From 3e32b7e12701de772b1cdf855b42253650a1e997 Mon Sep 17 00:00:00 2001
From: Juneyoung Lee <aqjune@gmail.com>
Date: Wed, 15 Jan 2020 01:11:20 +0900
Subject: [PATCH] [InstCombine] Let combineLoadToNewType preserve ABI alignment
 of the load (PR44543)

Summary:
If aligment on `LoadInst` isn't specified, load is assumed to be ABI-aligned.
And said aligment may be different for different types.
So if we change load type, but don't pay extra attention to the aligment
(i.e. keep it unspecified), we may either overpromise (if the default aligment
of the new type is higher), or underpromise (if the default aligment
of the new type is smaller).

Thus, if no alignment is specified, we need to manually preserve the implied ABI alignment.

This addresses https://bugs.llvm.org/show_bug.cgi?id=44543 by making combineLoadToNewType preserve ABI alignment of the load.

Reviewers: spatel, lebedev.ri

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72710
---
 .../InstCombine/InstCombineLoadStoreAlloca.cpp           | 9 ++++++++-
 llvm/test/Transforms/InstCombine/load-bitcast64.ll       | 6 +++---
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index dc1ccc4bafcd..f2c2d1cdf5ae 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -462,8 +462,15 @@ static LoadInst *combineLoadToNewType(InstCombiner &IC, LoadInst &LI, Type *NewT
         NewPtr->getType()->getPointerAddressSpace() == AS))
     NewPtr = IC.Builder.CreateBitCast(Ptr, NewTy->getPointerTo(AS));
 
+  unsigned Align = LI.getAlignment();
+  if (!Align)
+    // If old load did not have an explicit alignment specified,
+    // manually preserve the implied (ABI) alignment of the load.
+    // Else we may inadvertently incorrectly over-promise alignment.
+    Align = IC.getDataLayout().getABITypeAlignment(LI.getType());
+
   LoadInst *NewLoad = IC.Builder.CreateAlignedLoad(
-      NewTy, NewPtr, LI.getAlignment(), LI.isVolatile(), LI.getName() + Suffix);
+      NewTy, NewPtr, Align, LI.isVolatile(), LI.getName() + Suffix);
   NewLoad->setAtomic(LI.getOrdering(), LI.getSyncScopeID());
   copyMetadataForLoad(*NewLoad, LI);
   return NewLoad;
diff --git a/llvm/test/Transforms/InstCombine/load-bitcast64.ll b/llvm/test/Transforms/InstCombine/load-bitcast64.ll
index 58bc923da699..270bd6f5f811 100644
--- a/llvm/test/Transforms/InstCombine/load-bitcast64.ll
+++ b/llvm/test/Transforms/InstCombine/load-bitcast64.ll
@@ -8,7 +8,7 @@ define i64* @test1(i8* %x) {
 ; CHECK-LABEL: @test1(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64**
-; CHECK-NEXT:    [[B1:%.*]] = load i64*, i64** [[TMP0]], align 8
+; CHECK-NEXT:    [[B1:%.*]] = load i64*, i64** [[TMP0]], align 4
 ; CHECK-NEXT:    ret i64* [[B1]]
 ;
 entry:
@@ -57,7 +57,7 @@ define i64 @test4(i8* %x) {
 ; CHECK-LABEL: @test4(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64*
-; CHECK-NEXT:    [[B1:%.*]] = load i64, i64* [[TMP0]], align 4
+; CHECK-NEXT:    [[B1:%.*]] = load i64, i64* [[TMP0]], align 8
 ; CHECK-NEXT:    ret i64 [[B1]]
 ;
 entry:
@@ -88,7 +88,7 @@ define i64 @test6(i8* %x) {
 ; CHECK-LABEL: @test6(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = bitcast i8* [[X:%.*]] to i64*
-; CHECK-NEXT:    [[B1:%.*]] = load i64, i64* [[TMP0]], align 4
+; CHECK-NEXT:    [[B1:%.*]] = load i64, i64* [[TMP0]], align 8
 ; CHECK-NEXT:    ret i64 [[B1]]
 ;
 entry: