[AggressiveInstCombine] Use AA during store merge (#149992)

This is a small extension of #147540, resolving one of the FIXMEs.
Instead of bailing out on any instruction that may read/write memory,
use AA to check whether it can alias the stored parts. Do this using a
crude check based on the underlying object only.

This pattern occurs rarely in practice, but at the same time it also doesn't
seem to add any compile-time cost, so it's probably worth handling.

Author: nikic

llvm/lib/Transforms/AggressiveInstCombine/AggressiveInstCombine.cpp

@@ -965,6 +965,7 @@ static bool foldConsecutiveStores(BasicBlock &BB, const DataLayout &DL,
   if (DL.isBigEndian())
     return false;
 
+  BatchAAResults BatchAA(AA);
   SmallVector<PartStore, 8> Parts;
   bool MadeChange = false;
   for (Instruction &I : make_early_inc_range(BB)) {
@@ -980,8 +981,13 @@ static bool foldConsecutiveStores(BasicBlock &BB, const DataLayout &DL,
       continue;
     }
 
-    // FIXME: Use AA to make this more precise.
-    if (I.mayReadOrWriteMemory() || I.mayThrow()) {
+    if (Parts.empty())
+      continue;
+
+    if (I.mayThrow() ||
+        (I.mayReadOrWriteMemory() &&
+         isModOrRefSet(BatchAA.getModRefInfo(
+             &I, MemoryLocation::getBeforeOrAfter(Parts[0].PtrBase))))) {
       MadeChange |= mergePartStores(Parts, DL, TTI);
       Parts.clear();
       continue;

llvm/test/Transforms/AggressiveInstCombine/X86/store-merge.ll

@@ -359,13 +359,8 @@ define void @test_aliasing_store(i16 %x, ptr %p, ptr %p2) {
 define void @test_non_aliasing_store(i16 %x, ptr noalias %p, ptr noalias %p2) {
 ; CHECK-LABEL: define void @test_non_aliasing_store(
 ; CHECK-SAME: i16 [[X:%.*]], ptr noalias [[P:%.*]], ptr noalias [[P2:%.*]]) {
-; CHECK-NEXT:    [[X_0:%.*]] = trunc i16 [[X]] to i8
-; CHECK-NEXT:    store i8 [[X_0]], ptr [[P]], align 1
+; CHECK-NEXT:    store i16 [[X]], ptr [[P]], align 1
 ; CHECK-NEXT:    store i8 0, ptr [[P2]], align 1
-; CHECK-NEXT:    [[SHR_1:%.*]] = lshr i16 [[X]], 8
-; CHECK-NEXT:    [[X_1:%.*]] = trunc i16 [[SHR_1]] to i8
-; CHECK-NEXT:    [[GEP_1:%.*]] = getelementptr i8, ptr [[P]], i64 1
-; CHECK-NEXT:    store i8 [[X_1]], ptr [[GEP_1]], align 1
 ; CHECK-NEXT:    ret void
 ;
   %x.0 = trunc i16 %x to i8
@@ -403,13 +398,8 @@ define i8 @test_aliasing_load(i16 %x, ptr %p, ptr %p2) {
 define i8 @test_non_aliasing_load(i16 %x, ptr noalias %p, ptr noalias %p2) {
 ; CHECK-LABEL: define i8 @test_non_aliasing_load(
 ; CHECK-SAME: i16 [[X:%.*]], ptr noalias [[P:%.*]], ptr noalias [[P2:%.*]]) {
-; CHECK-NEXT:    [[X_0:%.*]] = trunc i16 [[X]] to i8
-; CHECK-NEXT:    store i8 [[X_0]], ptr [[P]], align 1
+; CHECK-NEXT:    store i16 [[X]], ptr [[P]], align 1
 ; CHECK-NEXT:    [[V:%.*]] = load i8, ptr [[P2]], align 1
-; CHECK-NEXT:    [[SHR_1:%.*]] = lshr i16 [[X]], 8
-; CHECK-NEXT:    [[X_1:%.*]] = trunc i16 [[SHR_1]] to i8
-; CHECK-NEXT:    [[GEP_1:%.*]] = getelementptr i8, ptr [[P]], i64 1
-; CHECK-NEXT:    store i8 [[X_1]], ptr [[GEP_1]], align 1
 ; CHECK-NEXT:    ret i8 [[V]]
 ;
   %x.0 = trunc i16 %x to i8