commit 5a663bd77acbd536f16af7829c5f3c4efa14ffe1
author Roman Lebedev <lebedev.ri@gmail.com> Sun Jun 16 20:39:45 2019 +0000
committer Roman Lebedev <lebedev.ri@gmail.com> Sun Jun 16 20:39:45 2019 +0000
tree b38d325afe588bb773e02dbe35146c64f751d16f
parent 9b2d96024ae604aab0e050829d6ad5846166576d

[InstSimplify] Fix addo/subo undef folds (PR42209)

Fix folds of addo and subo with an undef operand to be:

`@llvm.{u,s}{add,sub}.with.overflow` all fold to `{ undef, false }`,
 as per LLVM undef rules.
Same for commuted variants.

Based on the original version of the patch by @nikic.

Fixes [[ https://bugs.llvm.org/show_bug.cgi?id=42209 | PR42209 ]]

Differential Revision: https://reviews.llvm.org/D63065

llvm-svn: 363522

llvm/lib/Analysis/InstructionSimplify.cpp

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index d0b9549d..ba76c5b 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -4843,16 +4843,19 @@
     // X - X -> { 0, false }
     if (Op0 == Op1)
       return Constant::getNullValue(ReturnType);
-    // X - undef -> undef
-    // undef - X -> undef
-    if (isa<UndefValue>(Op0) || isa<UndefValue>(Op1))
-      return UndefValue::get(ReturnType);
-    break;
+    LLVM_FALLTHROUGH;
   case Intrinsic::uadd_with_overflow:
   case Intrinsic::sadd_with_overflow:
-    // X + undef -> undef
-    if (isa<UndefValue>(Op0) || isa<UndefValue>(Op1))
-      return UndefValue::get(ReturnType);
+    // X - undef -> { undef, false }
+    // undef - X -> { undef, false }
+    // X + undef -> { undef, false }
+    // undef + x -> { undef, false }
+    if (isa<UndefValue>(Op0) || isa<UndefValue>(Op1)) {
+      return ConstantStruct::get(
+          cast<StructType>(ReturnType),
+          {UndefValue::get(ReturnType->getStructElementType(0)),
+           Constant::getNullValue(ReturnType->getStructElementType(1))});
+    }
     break;
   case Intrinsic::umul_with_overflow:
   case Intrinsic::smul_with_overflow: