[MemCpyOpt] Avoid infinite loops in `MemCpyOptPass::processMemCpyMemC…

…pyDependence` (#103218)

Closes #102994.

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1193,6 +1193,10 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
       CopySourceAlign = commonAlignment(*CopySourceAlign, MForwardOffset);
   }
 
+  // Avoid infinite loops
+  if (BAA.isMustAlias(M->getSource(), CopySource))
+    return false;
+
   // Verify that the copied-from memory doesn't change in between the two
   // transfers.  For example, in:
   //    memcpy(a <- b)

llvm/test/Transforms/MemCpyOpt/pr102994.ll

@@ -0,0 +1,39 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=memcpyopt < %s | FileCheck %s
+
+@g1 = external global i8
+@g2 = external global [64 x i8]
+@g3 = global i8 0, align 1
+
+define void @func() {
+; CHECK-LABEL: define void @func() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr getelementptr inbounds (i8, ptr @g2, i64 16), ptr getelementptr inbounds nuw (i8, ptr @g2, i64 16), i64 20, i1 false)
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr @g1, ptr getelementptr inbounds (i8, ptr @g2, i64 24), i64 1, i1 false)
+; CHECK-NEXT:    ret void
+;
+entry:
+  call void @llvm.memcpy.p0.p0.i64(ptr getelementptr inbounds (i8, ptr @g2, i64 16), ptr getelementptr inbounds nuw (i8, ptr @g2, i64 16), i64 20, i1 false)
+  call void @llvm.memcpy.p0.p0.i64(ptr @g1, ptr getelementptr inbounds (i8, ptr @g2, i64 24), i64 1, i1 false)
+  ret void
+}
+
+define void @func2(ptr %p) {
+; CHECK-LABEL: define void @func2(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr i8, ptr [[P]], i64 32
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i8, ptr [[P]], i64 34
+; CHECK-NEXT:    [[GEP3:%.*]] = getelementptr i8, ptr [[P]], i64 32
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[GEP1]], ptr [[GEP3]], i64 32, i1 false)
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr @g3, ptr [[GEP2]], i64 1, i1 false)
+; CHECK-NEXT:    ret void
+;
+entry:
+  %gep1 = getelementptr i8, ptr %p, i64 32
+  %gep2 = getelementptr i8, ptr %p, i64 34
+  %gep3 = getelementptr i8, ptr %p, i64 32
+  call void @llvm.memcpy.p0.p0.i64(ptr %gep1, ptr %gep3, i64 32, i1 false)
+  call void @llvm.memcpy.p0.p0.i64(ptr @g3, ptr %gep2, i64 1, i1 false)
+  ret void
+}