Re: [multiprecision] Getting the best performance on windows for uint128_t etc

From: John Maddock (jz.maddock_at_[hidden])
Date: 2019-07-12 10:21:30

On 12/07/2019 09:12, degski via Boost-users wrote:
> On Fri, 12 Jul 2019 at 10:38, John Maddock via Boost-users 
> <boost-users_at_[hidden] <mailto:boost-users_at_[hidden]>> wrote:
>
>     Aside: adding a native population count function might well be a
>     useful
>     addition to the library...
>
>
> Native clz/ctz (_BitScanReverse/_BitScanForward) might be useful as well.

That's done as msb/lsb (most/least significant bit).

>     The limb size is always half the width of the largest compiler
>     supported
>     integer type - this is a necessary condition to implement arithmetic
>     entirely within the language.
>
>
> This seems a high price to pay [I guess the 'within the language' is 
> key here]. At some point in time [when 64-bit started to be a general 
> thing] there was some info regarding this on gmplib.org 
> <http://gmplib.org>, which basically stated that 64-bit limbs [on x64] 
> were 4 times faster than 32-bit limbs. GMP (MPIR) does 64-bit limbs on 
> Windows, no problem [but yes it's C].

That's only possible with assembly level add/multiply/divide.

Ah OK, just looked at MPIR on win64 with the mpir_gc backend, and they 
do use 64-bit limbs, but rely on GNU's longlong.h for 
add/subtract/multiply which appears not to use inline assembly for msvc, 
but instead breaks the inputs into high/low parts and then does 
schoolbook multplication.  I would expect that to be quite a bit slower 
than just using 32-bit limbs in the first place, but no doubt you would 
make gains on the bitwise operations, likewise if you're using an 
assembly level backend.

Best, John.

>
>     If clang-win can have __int128 support enabled
>
>
> clang-cl can/will have __int128 support enabled by passing it '-Xclang 
> -fforce-enable-int128', i.e. pass '-fforce-enable-int128' to clang++.
>
> degski
> -- 
> @realdegski
> https://edition.cnn.com/interactive/2019/06/middleeast/saudi-teen-death-penalty-intl/
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> finite world is either a madman or an economist" - Kenneth E. Boulding
>
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