llvm dev - Dec 2018 - [cfe-dev] Portable multiplication 64 x 64 -> 128 for int128 reimplementation

Hi,

For some maybe dumb reasons I try to write a portable version of int128.

What is very valuable for this implementation is access to MUL instruction
on x86 which provides full 64 x 64 -> 128 bit multiplication. An equally
useful on ARM would be UMULH instruction.

Well, the way you can access this on clang / GCC is to use __int128 type or
use inline assembly. MSVC provides an intrinsic for this instruction. This
defeats the idea of portable int128 reimplementation and makes constexpr
implementation of multiplication at least inconvenient.

Maybe there is a hope for me in LLVM. Is there any pattern matcher that is
producing MUL instruction of bigger type?
If not, would it be good idea to teach LLVM about it?

Bests,
Paweł
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20181230/4aa0ec7a/attachment.html>

Hi Pawel,

There is the _mulx_u64 intrinsic, but it currently requires the hardware
flag "-mbmi2".
https://github.com/Quuxplusone/WideIntProofOfConcept/blob/master/wider.h#L89-L99

On Clang 3.8.1 and earlier, the _addcarry_u64 and _subborrow_u64 intrinsics
required the hardware flag `-madx`, even though they didn't use the
hardware ADX/ADOX instructions. Modern GCC and Clang permit the use of
these intrinsics (to generate ADC) even in the absence of `-madx`.

I think it would be a very good idea for Clang to support _mulx_u64 (to
generate MUL) even in the absence of `-mbmi2`.

–Arthur


On Sat, Dec 29, 2018 at 6:03 PM Paweł Bylica via cfe-dev <
cfe-dev at lists.llvm.org> wrote:
> Hi,
>
> For some maybe dumb reasons I try to write a portable version of int128.
>
> What is very valuable for this implementation is access to MUL instruction
> on x86 which provides full 64 x 64 -> 128 bit multiplication. An equally
> useful on ARM would be UMULH instruction.
>
> Well, the way you can access this on clang / GCC is to use __int128 type
> or use inline assembly. MSVC provides an intrinsic for this instruction.
> This defeats the idea of portable int128 reimplementation and makes
> constexpr implementation of multiplication at least inconvenient.
>
> Maybe there is a hope for me in LLVM. Is there any pattern matcher that is
> producing MUL instruction of bigger type?
> If not, would it be good idea to teach LLVM about it?
>
> Bests,
> Paweł
> _______________________________________________
> cfe-dev mailing list
> cfe-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20181229/a749ca44/attachment.html>

_mulx_u64 only exists when the target is x86_64. That's still not very
portable. I'm not opposed to removing the bmi2 check, but gcc also has the
same check so it doesn't improve portability much.

~Craig


On Sat, Dec 29, 2018 at 4:44 PM Arthur O'Dwyer via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi Pawel,
>
> There is the _mulx_u64 intrinsic, but it currently requires the hardware
> flag "-mbmi2".
>
>
https://github.com/Quuxplusone/WideIntProofOfConcept/blob/master/wider.h#L89-L99
>
> On Clang 3.8.1 and earlier, the _addcarry_u64 and _subborrow_u64
> intrinsics required the hardware flag `-madx`, even though they didn't
use
> the hardware ADX/ADOX instructions. Modern GCC and Clang permit the use of
> these intrinsics (to generate ADC) even in the absence of `-madx`.
>
> I think it would be a very good idea for Clang to support _mulx_u64 (to
> generate MUL) even in the absence of `-mbmi2`.
>
> –Arthur
>
>
> On Sat, Dec 29, 2018 at 6:03 PM Paweł Bylica via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> For some maybe dumb reasons I try to write a portable version of
int128.
>>
>> What is very valuable for this implementation is access to MUL
>> instruction on x86 which provides full 64 x 64 -> 128 bit
multiplication.
>> An equally useful on ARM would be UMULH instruction.
>>
>> Well, the way you can access this on clang / GCC is to use __int128
type
>> or use inline assembly. MSVC provides an intrinsic for this
instruction.
>> This defeats the idea of portable int128 reimplementation and makes
>> constexpr implementation of multiplication at least inconvenient.
>>
>> Maybe there is a hope for me in LLVM. Is there any pattern matcher that
>> is producing MUL instruction of bigger type?
>> If not, would it be good idea to teach LLVM about it?
>>
>> Bests,
>> Paweł
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://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/20181229/fc8f9f4e/attachment.html>