[DA] Dependence analysis does not handle array accesses of different sizes

This fixes bug #16183
where the elements of a same array are accesses as i32 and i64.
This is not handled by the array dependence analysis.

Author: sebpop

llvm/lib/Analysis/DependenceAnalysis.cpp

@@ -722,9 +722,14 @@ static AliasResult underlyingObjectsAlias(AAResults *AA, LoopInfo *LI,
   const Value *AObj = getUnderlyingObject(LocA.Ptr);
   const Value *BObj = getUnderlyingObject(LocB.Ptr);
 
-  // If the underlying objects are the same, they must alias.
-  if (AObj == BObj)
+  if (AObj == BObj) {
+    // The dependence test gets confused if the size of the memory accesses differ.
+    if (LocA.Size != LocB.Size)
+      return AliasResult::MayAlias;
+
+    // If the underlying objects are the same, they must alias.
     return AliasResult::MustAlias;
+  }
 
   if (auto *APhi = dyn_cast<PHINode>(AObj)) {
     if (auto *BPhi = dyn_cast<PHINode>(BObj)) {

llvm/test/Analysis/DependenceAnalysis/DifferentAccessSize.ll

@@ -0,0 +1,15 @@
+; RUN: opt < %s -disable-output "-passes=print<da>" -aa-pipeline=basic-aa 2>&1 \
+; RUN: | FileCheck %s
+
+; The dependence test does not handle array accesses of different sizes: i32 and i64.
+; Bug 16183 - https://github.com/llvm/llvm-project/issues/16183
+; CHECK-LABEL: bug16183_alias
+; CHECK: da analyze - confused!
+
+define i64 @bug16183_alias(i32* nocapture %A) {
+entry:
+  %arrayidx = getelementptr inbounds i32, ptr %A, i64 1
+  store i32 2, ptr %arrayidx, align 4
+  %0 = load i64, ptr %A, align 8
+  ret i64 %0
+}