[llvm-dev] Need help in understanding llvm optimization

[SANGEETA CHOWDHARY via llvm-dev]

Yes, with O1. Is there any way to disable constant folding for floating
point computations?

On Sat, Aug 11, 2018, 4:30 PM Craig Topper <craig.topper at gmail.com> wrote:

> The compiler constant folded it as an optimization. Was this compiled with
> optimization’s enabled?
>
> On Sat, Aug 11, 2018 at 1:15 PM SANGEETA CHOWDHARY <
> sc1696 at scarletmail.rutgers.edu> wrote:
>
>> I am sorry to not make my question clear. My question is how this value
>> is calculated without having fadd and fsub in IR?
>>
>> On Sat, Aug 11, 2018 at 4:06 PM, Craig Topper <craig.topper at gmail.com>
>> wrote:
>>
>>> Neither 1e16 or 1e16+1 can be accurately represented in a double. The
>>> largest integer than be accurately represented is 2^53. As the number gets
>>> larger floating sacrifices precision in the lower digits. Because of this
>>> 1e16 and 1e16+1 end up having the same representation. So the result of
>>> subtracting them is 0.0.
>>>
>>> On Sat, Aug 11, 2018 at 12:30 PM SANGEETA CHOWDHARY via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> I have below code in C -
>>>>
>>>> int main() {
>>>>
>>>>   double x,y;
>>>>
>>>>   x = 1e16;
>>>>
>>>>   y = (x + 1) - x;
>>>>
>>>>   printf("y:%e\n", y);
>>>>
>>>>   return 0;
>>>>
>>>> }
>>>>
>>>> llvm bitcode looks like this for this function -
>>>>
>>>>
>>>> ; Function Attrs: nounwind uwtable
>>>>
>>>> define dso_local i32 @main() local_unnamed_addr #0 {
>>>>
>>>> entry:
>>>>
>>>>   %call = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds
>>>> ([4 x i8], [4 x i8]* @.str, i64 0, i64 0), double 0.000000e+00)
>>>>
>>>>   ret i32 0
>>>>
>>>> }
>>>>
>>>> I am not able to understand how addition and subtraction are performed
>>>> in this code. There is no fadd or fsub instruction. How llvm knows that
>>>> result of y is 0?
>>>>
>>>> Is there any way to disable this in llvm?
>>>>
>>>>
>>>> Any help would be much appreciated.
>>>>
>>>>
>>>> Regards,
>>>>
>>>> Sangeeta
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>> --
>>> ~Craig
>>>
>>
>> --
> ~Craig
>
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