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Return undef on FP <-> Int conversions that overflow (PR21330).

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The LLVM Lang Ref states for signed/unsigned int to float conversions:
"If the value cannot fit in the floating point value, the results are undefined."

And for FP to signed/unsigned int:
"If the value cannot fit in ty2, the results are undefined."

This matches the C definitions.

The existing behavior pins to infinity or a max int value, but that may just
lead to more confusion as seen in:
http://llvm.org/bugs/show_bug.cgi?id=21130

Returning undef will hopefully lead to a less silent failure.

Differential Revision: http://reviews.llvm.org/D5603



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219542 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2014-10-10 23:00:21 +00:00
parent fab0a999d7
commit add74ff5ff
2 changed files with 48 additions and 5 deletions
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19
lib/IR/ConstantFold.cpp
View File

@ -593,8 +593,13 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
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 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
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;

34
test/Transforms/InstCombine/cast.ll
View File

@ -1050,3 +1050,37 @@ define i8 @test85(i32 %a) {
; 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
}