360 lines
14 KiB
C++
360 lines
14 KiB
C++
//===----------------------- AlignmentFromAssumptions.cpp -----------------===//
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// Set Load/Store Alignments From Assumptions
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a ScalarEvolution-based transformation to set
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// the alignments of load, stores and memory intrinsics based on the truth
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// expressions of assume intrinsics. The primary motivation is to handle
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// complex alignment assumptions that apply to vector loads and stores that
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// appear after vectorization and unrolling.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/IR/Instructions.h"
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#include "llvm/InitializePasses.h"
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#define AA_NAME "alignment-from-assumptions"
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#define DEBUG_TYPE AA_NAME
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#include "llvm/Transforms/Scalar/AlignmentFromAssumptions.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/AssumptionCache.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/Analysis/ValueTracking.h"
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#include "llvm/IR/Constant.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Instruction.h"
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#include "llvm/IR/IntrinsicInst.h"
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#include "llvm/IR/Intrinsics.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Scalar.h"
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using namespace llvm;
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STATISTIC(NumLoadAlignChanged,
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"Number of loads changed by alignment assumptions");
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STATISTIC(NumStoreAlignChanged,
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"Number of stores changed by alignment assumptions");
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STATISTIC(NumMemIntAlignChanged,
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"Number of memory intrinsics changed by alignment assumptions");
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namespace {
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struct AlignmentFromAssumptions : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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AlignmentFromAssumptions() : FunctionPass(ID) {
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initializeAlignmentFromAssumptionsPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<AssumptionCacheTracker>();
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AU.addRequired<ScalarEvolutionWrapperPass>();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.setPreservesCFG();
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AU.addPreserved<AAResultsWrapperPass>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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AU.addPreserved<LoopInfoWrapperPass>();
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AU.addPreserved<DominatorTreeWrapperPass>();
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AU.addPreserved<ScalarEvolutionWrapperPass>();
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}
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AlignmentFromAssumptionsPass Impl;
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};
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}
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char AlignmentFromAssumptions::ID = 0;
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static const char aip_name[] = "Alignment from assumptions";
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INITIALIZE_PASS_BEGIN(AlignmentFromAssumptions, AA_NAME,
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aip_name, false, false)
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INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
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INITIALIZE_PASS_END(AlignmentFromAssumptions, AA_NAME,
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aip_name, false, false)
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FunctionPass *llvm::createAlignmentFromAssumptionsPass() {
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return new AlignmentFromAssumptions();
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}
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// Given an expression for the (constant) alignment, AlignSCEV, and an
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// expression for the displacement between a pointer and the aligned address,
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// DiffSCEV, compute the alignment of the displaced pointer if it can be reduced
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// to a constant. Using SCEV to compute alignment handles the case where
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// DiffSCEV is a recurrence with constant start such that the aligned offset
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// is constant. e.g. {16,+,32} % 32 -> 16.
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static MaybeAlign getNewAlignmentDiff(const SCEV *DiffSCEV,
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const SCEV *AlignSCEV,
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ScalarEvolution *SE) {
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// DiffUnits = Diff % int64_t(Alignment)
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const SCEV *DiffUnitsSCEV = SE->getURemExpr(DiffSCEV, AlignSCEV);
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LLVM_DEBUG(dbgs() << "\talignment relative to " << *AlignSCEV << " is "
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<< *DiffUnitsSCEV << " (diff: " << *DiffSCEV << ")\n");
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if (const SCEVConstant *ConstDUSCEV =
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dyn_cast<SCEVConstant>(DiffUnitsSCEV)) {
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int64_t DiffUnits = ConstDUSCEV->getValue()->getSExtValue();
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// If the displacement is an exact multiple of the alignment, then the
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// displaced pointer has the same alignment as the aligned pointer, so
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// return the alignment value.
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if (!DiffUnits)
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return cast<SCEVConstant>(AlignSCEV)->getValue()->getAlignValue();
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// If the displacement is not an exact multiple, but the remainder is a
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// constant, then return this remainder (but only if it is a power of 2).
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uint64_t DiffUnitsAbs = std::abs(DiffUnits);
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if (isPowerOf2_64(DiffUnitsAbs))
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return Align(DiffUnitsAbs);
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}
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return None;
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}
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// There is an address given by an offset OffSCEV from AASCEV which has an
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// alignment AlignSCEV. Use that information, if possible, to compute a new
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// alignment for Ptr.
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static Align getNewAlignment(const SCEV *AASCEV, const SCEV *AlignSCEV,
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const SCEV *OffSCEV, Value *Ptr,
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ScalarEvolution *SE) {
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const SCEV *PtrSCEV = SE->getSCEV(Ptr);
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// On a platform with 32-bit allocas, but 64-bit flat/global pointer sizes
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// (*cough* AMDGPU), the effective SCEV type of AASCEV and PtrSCEV
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// may disagree. Trunc/extend so they agree.
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PtrSCEV = SE->getTruncateOrZeroExtend(
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PtrSCEV, SE->getEffectiveSCEVType(AASCEV->getType()));
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const SCEV *DiffSCEV = SE->getMinusSCEV(PtrSCEV, AASCEV);
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// On 32-bit platforms, DiffSCEV might now have type i32 -- we've always
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// sign-extended OffSCEV to i64, so make sure they agree again.
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DiffSCEV = SE->getNoopOrSignExtend(DiffSCEV, OffSCEV->getType());
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// What we really want to know is the overall offset to the aligned
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// address. This address is displaced by the provided offset.
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DiffSCEV = SE->getMinusSCEV(DiffSCEV, OffSCEV);
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LLVM_DEBUG(dbgs() << "AFI: alignment of " << *Ptr << " relative to "
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<< *AlignSCEV << " and offset " << *OffSCEV
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<< " using diff " << *DiffSCEV << "\n");
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if (MaybeAlign NewAlignment = getNewAlignmentDiff(DiffSCEV, AlignSCEV, SE)) {
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LLVM_DEBUG(dbgs() << "\tnew alignment: " << DebugStr(NewAlignment) << "\n");
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return *NewAlignment;
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}
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if (const SCEVAddRecExpr *DiffARSCEV = dyn_cast<SCEVAddRecExpr>(DiffSCEV)) {
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// The relative offset to the alignment assumption did not yield a constant,
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// but we should try harder: if we assume that a is 32-byte aligned, then in
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// for (i = 0; i < 1024; i += 4) r += a[i]; not all of the loads from a are
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// 32-byte aligned, but instead alternate between 32 and 16-byte alignment.
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// As a result, the new alignment will not be a constant, but can still
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// be improved over the default (of 4) to 16.
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const SCEV *DiffStartSCEV = DiffARSCEV->getStart();
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const SCEV *DiffIncSCEV = DiffARSCEV->getStepRecurrence(*SE);
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LLVM_DEBUG(dbgs() << "\ttrying start/inc alignment using start "
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<< *DiffStartSCEV << " and inc " << *DiffIncSCEV << "\n");
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// Now compute the new alignment using the displacement to the value in the
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// first iteration, and also the alignment using the per-iteration delta.
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// If these are the same, then use that answer. Otherwise, use the smaller
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// one, but only if it divides the larger one.
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MaybeAlign NewAlignment = getNewAlignmentDiff(DiffStartSCEV, AlignSCEV, SE);
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MaybeAlign NewIncAlignment =
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getNewAlignmentDiff(DiffIncSCEV, AlignSCEV, SE);
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LLVM_DEBUG(dbgs() << "\tnew start alignment: " << DebugStr(NewAlignment)
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<< "\n");
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LLVM_DEBUG(dbgs() << "\tnew inc alignment: " << DebugStr(NewIncAlignment)
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<< "\n");
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if (!NewAlignment || !NewIncAlignment)
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return Align(1);
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const Align NewAlign = *NewAlignment;
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const Align NewIncAlign = *NewIncAlignment;
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if (NewAlign > NewIncAlign) {
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LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: "
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<< DebugStr(NewIncAlign) << "\n");
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return NewIncAlign;
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}
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if (NewIncAlign > NewAlign) {
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LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: " << DebugStr(NewAlign)
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<< "\n");
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return NewAlign;
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}
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assert(NewIncAlign == NewAlign);
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LLVM_DEBUG(dbgs() << "\tnew start/inc alignment: " << DebugStr(NewAlign)
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<< "\n");
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return NewAlign;
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}
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return Align(1);
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}
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bool AlignmentFromAssumptionsPass::extractAlignmentInfo(CallInst *I,
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unsigned Idx,
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Value *&AAPtr,
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const SCEV *&AlignSCEV,
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const SCEV *&OffSCEV) {
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Type *Int64Ty = Type::getInt64Ty(I->getContext());
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OperandBundleUse AlignOB = I->getOperandBundleAt(Idx);
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if (AlignOB.getTagName() != "align")
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return false;
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assert(AlignOB.Inputs.size() >= 2);
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AAPtr = AlignOB.Inputs[0].get();
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// TODO: Consider accumulating the offset to the base.
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AAPtr = AAPtr->stripPointerCastsSameRepresentation();
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AlignSCEV = SE->getSCEV(AlignOB.Inputs[1].get());
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AlignSCEV = SE->getTruncateOrZeroExtend(AlignSCEV, Int64Ty);
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if (AlignOB.Inputs.size() == 3)
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OffSCEV = SE->getSCEV(AlignOB.Inputs[2].get());
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else
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OffSCEV = SE->getZero(Int64Ty);
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OffSCEV = SE->getTruncateOrZeroExtend(OffSCEV, Int64Ty);
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return true;
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}
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bool AlignmentFromAssumptionsPass::processAssumption(CallInst *ACall,
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unsigned Idx) {
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Value *AAPtr;
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const SCEV *AlignSCEV, *OffSCEV;
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if (!extractAlignmentInfo(ACall, Idx, AAPtr, AlignSCEV, OffSCEV))
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return false;
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// Skip ConstantPointerNull and UndefValue. Assumptions on these shouldn't
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// affect other users.
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if (isa<ConstantData>(AAPtr))
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return false;
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const SCEV *AASCEV = SE->getSCEV(AAPtr);
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// Apply the assumption to all other users of the specified pointer.
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SmallPtrSet<Instruction *, 32> Visited;
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SmallVector<Instruction*, 16> WorkList;
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for (User *J : AAPtr->users()) {
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if (J == ACall)
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continue;
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if (Instruction *K = dyn_cast<Instruction>(J))
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WorkList.push_back(K);
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}
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while (!WorkList.empty()) {
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Instruction *J = WorkList.pop_back_val();
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if (LoadInst *LI = dyn_cast<LoadInst>(J)) {
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if (!isValidAssumeForContext(ACall, J, DT))
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continue;
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Align NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
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LI->getPointerOperand(), SE);
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if (NewAlignment > LI->getAlign()) {
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LI->setAlignment(NewAlignment);
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++NumLoadAlignChanged;
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}
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} else if (StoreInst *SI = dyn_cast<StoreInst>(J)) {
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if (!isValidAssumeForContext(ACall, J, DT))
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continue;
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Align NewAlignment = getNewAlignment(AASCEV, AlignSCEV, OffSCEV,
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SI->getPointerOperand(), SE);
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if (NewAlignment > SI->getAlign()) {
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SI->setAlignment(NewAlignment);
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++NumStoreAlignChanged;
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}
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} else if (MemIntrinsic *MI = dyn_cast<MemIntrinsic>(J)) {
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if (!isValidAssumeForContext(ACall, J, DT))
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continue;
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Align NewDestAlignment =
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getNewAlignment(AASCEV, AlignSCEV, OffSCEV, MI->getDest(), SE);
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LLVM_DEBUG(dbgs() << "\tmem inst: " << DebugStr(NewDestAlignment)
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<< "\n";);
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if (NewDestAlignment > *MI->getDestAlign()) {
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MI->setDestAlignment(NewDestAlignment);
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++NumMemIntAlignChanged;
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}
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// For memory transfers, there is also a source alignment that
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// can be set.
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if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(MI)) {
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Align NewSrcAlignment =
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getNewAlignment(AASCEV, AlignSCEV, OffSCEV, MTI->getSource(), SE);
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LLVM_DEBUG(dbgs() << "\tmem trans: " << DebugStr(NewSrcAlignment)
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<< "\n";);
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if (NewSrcAlignment > *MTI->getSourceAlign()) {
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MTI->setSourceAlignment(NewSrcAlignment);
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++NumMemIntAlignChanged;
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}
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}
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}
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// Now that we've updated that use of the pointer, look for other uses of
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// the pointer to update.
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Visited.insert(J);
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for (User *UJ : J->users()) {
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Instruction *K = cast<Instruction>(UJ);
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if (!Visited.count(K))
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WorkList.push_back(K);
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}
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}
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return true;
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}
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bool AlignmentFromAssumptions::runOnFunction(Function &F) {
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if (skipFunction(F))
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return false;
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auto &AC = getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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ScalarEvolution *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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return Impl.runImpl(F, AC, SE, DT);
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}
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bool AlignmentFromAssumptionsPass::runImpl(Function &F, AssumptionCache &AC,
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ScalarEvolution *SE_,
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DominatorTree *DT_) {
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SE = SE_;
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DT = DT_;
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bool Changed = false;
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for (auto &AssumeVH : AC.assumptions())
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if (AssumeVH) {
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CallInst *Call = cast<CallInst>(AssumeVH);
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for (unsigned Idx = 0; Idx < Call->getNumOperandBundles(); Idx++)
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Changed |= processAssumption(Call, Idx);
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}
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return Changed;
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}
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PreservedAnalyses
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AlignmentFromAssumptionsPass::run(Function &F, FunctionAnalysisManager &AM) {
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AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F);
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ScalarEvolution &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
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DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
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if (!runImpl(F, AC, &SE, &DT))
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return PreservedAnalyses::all();
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PreservedAnalyses PA;
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PA.preserveSet<CFGAnalyses>();
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PA.preserve<AAManager>();
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PA.preserve<ScalarEvolutionAnalysis>();
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PA.preserve<GlobalsAA>();
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return PA;
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}
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