[NaryReassociate] avoid running forever

Avoid running forever by checking we are not reassociating an expression into
the same form.

Tested with @avoid_infinite_loops in nary-add.ll

llvm-svn: 237269

2 files changed, 19 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
index 7d3080dcd9b..af61068c97b 100644
--- a/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
+++ b/llvm/lib/Transforms/Scalar/NaryReassociate.cpp
@@ -217,10 +217,14 @@ Instruction *NaryReassociate::tryReassociateAdd(Value *LHS, Value *RHS,
     //   = (A + RHS) + B or (B + RHS) + A
     const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
     const SCEV *RHSExpr = SE->getSCEV(RHS);
-    if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(AExpr, RHSExpr), B, I))
-      return NewI;
-    if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(BExpr, RHSExpr), A, I))
-      return NewI;
+    if (BExpr != RHSExpr) {
+      if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(AExpr, RHSExpr), B, I))
+        return NewI;
+    }
+    if (AExpr != RHSExpr) {
+      if (auto *NewI = tryReassociatedAdd(SE->getAddExpr(BExpr, RHSExpr), A, I))
+        return NewI;
+    }
   }
   return nullptr;
 }
diff --git a/llvm/test/Transforms/NaryReassociate/nary-add.ll b/llvm/test/Transforms/NaryReassociate/nary-add.ll
index 39d7c59ef9d..b3093ff6ecd 100644
--- a/llvm/test/Transforms/NaryReassociate/nary-add.ll
+++ b/llvm/test/Transforms/NaryReassociate/nary-add.ll
@@ -196,3 +196,14 @@ define void @iterative(i32 %a, i32 %b, i32 %c) {
 
   ret void
 }
+
+define void @avoid_infinite_loop(i32 %a, i32 %b) {
+; CHECK-LABEL: @avoid_infinite_loop
+  %ab = add i32 %a, %b
+; CHECK-NEXT: %ab
+  %ab2 = add i32 %ab, %b
+; CHECK-NEXT: %ab2
+  call void @foo(i32 %ab2)
+; CHECK-NEXT: @foo(i32 %ab2)
+  ret void
+}