1//===- InferAlignment.cpp -------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// Infer alignment for load, stores and other memory operations based on
10// trailing zero known bits information.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/Transforms/Scalar/InferAlignment.h"
15#include "llvm/Analysis/AssumptionCache.h"
16#include "llvm/Analysis/ValueTracking.h"
17#include "llvm/IR/Instructions.h"
18#include "llvm/Support/KnownBits.h"
19#include "llvm/Transforms/Scalar.h"
20#include "llvm/Transforms/Utils/Local.h"
21
22using namespace llvm;
23
24static bool tryToImproveAlign(
25 const DataLayout &DL, Instruction *I,
26 function_ref<Align(Value *PtrOp, Align OldAlign, Align PrefAlign)> Fn) {
27
28 if (auto *PtrOp = getLoadStorePointerOperand(V: I)) {
29 Align OldAlign = getLoadStoreAlignment(I);
30 Align PrefAlign = DL.getPrefTypeAlign(Ty: getLoadStoreType(I));
31
32 Align NewAlign = Fn(PtrOp, OldAlign, PrefAlign);
33 if (NewAlign > OldAlign) {
34 setLoadStoreAlignment(I, NewAlign);
35 return true;
36 }
37 }
38 // TODO: Also handle memory intrinsics.
39 return false;
40}
41
42bool inferAlignment(Function &F, AssumptionCache &AC, DominatorTree &DT) {
43 const DataLayout &DL = F.getDataLayout();
44 bool Changed = false;
45
46 // Enforce preferred type alignment if possible. We do this as a separate
47 // pass first, because it may improve the alignments we infer below.
48 for (BasicBlock &BB : F) {
49 for (Instruction &I : BB) {
50 Changed |= tryToImproveAlign(
51 DL, I: &I, Fn: [&](Value *PtrOp, Align OldAlign, Align PrefAlign) {
52 if (PrefAlign > OldAlign)
53 return std::max(a: OldAlign,
54 b: tryEnforceAlignment(V: PtrOp, PrefAlign, DL));
55 return OldAlign;
56 });
57 }
58 }
59
60 // Compute alignment from known bits.
61 for (BasicBlock &BB : F) {
62 for (Instruction &I : BB) {
63 Changed |= tryToImproveAlign(
64 DL, I: &I, Fn: [&](Value *PtrOp, Align OldAlign, Align PrefAlign) {
65 KnownBits Known = computeKnownBits(V: PtrOp, DL, AC: &AC, CxtI: &I, DT: &DT);
66 unsigned TrailZ = std::min(a: Known.countMinTrailingZeros(),
67 b: +Value::MaxAlignmentExponent);
68 return Align(1ull << std::min(a: Known.getBitWidth() - 1, b: TrailZ));
69 });
70 }
71 }
72
73 return Changed;
74}
75
76PreservedAnalyses InferAlignmentPass::run(Function &F,
77 FunctionAnalysisManager &AM) {
78 AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(IR&: F);
79 DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
80 inferAlignment(F, AC, DT);
81 // Changes to alignment shouldn't invalidated analyses.
82 return PreservedAnalyses::all();
83}
84