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

Craig Topper via cfe-dev cfe-dev at lists.llvm.org
Sat Dec 29 17:34:33 PST 2018

_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.


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
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