diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -1116,8 +1116,12 @@
         return C1;
       // undef << X -> 0
       return Constant::getNullValue(C1->getType());
-    case Instruction::FAdd:
     case Instruction::FSub:
+      // -0.0 - undef --> undef (consistent with "fneg undef")
+      if (match(C1, m_NegZeroFP()) && isa<UndefValue>(C2))
+        return C2;
+      LLVM_FALLTHROUGH;
+    case Instruction::FAdd:
     case Instruction::FMul:
     case Instruction::FDiv:
     case Instruction::FRem:
diff --git a/llvm/test/Analysis/ConstantFolding/fp-undef.ll b/llvm/test/Analysis/ConstantFolding/fp-undef.ll
--- a/llvm/test/Analysis/ConstantFolding/fp-undef.ll
+++ b/llvm/test/Analysis/ConstantFolding/fp-undef.ll
@@ -235,7 +235,7 @@
 
 define double @fsub_undef_op1_constant_neg0(double %x) {
 ; CHECK-LABEL: @fsub_undef_op1_constant_neg0(
-; CHECK-NEXT:    ret double 0x7FF8000000000000
+; CHECK-NEXT:    ret double undef
 ;
   %r = fsub double 0x8000000000000000, undef
   ret double %r