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//===- LoopUnrollAnalyzer.cpp - Unrolling Effect Estimation -----*- C++ -*-===//
//
// 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 UnrolledInstAnalyzer class. It's used for predicting
// potential effects that loop unrolling might have, such as enabling constant
// propagation and other optimizations.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/LoopUnrollAnalyzer.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/Operator.h"
using namespace llvm;
/// Try to simplify instruction \param I using its SCEV expression.
///
/// The idea is that some AddRec expressions become constants, which then
/// could trigger folding of other instructions. However, that only happens
/// for expressions whose start value is also constant, which isn't always the
/// case. In another common and important case the start value is just some
/// address (i.e. SCEVUnknown) - in this case we compute the offset and save
/// it along with the base address instead.
bool UnrolledInstAnalyzer::simplifyInstWithSCEV(Instruction *I) {
if (!SE.isSCEVable(I->getType()))
return false;
const SCEV *S = SE.getSCEV(I);
if (auto *SC = dyn_cast<SCEVConstant>(S)) {
SimplifiedValues[I] = SC->getValue();
return true;
}
// If we have a loop invariant computation, we only need to compute it once.
// Given that, all but the first occurance are free.
if (!IterationNumber->isZero() && SE.isLoopInvariant(S, L))
return true;
auto *AR = dyn_cast<SCEVAddRecExpr>(S);
if (!AR || AR->getLoop() != L)
return false;
const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
// Check if the AddRec expression becomes a constant.
if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
SimplifiedValues[I] = SC->getValue();
return true;
}
// Check if the offset from the base address becomes a constant.
auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
if (!Base)
return false;
auto *Offset =
dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
if (!Offset)
return false;
SimplifiedAddress Address;
Address.Base = Base->getValue();
Address.Offset = Offset->getValue();
SimplifiedAddresses[I] = Address;
return false;
}
/// Try to simplify binary operator I.
///
/// TODO: Probably it's worth to hoist the code for estimating the
/// simplifications effects to a separate class, since we have a very similar
/// code in InlineCost already.
bool UnrolledInstAnalyzer::visitBinaryOperator(BinaryOperator &I) {
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
if (!isa<Constant>(LHS))
if (Value *SimpleLHS = SimplifiedValues.lookup(LHS))
LHS = SimpleLHS;
if (!isa<Constant>(RHS))
if (Value *SimpleRHS = SimplifiedValues.lookup(RHS))
RHS = SimpleRHS;
Value *SimpleV = nullptr;
const DataLayout &DL = I.getModule()->getDataLayout();
if (auto FI = dyn_cast<FPMathOperator>(&I))
SimpleV =
simplifyBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags(), DL);
else
SimpleV = simplifyBinOp(I.getOpcode(), LHS, RHS, DL);
if (SimpleV) {
SimplifiedValues[&I] = SimpleV;
return true;
}
return Base::visitBinaryOperator(I);
}
/// Try to fold load I.
bool UnrolledInstAnalyzer::visitLoad(LoadInst &I) {
Value *AddrOp = I.getPointerOperand();
auto AddressIt = SimplifiedAddresses.find(AddrOp);
if (AddressIt == SimplifiedAddresses.end())
return false;
ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;
auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
// We're only interested in loads that can be completely folded to a
// constant.
if (!GV || !GV->hasDefinitiveInitializer() || !GV->isConstant())
return false;
ConstantDataSequential *CDS =
dyn_cast<ConstantDataSequential>(GV->getInitializer());
if (!CDS)
return false;
// We might have a vector load from an array. FIXME: for now we just bail
// out in this case, but we should be able to resolve and simplify such
// loads.
if (CDS->getElementType() != I.getType())
return false;
unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
if (SimplifiedAddrOp->getValue().getActiveBits() > 64)
return false;
int64_t SimplifiedAddrOpV = SimplifiedAddrOp->getSExtValue();
if (SimplifiedAddrOpV < 0) {
// FIXME: For now we conservatively ignore out of bound accesses, but
// we're allowed to perform the optimization in this case.
return false;
}
uint64_t Index = static_cast<uint64_t>(SimplifiedAddrOpV) / ElemSize;
if (Index >= CDS->getNumElements()) {
// FIXME: For now we conservatively ignore out of bound accesses, but
// we're allowed to perform the optimization in this case.
return false;
}
Constant *CV = CDS->getElementAsConstant(Index);
assert(CV && "Constant expected.");
SimplifiedValues[&I] = CV;
return true;
}
/// Try to simplify cast instruction.
bool UnrolledInstAnalyzer::visitCastInst(CastInst &I) {
Value *Op = I.getOperand(0);
if (Value *Simplified = SimplifiedValues.lookup(Op))
Op = Simplified;
// The cast can be invalid, because SimplifiedValues contains results of SCEV
// analysis, which operates on integers (and, e.g., might convert i8* null to
// i32 0).
if (CastInst::castIsValid(I.getOpcode(), Op, I.getType())) {
const DataLayout &DL = I.getModule()->getDataLayout();
if (Value *V = simplifyCastInst(I.getOpcode(), Op, I.getType(), DL)) {
SimplifiedValues[&I] = V;
return true;
}
}
return Base::visitCastInst(I);
}
/// Try to simplify cmp instruction.
bool UnrolledInstAnalyzer::visitCmpInst(CmpInst &I) {
Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
// First try to handle simplified comparisons.
if (!isa<Constant>(LHS))
if (Value *SimpleLHS = SimplifiedValues.lookup(LHS))
LHS = SimpleLHS;
if (!isa<Constant>(RHS))
if (Value *SimpleRHS = SimplifiedValues.lookup(RHS))
RHS = SimpleRHS;
if (!isa<Constant>(LHS) && !isa<Constant>(RHS)) {
auto SimplifiedLHS = SimplifiedAddresses.find(LHS);
if (SimplifiedLHS != SimplifiedAddresses.end()) {
auto SimplifiedRHS = SimplifiedAddresses.find(RHS);
if (SimplifiedRHS != SimplifiedAddresses.end()) {
SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
if (LHSAddr.Base == RHSAddr.Base) {
LHS = LHSAddr.Offset;
RHS = RHSAddr.Offset;
}
}
}
}
const DataLayout &DL = I.getModule()->getDataLayout();
if (Value *V = simplifyCmpInst(I.getPredicate(), LHS, RHS, DL)) {
SimplifiedValues[&I] = V;
return true;
}
return Base::visitCmpInst(I);
}
bool UnrolledInstAnalyzer::visitPHINode(PHINode &PN) {
// Run base visitor first. This way we can gather some useful for later
// analysis information.
if (Base::visitPHINode(PN))
return true;
// The loop induction PHI nodes are definitionally free.
return PN.getParent() == L->getHeader();
}
bool UnrolledInstAnalyzer::visitInstruction(Instruction &I) {
return simplifyInstWithSCEV(&I);
}
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