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

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

For the record, MSVC has compiler intrinsics for double-wide arithmetics, both signed and unsigned. For example: https://docs.microsoft.com/en-us/cpp/intrinsics/mul128?view=vs-2019 -- Stian ________________________________ From: Boost-users on behalf of John Maddock via Boost-users Sent: Friday, July 12, 2019 12:21:30 PM To: degski via Boost-users Cc: John Maddock Subject: Re: [Boost-users] [multiprecision] Getting the best performance on windows for uint128_t etc On 12/07/2019 09:12, degski via Boost-users wrote:

On Fri, 12 Jul 2019 at 10:38, John Maddock via Boost-users mailto:boost-users@lists.boost.org> 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-pena... "Anyone who believes that exponential growth can go on forever in a finite world is either a madman or an economist" - Kenneth E. Boulding

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