->isInlineAsm())
continue;
// Ignore debugging intrinsics.
if (Insert->isDebugValue())
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 &Use = TreeWalker.pop();
// We're only interested in explicit virtual register operands.
if (!Use.isReg())
continue;
Register Reg = Use.getReg();
assert(Use.isUse() && "explicit_uses() should only iterate over uses");
assert(!Use.isImplicit() &&
"explicit_uses() should only iterate over explicit operands");
if (Register::isPhysicalRegister(Reg))
continue;
// Identify the definition for this register at this point.
MachineInstr *DefI = getVRegDef(Reg, Insert, MRI, LIS);
if (!DefI)
continue;
// Don't nest an INLINE_ASM def into anything, because we don't have
// constraints for $pop outputs.
if (DefI->isInlineAsm())
continue;
// Argument instructions represent live-in registers and not real
// instructions.
if (WebAssembly::isArgument(DefI->getOpcode()))
continue;
// Currently catch's return value register cannot be stackified, because
// the wasm LLVM backend currently does not support live-in values
// entering blocks, which is a part of multi-value proposal.
//
// Once we support live-in values of wasm blocks, this can be:
// catch ; push exnref value onto stack
// block exnref -> i32
// br_on_exn $__cpp_exception ; pop the exnref value
// end_block
//
// But because we don't support it yet, the catch instruction's dst
// register should be assigned to a local to be propagated across
// 'block' boundary now.
//
// TODO: Fix this once we support the multivalue blocks
if (DefI->getOpcode() == WebAssembly::CATCH)
continue;
MachineOperand *Def = DefI->findRegisterDefOperand(Reg);
assert(Def != nullptr);
// 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 = DefI->getParent() == &MBB;
bool CanMove = SameBlock &&
isSafeToMove(Def, &Use, Insert, AA, MFI, MRI) &&
!TreeWalker.isOnStack(Reg);
if (CanMove && hasOneUse(Reg, DefI, MRI, MDT, LIS)) {
Insert = moveForSingleUse(Reg, Use, DefI, MBB, Insert, LIS, MFI, MRI);
// If we are removing the frame base reg completely, remove the debug
// info as well.
// TODO: Encode this properly as a stackified value.
if (MFI.isFrameBaseVirtual() && MFI.getFrameBaseVreg() == Reg)
MFI.clearFrameBaseVreg();
} else if (shouldRematerialize(*DefI, AA, TII)) {
Insert =
rematerializeCheapDef(Reg, Use, *DefI, MBB, Insert->getIterator(),
LIS, MFI, MRI, TII, TRI);
} else if (CanMove && oneUseDominatesOtherUses(Reg, Use, MBB, MRI, MDT,
LIS, MFI)) {
Insert = moveAndTeeForMultiUse(Reg, Use, DefI, 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;
}
// Stackifying a multivalue def may unlock in-place stackification of
// subsequent defs. TODO: Handle the case where the consecutive uses are
// not all in the same instruction.
auto *SubsequentDef = Insert->defs().begin();
auto *SubsequentUse = &Use;
while (SubsequentDef != Insert->defs().end() &&
SubsequentUse != Use.getParent()->uses().end()) {
if (!SubsequentDef->isReg() || !SubsequentUse->isReg())
break;
unsigned DefReg = SubsequentDef->getReg();
unsigned UseReg = SubsequentUse->getReg();
// TODO: This single-use restriction could be relaxed by using tees
if (DefReg != UseReg || !MRI.hasOneUse(DefReg))
break;
MFI.stackifyVReg(MRI, DefReg);
++SubsequentDef;
++SubsequentUse;
}
// 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, LIS);
// 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.isDebugInstr())
continue;
for (MachineOperand &MO : reverse(MI.explicit_uses())) {
if (!MO.isReg())
continue;
Register Reg = MO.getReg();
if (MFI.isVRegStackified(Reg))
assert(Stack.pop_back_val() == Reg &&
"Register stack pop should be paired with a push");
}
for (MachineOperand &MO : MI.defs()) {
if (!MO.isReg())
continue;
Register Reg = MO.getReg();
if (MFI.isVRegStackified(Reg))
Stack.push_back(MO.getReg());
}
}
// 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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