Automerge: [IndVarSimplify] Handle the case where both operands are the same when widening IV (#135207)

`WidenIV::widenWithVariantUse` assumes that exactly one of the binop
operands is the IV to be widened. This miscompilation happens when it
tries to sign-extend the "NonIV" operand while the IV is zero-extended.
Closes llvm/llvm-project#135182.

Author: dtcxzyw

llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

@@ -1743,6 +1743,9 @@ bool WidenIV::widenWithVariantUse(WidenIV::NarrowIVDefUse DU) {
     // TODO: Support case for NarrowDef = NarrowUse->getOperand(1).
     if (NarrowUse->getOperand(0) != NarrowDef)
       return false;
+    // We cannot use a different extend kind for the same operand.
+    if (NarrowUse->getOperand(1) == NarrowDef)
+      return false;
     if (!SE->isKnownNegative(RHS))
       return false;
     bool ProvedSubNUW = SE->isKnownPredicateAt(ICmpInst::ICMP_UGE, LHS,

llvm/test/Transforms/IndVarSimplify/pr135182.ll

@@ -0,0 +1,27 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=indvars < %s | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
+
+define i32 @pr135182() {
+; CHECK-LABEL: define i32 @pr135182() {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    br i1 false, label %[[FOR_BODY]], label %[[FOR_END:.*]]
+; CHECK:       [[FOR_END]]:
+; CHECK-NEXT:    ret i32 65512
+;
+entry:
+  br label %for.body
+
+for.body:
+  %indvar = phi i16 [ -12, %entry ], [ %indvar.next, %for.body ]
+  %add = add i16 %indvar, %indvar
+  %ext = zext i16 %add to i32
+  %indvar.next = add i16 %indvar, 1
+  br i1 false, label %for.body, label %for.end
+
+for.end:
+  ret i32 %ext
+}