Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -169,7 +169,12 @@
       return V;
 
     // See note below regarding the PPC_FP128 restriction.
-    if (DestTy->isFloatingPointTy() && !DestTy->isPPC_FP128Ty())
+    // Also disable for x86_fp80 to avoid turning unnormals, a pseudo-denormals
+    // or pseudo-infinities into a nan which we can't convert back to the
+    // original value if it gets casted back to integer.
+    // FIXME: Can we preserve this information in APFloat?
+    if (DestTy->isFloatingPointTy() && !DestTy->isPPC_FP128Ty() &&
+        !DestTy->isX86_FP80Ty())
       return ConstantFP::get(DestTy->getContext(),
                              APFloat(DestTy->getFltSemantics(),
                                      CI->getValue()));
Index: llvm/test/Transforms/InstCombine/X86/2009-03-23-i80-fp80.ll
===================================================================
--- llvm/test/Transforms/InstCombine/X86/2009-03-23-i80-fp80.ll
+++ llvm/test/Transforms/InstCombine/X86/2009-03-23-i80-fp80.ll
@@ -13,7 +13,7 @@
 
 define x86_fp80 @to() {
 ; CHECK-LABEL: @to(
-; CHECK-NEXT:    ret x86_fp80 0xK40018000000000000000
+; CHECK-NEXT:    ret x86_fp80 bitcast (i80 302259125019767858003968 to x86_fp80)
 ;
   %tmp = bitcast i80 302259125019767858003968 to x86_fp80
   ret x86_fp80 %tmp