[AST] Fix size merging for MustAlias sets

AST checks aliasing with MustAlias sets by only checking the
representative pointer (getSomePointer). This is only correct
if the Size and AATags information of that pointer also includes
the Size/AATags of all other pointers in the set.

When we add a new pointer to the AliasSet, we do perform this
update (see the code in AliasSet::addPointer). However, if a
pointer already in the MustAlias set is used with a new size,
we currently do not update the representative pointer, resulting
in miscompilations. Fix this by adding the missing update.

This is targeted fix using the current representation. There are
a couple of alternatives:
 * For MustAlias sets, don't store per-pointer Size/AATags at all.
   This would make it clear that there is only one set of common
   Size/AATags for all pointers.
 * Check against all pointers in the set even for MustAlias. This
   is what llvm#65731
   proposes to do as part of a larger change to AST representation.

Fixes llvm#64897.

Author: nikic

llvm/lib/Analysis/AliasSetTracker.cpp

@@ -348,8 +348,16 @@ AliasSet &AliasSetTracker::getAliasSetFor(const MemoryLocation &MemLoc) {
     // due to a quirk of alias analysis behavior. Since alias(undef, undef)
     // is NoAlias, mergeAliasSetsForPointer(undef, ...) will not find the
     // the right set for undef, even if it exists.
-    if (Entry.updateSizeAndAAInfo(Size, AAInfo))
+    if (Entry.updateSizeAndAAInfo(Size, AAInfo)) {
       mergeAliasSetsForPointer(Pointer, Size, AAInfo, MustAliasAll);
+
+      // For MustAlias sets, also update Size/AAInfo of the representative
+      // pointer.
+      AliasSet &AS = *Entry.getAliasSet(*this);
+      if (AS.isMustAlias())
+        if (AliasSet::PointerRec *P = AS.getSomePointer())
+          P->updateSizeAndAAInfo(Size, AAInfo);
+    }
     // Return the set!
     return *Entry.getAliasSet(*this)->getForwardedTarget(*this);
   }

llvm/test/Transforms/LICM/pr64897.ll

@@ -1,7 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
 ; RUN: opt -S -passes=licm < %s | FileCheck %s
 
-; FIXME: This is a miscompile.
 define void @test(i1 %c, i8 %x) {
 ; CHECK-LABEL: define void @test(
 ; CHECK-SAME: i1 [[C:%.*]], i8 [[X:%.*]]) {
@@ -10,17 +9,16 @@ define void @test(i1 %c, i8 %x) {
 ; CHECK-NEXT:    [[P:%.*]] = getelementptr inbounds i8, ptr [[A]], i64 8
 ; CHECK-NEXT:    [[P_COPY:%.*]] = getelementptr inbounds i8, ptr [[A]], i64 8
 ; CHECK-NEXT:    [[P2:%.*]] = getelementptr inbounds i8, ptr [[A]], i64 12
-; CHECK-NEXT:    [[P2_PROMOTED:%.*]] = load i8, ptr [[P2]], align 1
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[TMP0:%.*]] = phi i8 [ 0, [[LOOP]] ], [ [[P2_PROMOTED]], [[START:%.*]] ]
 ; CHECK-NEXT:    store i32 286331153, ptr [[P]], align 4
 ; CHECK-NEXT:    store i32 34, ptr [[P_COPY]], align 4
 ; CHECK-NEXT:    store i64 3689348814741910323, ptr [[P_COPY]], align 4
-; CHECK-NEXT:    call void @use(i8 [[TMP0]])
+; CHECK-NEXT:    [[VAL:%.*]] = load i8, ptr [[P2]], align 1
+; CHECK-NEXT:    call void @use(i8 [[VAL]])
+; CHECK-NEXT:    store i8 0, ptr [[P2]], align 1
 ; CHECK-NEXT:    br i1 [[C]], label [[LOOP]], label [[EXIT:%.*]]
 ; CHECK:       exit:
-; CHECK-NEXT:    store i8 0, ptr [[P2]], align 1
 ; CHECK-NEXT:    ret void
 ;
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