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

566 lines
22 KiB
C++

//===-- 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 stack.
///
/// This is primarily a code size optimization, since temporary values on the
/// expression don't need to be named.
///
//===----------------------------------------------------------------------===//
#include "WebAssembly.h"
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h" // for WebAssembly::ARGUMENT_*
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineInstrBuilder.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 {
const char *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 EXPR_STACK register.
if (!MI->definesRegister(WebAssembly::EXPR_STACK))
MI->addOperand(MachineOperand::CreateReg(WebAssembly::EXPR_STACK,
/*isDef=*/true,
/*isImp=*/true));
// Also read the opaque EXPR_STACK register.
if (!MI->readsRegister(WebAssembly::EXPR_STACK))
MI->addOperand(MachineOperand::CreateReg(WebAssembly::EXPR_STACK,
/*isDef=*/false,
/*isImp=*/true));
}
// 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 LiveIntervals &LIS,
const MachineRegisterInfo &MRI) {
assert(Def->getParent() == Insert->getParent());
bool SawStore = false, SawSideEffects = false;
MachineBasicBlock::const_iterator D(Def), I(Insert);
// Check for register dependencies.
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)) {
// 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;
}
// Ask LiveIntervals whether moving this virtual register use or def to
// Insert will change value numbers are seen.
const LiveInterval &LI = LIS.getInterval(Reg);
VNInfo *DefVNI =
MO.isDef() ? LI.getVNInfoAt(LIS.getInstructionIndex(*Def).getRegSlot())
: LI.getVNInfoBefore(LIS.getInstructionIndex(*Def));
assert(DefVNI && "Instruction input missing value number");
VNInfo *InsVNI = LI.getVNInfoBefore(LIS.getInstructionIndex(*Insert));
if (InsVNI && DefVNI != InsVNI)
return false;
}
SawStore = Def->isCall() || Def->mayStore();
// Check for memory dependencies and side effects.
for (--I; I != D; --I)
SawSideEffects |= !I->isSafeToMove(&AA, SawStore);
return !(SawStore && Def->mayLoad() && !Def->isInvariantLoad(&AA)) &&
!(SawSideEffects && !Def->isSafeToMove(&AA, SawStore));
}
/// 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) {
for (const MachineOperand &Use : MRI.use_operands(Reg)) {
if (&Use == &OneUse)
continue;
const MachineInstr *UseInst = Use.getParent();
const MachineInstr *OneUseInst = OneUse.getParent();
if (UseInst->getOpcode() == TargetOpcode::PHI) {
// Test that the PHI use, which happens on the CFG edge rather than
// within the PHI's own block, is dominated by the one selected use.
const MachineBasicBlock *Pred =
UseInst->getOperand(&Use - &UseInst->getOperand(0) + 1).getMBB();
if (!MDT.dominates(&MBB, Pred))
return false;
} else 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.
if (!MDT.dominates(OneUseInst, UseInst))
return false;
}
}
return true;
}
/// Get the appropriate tee_local opcode for the given register class.
static unsigned GetTeeLocalOpcode(const TargetRegisterClass *RC) {
if (RC == &WebAssembly::I32RegClass)
return WebAssembly::TEE_LOCAL_I32;
if (RC == &WebAssembly::I64RegClass)
return WebAssembly::TEE_LOCAL_I64;
if (RC == &WebAssembly::F32RegClass)
return WebAssembly::TEE_LOCAL_F32;
if (RC == &WebAssembly::F64RegClass)
return WebAssembly::TEE_LOCAL_F64;
llvm_unreachable("Unexpected register class");
}
/// 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, MachineInstr *Def,
MachineBasicBlock &MBB,
MachineInstr *Insert, LiveIntervals &LIS,
WebAssemblyFunctionInfo &MFI) {
MBB.splice(Insert, &MBB, Def);
LIS.handleMove(*Def);
MFI.stackifyVReg(Reg);
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, MachineInstr *Insert,
LiveIntervals &LIS, WebAssemblyFunctionInfo &MFI,
MachineRegisterInfo &MRI, const WebAssemblyInstrInfo *TII,
const WebAssemblyRegisterInfo *TRI) {
unsigned NewReg = MRI.createVirtualRegister(MRI.getRegClass(Reg));
TII->reMaterialize(MBB, Insert, NewReg, 0, Def, *TRI);
Op.setReg(NewReg);
MachineInstr *Clone = &*std::prev(MachineBasicBlock::instr_iterator(Insert));
LIS.InsertMachineInstrInMaps(*Clone);
LIS.createAndComputeVirtRegInterval(NewReg);
MFI.stackifyVReg(NewReg);
ImposeStackOrdering(Clone);
// If that was the last use of the original, delete the original.
// Otherwise shrink the LiveInterval.
if (MRI.use_empty(Reg)) {
SlotIndex Idx = LIS.getInstructionIndex(*Def).getRegSlot();
LIS.removePhysRegDefAt(WebAssembly::ARGUMENTS, Idx);
LIS.removeVRegDefAt(LIS.getInterval(Reg), Idx);
LIS.removeInterval(Reg);
LIS.RemoveMachineInstrFromMaps(*Def);
Def->eraseFromParent();
} else {
LIS.shrinkToUses(&LIS.getInterval(Reg));
}
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_local 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, NewReg = TEE_LOCAL_... DefReg
/// INST ..., TeeReg, ... // Insert
/// INST ..., NewReg, ...
/// INST ..., NewReg, ...
///
/// 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) {
MBB.splice(Insert, &MBB, Def);
LIS.handleMove(*Def);
const auto *RegClass = MRI.getRegClass(Reg);
unsigned NewReg = MRI.createVirtualRegister(RegClass);
unsigned TeeReg = MRI.createVirtualRegister(RegClass);
unsigned DefReg = MRI.createVirtualRegister(RegClass);
MRI.replaceRegWith(Reg, NewReg);
MachineInstr *Tee = BuildMI(MBB, Insert, Insert->getDebugLoc(),
TII->get(GetTeeLocalOpcode(RegClass)), TeeReg)
.addReg(NewReg, RegState::Define)
.addReg(DefReg);
Op.setReg(TeeReg);
Def->getOperand(0).setReg(DefReg);
LIS.InsertMachineInstrInMaps(*Tee);
LIS.removeInterval(Reg);
LIS.createAndComputeVirtRegInterval(NewReg);
LIS.createAndComputeVirtRegInterval(TeeReg);
LIS.createAndComputeVirtRegInterval(DefReg);
MFI.stackifyVReg(DefReg);
MFI.stackifyVReg(TeeReg);
ImposeStackOrdering(Def);
ImposeStackOrdering(Tee);
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.
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>();
// 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 a phi.
if (Insert->getOpcode() == TargetOpcode::PHI)
break;
// 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. Most
// registers are in SSA form here so we try a quick MRI query first.
MachineInstr *Def = MRI.getUniqueVRegDef(Reg);
if (!Def) {
// MRI doesn't know what the Def is. Try asking LIS.
const VNInfo *ValNo = LIS.getInterval(Reg).getVNInfoBefore(
LIS.getInstructionIndex(*Insert));
if (!ValNo)
continue;
Def = LIS.getInstructionFromIndex(ValNo->def);
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;
// Don't nest PHIs inside of anything.
if (Def->getOpcode() == TargetOpcode::PHI)
continue;
// Argument instructions represent live-in registers and not real
// instructions.
if (Def->getOpcode() == WebAssembly::ARGUMENT_I32 ||
Def->getOpcode() == WebAssembly::ARGUMENT_I64 ||
Def->getOpcode() == WebAssembly::ARGUMENT_F32 ||
Def->getOpcode() == WebAssembly::ARGUMENT_F64)
continue;
// Decide which strategy to take. Prefer to move a single-use value
// over cloning it, and prefer cloning over introducing a tee_local.
// 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, LIS, MRI) &&
!TreeWalker.IsOnStack(Reg);
if (CanMove && MRI.hasOneUse(Reg)) {
Insert = MoveForSingleUse(Reg, Def, MBB, Insert, LIS, MFI);
} else if (Def->isAsCheapAsAMove() &&
TII->isTriviallyReMaterializable(Def, &AA)) {
Insert = RematerializeCheapDef(Reg, Op, Def, MBB, Insert, LIS, MFI,
MRI, TII, TRI);
} else if (CanMove &&
OneUseDominatesOtherUses(Reg, Op, MBB, MRI, MDT)) {
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;
}
// 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 = std::prev(
llvm::make_reverse_iterator(MachineBasicBlock::iterator(Insert)));
Changed = true;
}
}
}
// If we used EXPR_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::EXPR_STACK);
for (MachineBasicBlock &MBB : MF)
MBB.addLiveIn(WebAssembly::EXPR_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) {
for (MachineOperand &MO : reverse(MI.explicit_operands())) {
if (!MO.isReg())
continue;
unsigned Reg = MO.getReg();
// Don't stackify physregs like SP or FP.
if (!TargetRegisterInfo::isVirtualRegister(Reg))
continue;
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;
}