Index: llvm/trunk/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/trunk/lib/IR/ConstantFold.cpp
+++ llvm/trunk/lib/IR/ConstantFold.cpp
@@ -593,8 +593,13 @@
       bool ignored;
       uint64_t x[2]; 
       uint32_t DestBitWidth = cast<IntegerType>(DestTy)->getBitWidth();
-      (void) V.convertToInteger(x, DestBitWidth, opc==Instruction::FPToSI,
-                                APFloat::rmTowardZero, &ignored);
+      if (APFloat::opInvalidOp ==
+          V.convertToInteger(x, DestBitWidth, opc==Instruction::FPToSI,
+                             APFloat::rmTowardZero, &ignored)) {
+        // Undefined behavior invoked - the destination type can't represent
+        // the input constant.
+        return UndefValue::get(DestTy);
+      }
       APInt Val(DestBitWidth, x);
       return ConstantInt::get(FPC->getContext(), Val);
     }
@@ -653,9 +658,13 @@
       APInt api = CI->getValue();
       APFloat apf(DestTy->getFltSemantics(),
                   APInt::getNullValue(DestTy->getPrimitiveSizeInBits()));
-      (void)apf.convertFromAPInt(api, 
-                                 opc==Instruction::SIToFP,
-                                 APFloat::rmNearestTiesToEven);
+      if (APFloat::opOverflow &
+          apf.convertFromAPInt(api, opc==Instruction::SIToFP,
+                              APFloat::rmNearestTiesToEven)) {
+        // Undefined behavior invoked - the destination type can't represent
+        // the input constant.
+        return UndefValue::get(DestTy);
+      }
       return ConstantFP::get(V->getContext(), apf);
     }
     return nullptr;
Index: llvm/trunk/test/Transforms/InstCombine/cast.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/cast.ll
+++ llvm/trunk/test/Transforms/InstCombine/cast.ll
@@ -1050,3 +1050,37 @@
 ; CHECK: [[SHR:%.*]] = lshr exact i32 [[ADD]], 23
 ; CHECK: [[CST:%.*]] = trunc i32 [[SHR]] to i8
 }
+
+; Overflow on a float to int or int to float conversion is undefined (PR21130).
+
+define i8 @overflow_fptosi() {
+  %i = fptosi double 1.56e+02 to i8
+  ret i8 %i
+; CHECK-LABEL: @overflow_fptosi(
+; CHECK-NEXT: ret i8 undef 
+}
+
+define i8 @overflow_fptoui() {
+  %i = fptoui double 2.56e+02 to i8
+  ret i8 %i
+; CHECK-LABEL: @overflow_fptoui(
+; CHECK-NEXT: ret i8 undef 
+}
+
+; The maximum float is approximately 2 ** 128 which is 3.4E38. 
+; The constant below is 4E38. Use a 130 bit integer to hold that
+; number; 129-bits for the value + 1 bit for the sign.
+define float @overflow_uitofp() {
+  %i = uitofp i130 400000000000000000000000000000000000000 to float
+  ret float %i
+; CHECK-LABEL: @overflow_uitofp(
+; CHECK-NEXT: ret float undef 
+}
+
+define float @overflow_sitofp() {
+  %i = sitofp i130 400000000000000000000000000000000000000 to float
+  ret float %i
+; CHECK-LABEL: @overflow_sitofp(
+; CHECK-NEXT: ret float undef 
+}
+