[llvm-dev] on division of __int128 bit integer

[Vivek Pandey via llvm-dev]

Dear Paweł,

Thanks a lot! The solution helped!

Thank you!
Regards,
Vivek JP Pandey,
Software Professional,
*+91-7775054441*


On Thu, May 21, 2020 at 7:08 PM Paweł Bylica <chfast at gmail.com> wrote:

> Hi,
>
> First of all, the code in compiler-rt/builtins is rather confusing for me.
> No idea what `tu_int` is but I'm guessing this must be `unsigned __int128`,
> not `unsigned long long`.
>
> https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/builtins/udivmodti4.c#L22
>
> Secondly the "slow" case proceeds single bit at a time, so this loop may
> need ~64 iterations in worst case:
>
> https://github.com/llvm/llvm-project/blob/master/compiler-rt/lib/builtins/udivmodti4.c#L167-L188
>
> From measurements, it looks GCC has it implemented more efficiently
> although I have no idea where do they have the code for it.
>
> If you are interested, I have division implemented for unsigned 128-bit
> integer here:
> https://github.com/chfast/intx/blob/master/include/intx/int128.hpp#L632-L687
> From quick check it takes 16 ns, GCC runtime lib: 25 ns, GMP: 20 ns.
>
> It follows "Improved division by invariant integers"
> https://gmplib.org/~tege/division-paper.pdf
>
> If make sense I can contribute this implementation as replacement for
> `__udivmodti4`. Although I'm not sure what requirements for __udivmodti4
> are.
>
> --
> Paweł
>
>
> On Thu, May 21, 2020 at 11:30 AM Vivek Pandey via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi Team,
>>
>> I observer that division of __int128 bit is very heavy operation.
>> It internally call a routine '__udivti3', which internally call '
>> __udivmodti4'.
>>
>> Due to it the overall performance is much much slower (almost 15 time
>> slower than if I do it via a combination of 64-bit or microsoft '_udiv128
>> ').
>>
>> Also what to know if I can directly call below routine directly from my
>> code :
>> *unsigned long long* *__udivmodti4* *(unsigned long long *a*, unsigned
>> long long *b*, unsigned long long **c*)*
>>
>> This is against statement  "Conceptually,in operation, like ‘/’ or ‘%’,
>> no function is called, so no header is provided for *__udivmodti4* etc.
>> – and we should not call them explicitly in our code." but wanted to
>> know more on it.
>>
>> Thank you!
>> Regards,
>> Vivek JP Pandey,
>> Software Professional,
>> *+91-7775054441*
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200603/8d9702e2/attachment.html>