[ConstantFold] Fix misfolding fcmp of a ConstantExpr NaN with itself.

The code incorrectly inferred that the relationship of a constant expression
to itself is FCMP_OEQ (ordered and equal), when it's actually FCMP_UEQ
(unordered *or* equal). This change corrects that, and adds some more limited
folds that can be done in this case.

Differential revision: https://reviews.llvm.org/D51216

llvm-svn: 354381

llvm/lib/IR/ConstantFold.cpp

@@ -1359,8 +1359,9 @@ static FCmpInst::Predicate evaluateFCmpRelation(Constant *V1, Constant *V2) {
   assert(V1->getType() == V2->getType() &&
          "Cannot compare values of different types!");
 
-  // Handle degenerate case quickly
-  if (V1 == V2) return FCmpInst::FCMP_OEQ;
+  // We do not know if a constant expression will evaluate to a number or NaN.
+  // Therefore, we can only say that the relation is unordered or equal.
+  if (V1 == V2) return FCmpInst::FCMP_UEQ;
 
   if (!isa<ConstantExpr>(V1)) {
     if (!isa<ConstantExpr>(V2)) {
@@ -1855,7 +1856,6 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
     default: llvm_unreachable("Unknown relation!");
     case FCmpInst::FCMP_UNO:
     case FCmpInst::FCMP_ORD:
-    case FCmpInst::FCMP_UEQ:
     case FCmpInst::FCMP_UNE:
     case FCmpInst::FCMP_ULT:
     case FCmpInst::FCMP_UGT:
@@ -1901,6 +1901,13 @@ Constant *llvm::ConstantFoldCompareInstruction(unsigned short pred,
       else if (pred == FCmpInst::FCMP_ONE || pred == FCmpInst::FCMP_UNE)
         Result = 1;
       break;
+    case FCmpInst::FCMP_UEQ: // We know that C1 == C2 || isUnordered(C1, C2).
+      // We can only partially decide this relation.
+      if (pred == FCmpInst::FCMP_ONE)
+        Result = 0;
+      else if (pred == FCmpInst::FCMP_UEQ)
+        Result = 1;
+      break;
     }
 
     // If we evaluated the result, return it now.

llvm/test/Transforms/ConstProp/bitcast.ll

@@ -36,21 +36,21 @@ define i1 @bad_fcmp_constexpr_bitcast() {
   ret i1 %cmp
 }
 
-; FIXME: If the bitcasts result in a NaN FP value, then "ordered and equal" would be false.
+; Ensure that an "ordered and equal" fcmp of a ConstantExpr to itself is not folded, since the ConstantExpr may be a NaN.
 
 define i1 @fcmp_constexpr_oeq(float %conv) {
 ; CHECK-LABEL: @fcmp_constexpr_oeq(
-; CHECK-NEXT:    ret i1 true
+; CHECK-NEXT:    ret i1 fcmp oeq (float bitcast (i32 ptrtoint (i16* @a to i32) to float), float bitcast (i32 ptrtoint (i16* @a to i32) to float))
 ;
   %cmp = fcmp oeq float bitcast (i32 ptrtoint (i16* @a to i32) to float), bitcast (i32 ptrtoint (i16* @a to i32) to float)
   ret i1 %cmp
 }
 
-; FIXME: If the bitcasts result in a NaN FP value, then "unordered or not equal" would be true.
+; Ensure that an "unordered or not equal" fcmp of a ConstantExpr to itself is not folded, since the ConstantExpr may be a NaN.
 
 define i1 @fcmp_constexpr_une(float %conv) {
 ; CHECK-LABEL: @fcmp_constexpr_une(
-; CHECK-NEXT:    ret i1 false
+; CHECK-NEXT:    ret i1 fcmp une (float bitcast (i32 ptrtoint (i16* @a to i32) to float), float bitcast (i32 ptrtoint (i16* @a to i32) to float))
 ;
   %cmp = fcmp une float bitcast (i32 ptrtoint (i16* @a to i32) to float), bitcast (i32 ptrtoint (i16* @a to i32) to float)
   ret i1 %cmp