[ConstantFolding] Fixing addo/subo with undef

[georgemitenkov]

Summary

When folding addo/subo with undef, the current
convention is to use { -1, false } for addo and
{ 0, false } for subo. This was fixed for InstSimplify in
https://reviews.llvm.org/rGf094d65beaa492e845b03561eddd75b5be653a01,
but not in ConstantFolding.

[nikic]

LGTM

llvm/lib/Analysis/ConstantFolding.cpp

case Intrinsic::umin:
       if (!C0 && !C1)
         return UndefValue::get(Ty);
       if (!C0 || !C1)
         return ConstantInt::get(Ty, APInt::getMinValue(BitWidth));
       return ConstantInt::get(Ty, C0->ult(*C1) ? *C0 : *C1);
 
     case Intrinsic::usub_with_overflow:
     case Intrinsic::ssub_with_overflow:
+      // X - undef -> { 0, false }
+      // undef - X -> { 0, false }
+      if (!C0 || !C1)
+        return Constant::getNullValue(Ty);
+      LLVM_FALLTHROUGH;
     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 }
+      // X + undef -> { -1, false }
+      // undef + x -> { -1, false }
       if (!C0 || !C1) {
         return ConstantStruct::get(
             cast<StructType>(Ty),
-            {UndefValue::get(Ty->getStructElementType(0)),
+            {Constant::getAllOnesValue(Ty->getStructElementType(0)),
              Constant::getNullValue(Ty->getStructElementType(1))});
       }
       LLVM_FALLTHROUGH;
     case Intrinsic::smul_with_overflow:
     case Intrinsic::umul_with_overflow: {
       // undef * X -> { 0, false }
       // X * undef -> { 0, false }
       if (!C0 || !C1)

llvm/test/Transforms/InstSimplify/ConstProp/overflow-ops.ll

 ; CHECK-NEXT:    ret { i8, i1 } { i8 6, i1 true }
 ;
   %t = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 142, i8 120)
   ret {i8, i1} %t
 }
 
 define {i8, i1} @uadd_undef() nounwind {
 ; CHECK-LABEL: @uadd_undef(
-; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
+; CHECK-NEXT:    ret { i8, i1 } { i8 -1, i1 false }
 ;
   %t = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 142, i8 undef)
   ret {i8, i1} %t
 }
 
 ;;-----------------------------
 ;; usub
 ;;-----------------------------

 ; CHECK-NEXT:    ret { i8, i1 } { i8 -2, i1 true }
 ;
   %t = call {i8, i1} @llvm.usub.with.overflow.i8(i8 4, i8 6)
   ret {i8, i1} %t
 }
 
 define {i8, i1} @usub_undef() nounwind {
 ; CHECK-LABEL: @usub_undef(
-; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %t = call {i8, i1} @llvm.usub.with.overflow.i8(i8 4, i8 undef)
   ret {i8, i1} %t
 }
 
 ;;-----------------------------
 ;; umul
 ;;-----------------------------

 ; CHECK-NEXT:    ret { i8, i1 } { i8 -8, i1 false }
 ;
   %t = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 2, i8 -10)
   ret {i8, i1} %t
 }
 
 define {i8, i1} @sadd_undef() nounwind {
 ; CHECK-LABEL: @sadd_undef(
-; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
+; CHECK-NEXT:    ret { i8, i1 } { i8 -1, i1 false }
 ;
   %t = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 undef, i8 -10)
   ret {i8, i1} %t
 }
 
 ;;-----------------------------
 ;; ssub
 ;;-----------------------------

 ; CHECK-NEXT:    ret { i8, i1 } { i8 -10, i1 false }
 ;
   %t = call {i8, i1} @llvm.ssub.with.overflow.i8(i8 -20, i8 -10)
   ret {i8, i1} %t
 }
 
 define {i8, i1} @ssub_undef() nounwind {
 ; CHECK-LABEL: @ssub_undef(
-; CHECK-NEXT:    ret { i8, i1 } { i8 undef, i1 false }
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %t = call {i8, i1} @llvm.ssub.with.overflow.i8(i8 undef, i8 -10)
   ret {i8, i1} %t
 }
 
 ;;-----------------------------
 ;; smul
 ;;-----------------------------