On Monday, 19 July 2021 at 16:44:35 UTC, Guillaume Piolat wrote:
> On Monday, 19 July 2021 at 10:49:56 UTC, kinke wrote:
> This workaround is actually missing the clobber constraint for %2, which might be problematic after inlining.
An unrelated other issue with asm/__asm is that it doesn't follow consistent VEX encoding compared to normal compiler output.
sometimes you might want: paddq x, y
at other times: vpaddq x, y, z
but rarely both in the same program.
So this can easily nullify any gain obtained with VEX transition costs (if they are still a thing).
You know that asm is to be avoided whenever possible, but unfortunately, AFAIK intel-intrinsics doesn't fit the usual 'don't worry, simply compile all your code with an appropriate -mattr/-mcpu option' recommendation, as it employs runtime detection of available CPU instructions.
I've just tried another option, but that doesn't play nice with inlining:
import core.simd;
import ldc.attributes;
@target("sse2") // use SSE2 for this function
int4 _mm_add_int4(int4 a, int4 b)
{
return a + b; // perfect: paddd %xmm1, %xmm0
}
int4 wrapper(int4 a, int4 b)
{
return _mm_add_int4(a, b);
}
Compiling with -O -mtriple=i686-linux-gnu -mcpu=i686 (=> no SSE2 by default) shows that the inlined version inside wrapper() is the mega slow one, so the extra instructions aren't applied transitively unfortunately.
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