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//===- InferAlignment.cpp -------------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// Infer alignment for load, stores and other memory operations based on
// trailing zero known bits information.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar/InferAlignment.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Instructions.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
static bool tryToImproveAlign(
const DataLayout &DL, Instruction *I,
function_ref<Align(Value *PtrOp, Align OldAlign, Align PrefAlign)> Fn) {
if (auto *PtrOp = getLoadStorePointerOperand(I)) {
Align OldAlign = getLoadStoreAlignment(I);
Align PrefAlign = DL.getPrefTypeAlign(getLoadStoreType(I));
Align NewAlign = Fn(PtrOp, OldAlign, PrefAlign);
if (NewAlign > OldAlign) {
setLoadStoreAlignment(I, NewAlign);
return true;
}
}
// TODO: Also handle memory intrinsics.
return false;
}
bool inferAlignment(Function &F, AssumptionCache &AC, DominatorTree &DT) {
const DataLayout &DL = F.getDataLayout();
bool Changed = false;
// Enforce preferred type alignment if possible. We do this as a separate
// pass first, because it may improve the alignments we infer below.
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
Changed |= tryToImproveAlign(
DL, &I, [&](Value *PtrOp, Align OldAlign, Align PrefAlign) {
if (PrefAlign > OldAlign)
return std::max(OldAlign,
tryEnforceAlignment(PtrOp, PrefAlign, DL));
return OldAlign;
});
}
}
// Compute alignment from known bits.
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
Changed |= tryToImproveAlign(
DL, &I, [&](Value *PtrOp, Align OldAlign, Align PrefAlign) {
KnownBits Known = computeKnownBits(PtrOp, DL, 0, &AC, &I, &DT);
unsigned TrailZ = std::min(Known.countMinTrailingZeros(),
+Value::MaxAlignmentExponent);
return Align(1ull << std::min(Known.getBitWidth() - 1, TrailZ));
});
}
}
return Changed;
}
PreservedAnalyses InferAlignmentPass::run(Function &F,
FunctionAnalysisManager &AM) {
AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F);
DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
inferAlignment(F, AC, DT);
// Changes to alignment shouldn't invalidated analyses.
return PreservedAnalyses::all();
}
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