llvm-project/llvm/lib/Transforms/Scalar/Float2Int.cpp

553 lines
18 KiB
C++

//===- Float2Int.cpp - Demote floating point ops to work on integers ------===//
//
// 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 implements the Float2Int pass, which aims to demote floating
// point operations to work on integers, where that is losslessly possible.
//
//===----------------------------------------------------------------------===//
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#define DEBUG_TYPE "float2int"
#include "llvm/Transforms/Scalar/Float2Int.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/APSInt.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include <deque>
#include <functional> // For std::function
using namespace llvm;
// The algorithm is simple. Start at instructions that convert from the
// float to the int domain: fptoui, fptosi and fcmp. Walk up the def-use
// graph, using an equivalence datastructure to unify graphs that interfere.
//
// Mappable instructions are those with an integer corrollary that, given
// integer domain inputs, produce an integer output; fadd, for example.
//
// If a non-mappable instruction is seen, this entire def-use graph is marked
// as non-transformable. If we see an instruction that converts from the
// integer domain to FP domain (uitofp,sitofp), we terminate our walk.
/// The largest integer type worth dealing with.
static cl::opt<unsigned>
MaxIntegerBW("float2int-max-integer-bw", cl::init(64), cl::Hidden,
cl::desc("Max integer bitwidth to consider in float2int"
"(default=64)"));
namespace {
struct Float2IntLegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
Float2IntLegacyPass() : FunctionPass(ID) {
initializeFloat2IntLegacyPassPass(*PassRegistry::getPassRegistry());
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
const DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
return Impl.runImpl(F, DT);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<GlobalsAAWrapperPass>();
}
private:
Float2IntPass Impl;
};
}
char Float2IntLegacyPass::ID = 0;
INITIALIZE_PASS(Float2IntLegacyPass, "float2int", "Float to int", false, false)
// Given a FCmp predicate, return a matching ICmp predicate if one
// exists, otherwise return BAD_ICMP_PREDICATE.
static CmpInst::Predicate mapFCmpPred(CmpInst::Predicate P) {
switch (P) {
case CmpInst::FCMP_OEQ:
case CmpInst::FCMP_UEQ:
return CmpInst::ICMP_EQ;
case CmpInst::FCMP_OGT:
case CmpInst::FCMP_UGT:
return CmpInst::ICMP_SGT;
case CmpInst::FCMP_OGE:
case CmpInst::FCMP_UGE:
return CmpInst::ICMP_SGE;
case CmpInst::FCMP_OLT:
case CmpInst::FCMP_ULT:
return CmpInst::ICMP_SLT;
case CmpInst::FCMP_OLE:
case CmpInst::FCMP_ULE:
return CmpInst::ICMP_SLE;
case CmpInst::FCMP_ONE:
case CmpInst::FCMP_UNE:
return CmpInst::ICMP_NE;
default:
return CmpInst::BAD_ICMP_PREDICATE;
}
}
// Given a floating point binary operator, return the matching
// integer version.
static Instruction::BinaryOps mapBinOpcode(unsigned Opcode) {
switch (Opcode) {
default: llvm_unreachable("Unhandled opcode!");
case Instruction::FAdd: return Instruction::Add;
case Instruction::FSub: return Instruction::Sub;
case Instruction::FMul: return Instruction::Mul;
}
}
// Find the roots - instructions that convert from the FP domain to
// integer domain.
void Float2IntPass::findRoots(Function &F, const DominatorTree &DT,
SmallPtrSet<Instruction*,8> &Roots) {
for (BasicBlock &BB : F) {
// Unreachable code can take on strange forms that we are not prepared to
// handle. For example, an instruction may have itself as an operand.
if (!DT.isReachableFromEntry(&BB))
continue;
for (Instruction &I : BB) {
if (isa<VectorType>(I.getType()))
continue;
switch (I.getOpcode()) {
default: break;
case Instruction::FPToUI:
case Instruction::FPToSI:
Roots.insert(&I);
break;
case Instruction::FCmp:
if (mapFCmpPred(cast<CmpInst>(&I)->getPredicate()) !=
CmpInst::BAD_ICMP_PREDICATE)
Roots.insert(&I);
break;
}
}
}
}
// Helper - mark I as having been traversed, having range R.
void Float2IntPass::seen(Instruction *I, ConstantRange R) {
LLVM_DEBUG(dbgs() << "F2I: " << *I << ":" << R << "\n");
auto IT = SeenInsts.find(I);
if (IT != SeenInsts.end())
IT->second = std::move(R);
else
SeenInsts.insert(std::make_pair(I, std::move(R)));
}
// Helper - get a range representing a poison value.
ConstantRange Float2IntPass::badRange() {
return ConstantRange::getFull(MaxIntegerBW + 1);
}
ConstantRange Float2IntPass::unknownRange() {
return ConstantRange::getEmpty(MaxIntegerBW + 1);
}
ConstantRange Float2IntPass::validateRange(ConstantRange R) {
if (R.getBitWidth() > MaxIntegerBW + 1)
return badRange();
return R;
}
// The most obvious way to structure the search is a depth-first, eager
// search from each root. However, that require direct recursion and so
// can only handle small instruction sequences. Instead, we split the search
// up into two phases:
// - walkBackwards: A breadth-first walk of the use-def graph starting from
// the roots. Populate "SeenInsts" with interesting
// instructions and poison values if they're obvious and
// cheap to compute. Calculate the equivalance set structure
// while we're here too.
// - walkForwards: Iterate over SeenInsts in reverse order, so we visit
// defs before their uses. Calculate the real range info.
// Breadth-first walk of the use-def graph; determine the set of nodes
// we care about and eagerly determine if some of them are poisonous.
void Float2IntPass::walkBackwards(const SmallPtrSetImpl<Instruction*> &Roots) {
std::deque<Instruction*> Worklist(Roots.begin(), Roots.end());
while (!Worklist.empty()) {
Instruction *I = Worklist.back();
Worklist.pop_back();
if (SeenInsts.find(I) != SeenInsts.end())
// Seen already.
continue;
switch (I->getOpcode()) {
// FIXME: Handle select and phi nodes.
default:
// Path terminated uncleanly.
seen(I, badRange());
break;
case Instruction::UIToFP:
case Instruction::SIToFP: {
// Path terminated cleanly - use the type of the integer input to seed
// the analysis.
unsigned BW = I->getOperand(0)->getType()->getPrimitiveSizeInBits();
auto Input = ConstantRange::getFull(BW);
auto CastOp = (Instruction::CastOps)I->getOpcode();
seen(I, validateRange(Input.castOp(CastOp, MaxIntegerBW+1)));
continue;
}
case Instruction::FNeg:
case Instruction::FAdd:
case Instruction::FSub:
case Instruction::FMul:
case Instruction::FPToUI:
case Instruction::FPToSI:
case Instruction::FCmp:
seen(I, unknownRange());
break;
}
for (Value *O : I->operands()) {
if (Instruction *OI = dyn_cast<Instruction>(O)) {
// Unify def-use chains if they interfere.
ECs.unionSets(I, OI);
if (SeenInsts.find(I)->second != badRange())
Worklist.push_back(OI);
} else if (!isa<ConstantFP>(O)) {
// Not an instruction or ConstantFP? we can't do anything.
seen(I, badRange());
}
}
}
}
// Walk forwards down the list of seen instructions, so we visit defs before
// uses.
void Float2IntPass::walkForwards() {
for (auto &It : reverse(SeenInsts)) {
if (It.second != unknownRange())
continue;
Instruction *I = It.first;
std::function<ConstantRange(ArrayRef<ConstantRange>)> Op;
switch (I->getOpcode()) {
// FIXME: Handle select and phi nodes.
default:
case Instruction::UIToFP:
case Instruction::SIToFP:
llvm_unreachable("Should have been handled in walkForwards!");
case Instruction::FNeg:
Op = [](ArrayRef<ConstantRange> Ops) {
assert(Ops.size() == 1 && "FNeg is a unary operator!");
unsigned Size = Ops[0].getBitWidth();
auto Zero = ConstantRange(APInt::getNullValue(Size));
return Zero.sub(Ops[0]);
};
break;
case Instruction::FAdd:
case Instruction::FSub:
case Instruction::FMul:
Op = [I](ArrayRef<ConstantRange> Ops) {
assert(Ops.size() == 2 && "its a binary operator!");
auto BinOp = (Instruction::BinaryOps) I->getOpcode();
return Ops[0].binaryOp(BinOp, Ops[1]);
};
break;
//
// Root-only instructions - we'll only see these if they're the
// first node in a walk.
//
case Instruction::FPToUI:
case Instruction::FPToSI:
Op = [I](ArrayRef<ConstantRange> Ops) {
assert(Ops.size() == 1 && "FPTo[US]I is a unary operator!");
// Note: We're ignoring the casts output size here as that's what the
// caller expects.
auto CastOp = (Instruction::CastOps)I->getOpcode();
return Ops[0].castOp(CastOp, MaxIntegerBW+1);
};
break;
case Instruction::FCmp:
Op = [](ArrayRef<ConstantRange> Ops) {
assert(Ops.size() == 2 && "FCmp is a binary operator!");
return Ops[0].unionWith(Ops[1]);
};
break;
}
bool Abort = false;
SmallVector<ConstantRange,4> OpRanges;
for (Value *O : I->operands()) {
if (Instruction *OI = dyn_cast<Instruction>(O)) {
assert(SeenInsts.find(OI) != SeenInsts.end() &&
"def not seen before use!");
OpRanges.push_back(SeenInsts.find(OI)->second);
} else if (ConstantFP *CF = dyn_cast<ConstantFP>(O)) {
// Work out if the floating point number can be losslessly represented
// as an integer.
// APFloat::convertToInteger(&Exact) purports to do what we want, but
// the exactness can be too precise. For example, negative zero can
// never be exactly converted to an integer.
//
// Instead, we ask APFloat to round itself to an integral value - this
// preserves sign-of-zero - then compare the result with the original.
//
const APFloat &F = CF->getValueAPF();
// First, weed out obviously incorrect values. Non-finite numbers
// can't be represented and neither can negative zero, unless
// we're in fast math mode.
if (!F.isFinite() ||
(F.isZero() && F.isNegative() && isa<FPMathOperator>(I) &&
!I->hasNoSignedZeros())) {
seen(I, badRange());
Abort = true;
break;
}
APFloat NewF = F;
auto Res = NewF.roundToIntegral(APFloat::rmNearestTiesToEven);
if (Res != APFloat::opOK || NewF.compare(F) != APFloat::cmpEqual) {
seen(I, badRange());
Abort = true;
break;
}
// OK, it's representable. Now get it.
APSInt Int(MaxIntegerBW+1, false);
bool Exact;
CF->getValueAPF().convertToInteger(Int,
APFloat::rmNearestTiesToEven,
&Exact);
OpRanges.push_back(ConstantRange(Int));
} else {
llvm_unreachable("Should have already marked this as badRange!");
}
}
// Reduce the operands' ranges to a single range and return.
if (!Abort)
seen(I, Op(OpRanges));
}
}
// If there is a valid transform to be done, do it.
bool Float2IntPass::validateAndTransform() {
bool MadeChange = false;
// Iterate over every disjoint partition of the def-use graph.
for (auto It = ECs.begin(), E = ECs.end(); It != E; ++It) {
ConstantRange R(MaxIntegerBW + 1, false);
bool Fail = false;
Type *ConvertedToTy = nullptr;
// For every member of the partition, union all the ranges together.
for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
MI != ME; ++MI) {
Instruction *I = *MI;
auto SeenI = SeenInsts.find(I);
if (SeenI == SeenInsts.end())
continue;
R = R.unionWith(SeenI->second);
// We need to ensure I has no users that have not been seen.
// If it does, transformation would be illegal.
//
// Don't count the roots, as they terminate the graphs.
if (Roots.count(I) == 0) {
// Set the type of the conversion while we're here.
if (!ConvertedToTy)
ConvertedToTy = I->getType();
for (User *U : I->users()) {
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI || SeenInsts.find(UI) == SeenInsts.end()) {
LLVM_DEBUG(dbgs() << "F2I: Failing because of " << *U << "\n");
Fail = true;
break;
}
}
}
if (Fail)
break;
}
// If the set was empty, or we failed, or the range is poisonous,
// bail out.
if (ECs.member_begin(It) == ECs.member_end() || Fail ||
R.isFullSet() || R.isSignWrappedSet())
continue;
assert(ConvertedToTy && "Must have set the convertedtoty by this point!");
// The number of bits required is the maximum of the upper and
// lower limits, plus one so it can be signed.
unsigned MinBW = std::max(R.getLower().getMinSignedBits(),
R.getUpper().getMinSignedBits()) + 1;
LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");
// If we've run off the realms of the exactly representable integers,
// the floating point result will differ from an integer approximation.
// Do we need more bits than are in the mantissa of the type we converted
// to? semanticsPrecision returns the number of mantissa bits plus one
// for the sign bit.
unsigned MaxRepresentableBits
= APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
if (MinBW > MaxRepresentableBits) {
LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
continue;
}
if (MinBW > 64) {
LLVM_DEBUG(
dbgs() << "F2I: Value requires more than 64 bits to represent!\n");
continue;
}
// OK, R is known to be representable. Now pick a type for it.
// FIXME: Pick the smallest legal type that will fit.
Type *Ty = (MinBW > 32) ? Type::getInt64Ty(*Ctx) : Type::getInt32Ty(*Ctx);
for (auto MI = ECs.member_begin(It), ME = ECs.member_end();
MI != ME; ++MI)
convert(*MI, Ty);
MadeChange = true;
}
return MadeChange;
}
Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
if (ConvertedInsts.find(I) != ConvertedInsts.end())
// Already converted this instruction.
return ConvertedInsts[I];
SmallVector<Value*,4> NewOperands;
for (Value *V : I->operands()) {
// Don't recurse if we're an instruction that terminates the path.
if (I->getOpcode() == Instruction::UIToFP ||
I->getOpcode() == Instruction::SIToFP) {
NewOperands.push_back(V);
} else if (Instruction *VI = dyn_cast<Instruction>(V)) {
NewOperands.push_back(convert(VI, ToTy));
} else if (ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
bool Exact;
CF->getValueAPF().convertToInteger(Val,
APFloat::rmNearestTiesToEven,
&Exact);
NewOperands.push_back(ConstantInt::get(ToTy, Val));
} else {
llvm_unreachable("Unhandled operand type?");
}
}
// Now create a new instruction.
IRBuilder<> IRB(I);
Value *NewV = nullptr;
switch (I->getOpcode()) {
default: llvm_unreachable("Unhandled instruction!");
case Instruction::FPToUI:
NewV = IRB.CreateZExtOrTrunc(NewOperands[0], I->getType());
break;
case Instruction::FPToSI:
NewV = IRB.CreateSExtOrTrunc(NewOperands[0], I->getType());
break;
case Instruction::FCmp: {
CmpInst::Predicate P = mapFCmpPred(cast<CmpInst>(I)->getPredicate());
assert(P != CmpInst::BAD_ICMP_PREDICATE && "Unhandled predicate!");
NewV = IRB.CreateICmp(P, NewOperands[0], NewOperands[1], I->getName());
break;
}
case Instruction::UIToFP:
NewV = IRB.CreateZExtOrTrunc(NewOperands[0], ToTy);
break;
case Instruction::SIToFP:
NewV = IRB.CreateSExtOrTrunc(NewOperands[0], ToTy);
break;
case Instruction::FNeg:
NewV = IRB.CreateNeg(NewOperands[0], I->getName());
break;
case Instruction::FAdd:
case Instruction::FSub:
case Instruction::FMul:
NewV = IRB.CreateBinOp(mapBinOpcode(I->getOpcode()),
NewOperands[0], NewOperands[1],
I->getName());
break;
}
// If we're a root instruction, RAUW.
if (Roots.count(I))
I->replaceAllUsesWith(NewV);
ConvertedInsts[I] = NewV;
return NewV;
}
// Perform dead code elimination on the instructions we just modified.
void Float2IntPass::cleanup() {
for (auto &I : reverse(ConvertedInsts))
I.first->eraseFromParent();
}
bool Float2IntPass::runImpl(Function &F, const DominatorTree &DT) {
LLVM_DEBUG(dbgs() << "F2I: Looking at function " << F.getName() << "\n");
// Clear out all state.
ECs = EquivalenceClasses<Instruction*>();
SeenInsts.clear();
ConvertedInsts.clear();
Roots.clear();
Ctx = &F.getParent()->getContext();
findRoots(F, DT, Roots);
walkBackwards(Roots);
walkForwards();
bool Modified = validateAndTransform();
if (Modified)
cleanup();
return Modified;
}
namespace llvm {
FunctionPass *createFloat2IntPass() { return new Float2IntLegacyPass(); }
PreservedAnalyses Float2IntPass::run(Function &F, FunctionAnalysisManager &AM) {
const DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
if (!runImpl(F, DT))
return PreservedAnalyses::all();
PreservedAnalyses PA;
PA.preserveSet<CFGAnalyses>();
PA.preserve<GlobalsAA>();
return PA;
}
} // End namespace llvm