[ConstantFolding] Fix 'undef' folding for @llvm.[us]{add,sub}.with.overflow ops (PR43188)

As we have already established/fixed in
  https://bugs.llvm.org/show_bug.cgi?id=42209
  https://reviews.llvm.org/D63065
  https://reviews.llvm.org/rL363522
the InstSimplify handling for @llvm.with.overflow ops with undefs
is correct. Therefore if ConstantFolding produces different results,
then it is wrong.

This duplication of code hints at the need for some refactoring,
but for now address the brokenness of ConstantFolding by
copying the known-good handling from rL363522.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43188

llvm-svn: 370608

2 files changed, 26 insertions, 16 deletions

diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index f7cc53b345f..cedd788d226 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -2041,20 +2041,27 @@ static Constant *ConstantFoldScalarCall2(StringRef Name,
 
     switch (IntrinsicID) {
     default: break;
+    case Intrinsic::usub_with_overflow:
+    case Intrinsic::ssub_with_overflow:
+    case Intrinsic::uadd_with_overflow:
+    case Intrinsic::sadd_with_overflow:
+      // X - undef -> { undef, false }
+      // undef - X -> { undef, false }
+      // X + undef -> { undef, false }
+      // undef + x -> { undef, false }
+      if (!C0 || !C1) {
+        return ConstantStruct::get(
+            cast<StructType>(Ty),
+            {UndefValue::get(Ty->getStructElementType(0)),
+             Constant::getNullValue(Ty->getStructElementType(1))});
+      }
+      LLVM_FALLTHROUGH;
     case Intrinsic::smul_with_overflow:
-    case Intrinsic::umul_with_overflow:
-      // Even if both operands are undef, we cannot fold muls to undef
-      // in the general case. For example, on i2 there are no inputs
-      // that would produce { i2 -1, i1 true } as the result.
+    case Intrinsic::umul_with_overflow: {
+      // undef * X -> { 0, false }
+      // X * undef -> { 0, false }
       if (!C0 || !C1)
         return Constant::getNullValue(Ty);
-      LLVM_FALLTHROUGH;
-    case Intrinsic::sadd_with_overflow:
-    case Intrinsic::uadd_with_overflow:
-    case Intrinsic::ssub_with_overflow:
-    case Intrinsic::usub_with_overflow: {
-      if (!C0 || !C1)
-        return UndefValue::get(Ty);
 
       APInt Res;
       bool Overflow;
diff --git a/llvm/test/Transforms/ConstProp/overflow-ops.ll b/llvm/test/Transforms/ConstProp/overflow-ops.ll
index 303b3b90ab3..1d090d5d0ea 100644
--- a/llvm/test/Transforms/ConstProp/overflow-ops.ll
+++ b/llvm/test/Transforms/ConstProp/overflow-ops.ll
@@ -1,5 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -constprop -S | FileCheck %s
+; RUN: opt < %s -constprop -S | FileCheck %s --check-prefixes=CHECK,CONSTPROP
+; RUN: opt < %s -instsimplify -S | FileCheck %s --check-prefixes=CHECK,INSTSIMPLIFY
+; We must *NOT* have any check-lines with prefixes other than CHECK here.
+; If we do, that means the rules are different between the passes.
 
 declare {i8, i1} @llvm.uadd.with.overflow.i8(i8, i8)
 declare {i8, i1} @llvm.usub.with.overflow.i8(i8, i8)
@@ -31,7 +34,7 @@ define {i8, i1} @uadd_2() nounwind {
 
 define {i8, i1} @uadd_undef() nounwind {
 ; CHECK-LABEL: @uadd_undef(
-; CHECK-NEXT:    ret { i8, i1 } undef
+; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
 ;
   %t = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 142, i8 undef)
   ret {i8, i1} %t
@@ -59,7 +62,7 @@ define {i8, i1} @usub_2() nounwind {
 
 define {i8, i1} @usub_undef() nounwind {
 ; CHECK-LABEL: @usub_undef(
-; CHECK-NEXT:    ret { i8, i1 } undef
+; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
 ;
   %t = call {i8, i1} @llvm.usub.with.overflow.i8(i8 4, i8 undef)
   ret {i8, i1} %t
@@ -147,7 +150,7 @@ define {i8, i1} @sadd_5() nounwind {
 
 define {i8, i1} @sadd_undef() nounwind {
 ; CHECK-LABEL: @sadd_undef(
-; CHECK-NEXT:    ret { i8, i1 } undef
+; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
 ;
   %t = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 undef, i8 -10)
   ret {i8, i1} %t
@@ -215,7 +218,7 @@ define {i8, i1} @ssub_5() nounwind {
 
 define {i8, i1} @ssub_undef() nounwind {
 ; CHECK-LABEL: @ssub_undef(
-; CHECK-NEXT:    ret { i8, i1 } undef
+; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
 ;
   %t = call {i8, i1} @llvm.ssub.with.overflow.i8(i8 undef, i8 -10)
   ret {i8, i1} %t