I'd suspect stdc.math to be SSE3/SSE4 optimised assembly, where as std.math uses a very generic (works on almost every float format) implementation that is at least 'pure'.

Iain.
On 24 Mar 2015 00:30, "weaselcat via Digitalmars-d" <
digitalmars-d@puremagic.com> wrote:

> On Friday, 20 June 2014 at 12:32:39 UTC, Nick Treleaven wrote:
>
>> Hi,
>> A Perlin noise benchmark was quoted in this reddit thread:
>>
>> http://www.reddit.com/r/rust/comments/289enx/c0de517e_ where_is_my_c_replacement/cibn6sr
>>
>> It apparently shows the 3 main D compilers producing slower code than Go, Rust, gcc, clang, Nimrod:
>>
>> https://github.com/nsf/pnoise#readme
>>
>> I initially wondered about std.random, but got this response:
>>
>> "Yeah, but std.random is not used in that benchmark, it just initializes 256 random vectors and permutates 256 sequential integers. What spins in a loop is just plain FP math and array read/writes. I'm sure it can be done faster, maybe D compilers are bad at automatic inlining or something. "
>>
>> Obviously this is only one person's benchmark, but I wondered if people would like to check their code and suggest reasons for the speed deficit.
>>
>
> I saw this thread when searching for something on the site, been a few months since anyone posted-
>
> I fixed the D flags, gdc is now about 15% faster than the second fastest
> in the benchmark(C - gcc) which obviously puts D in first.
> some notes:
>
> LDC is missing _tons_ of inline opportunities, killing it in comparison to GDC. I think GDC inlined pretty much everything. LDC is about 50% slower.
>
> Also, AFAICT there's no fast-math switch for LDC(enabling this for GDC
> might actually be compromising it though : ) )
>
> I think LDC turns the floor in std.math into the same as the stdc one, but GDC does not. std.math.floor is still abysmally slow, I thought it was because it was still using reals but that does not seem to be the case. GDC slows to a crawl(10-20x slower) if you replace the stdc floor with the one in std.math(just remove the alias)
>
> I thought this might be interesting to someone(i.e, LDC/GDC folks or
> phobos math folks)
>
> bye.
>