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

[Arthur O'Dwyer via cfe-dev]

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
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