[PATCH] D51216: Fix IRBuilder.CreateFCmp(X, X) misfolding

[Andrew Scheidecker via Phabricator via llvm-commits]

AndrewScheidecker updated this revision to Diff 186817.
AndrewScheidecker changed the repository for this revision from rL LLVM to rG LLVM Github Monorepo.
AndrewScheidecker added a comment.
Herald added a subscriber: hiraditya.

Added comment, updated the new bitcast.ll tests to reflect the fix, and removed the superfluous IRBuilder tests.

I also moved the diff from rL <https://reviews.llvm.org/diffusion/L/> to rG <https://reviews.llvm.org/source/llvm-github/> because the old LLVM Github mirror I was using stopped updating (https://github.com/llvm-mirror/llvm).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D51216/new/

https://reviews.llvm.org/D51216

Files:
  llvm/lib/IR/ConstantFold.cpp
  llvm/test/Transforms/ConstProp/bitcast.ll


Index: llvm/test/Transforms/ConstProp/bitcast.ll
===================================================================
--- llvm/test/Transforms/ConstProp/bitcast.ll
+++ llvm/test/Transforms/ConstProp/bitcast.ll
@@ -23,21 +23,21 @@
   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
Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -1359,8 +1359,9 @@
   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 @@
     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 @@
       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.


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