Re: [eigen] Non-optimal sse assembly code with gcc

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

On 22.01.2012 17:32, Benjamin Schindler wrote:
> Hi
>
> I just had a close look at the assembly generated by the following
> function:
>
> bool particleCheckSpheric(Eigen::AlignedVector3<float> pos1,
> Eigen::AlignedVector3<float> pos2, float particleSize)
> {
> return particleSize*particleSize > (pos1-pos2).squaredNorm();
> }
>
> The assembly I got is the following (compiled on an amd64 machine using
> gcc 4.5.3, with -O2 -DNDEBUG):
>
> 01: movaps (%rdi), %xmm1
> 03: mulss %xmm0, %xmm0
> 04: subps (%rsi), %xmm1
> 05: mulps %xmm1, %xmm1
> 06: movaps %xmm1, %xmm2
> 07: movhlps %xmm1, %xmm2
> 08: addps %xmm1, %xmm2
> 09: movaps %xmm2, %xmm1
> 10: shufps $0x1, %xmm2, %xmm1
> 11: addss %xmm1, %xmm2
> 12: ucomiss %xmm2, %xmm0
> 13: seta %al
> 14: retq
>
> Notice line 6 (and 9): It seems to me that these copies are unnecessary
> as only the low quadword is really used. Is this a problem of the
> compiler is this an eigen issue?

Side-note: I guess, if you activate SSE3 line 6 to 11 will be replaced 
by just two haddps %xmm1, %xmm1


And I think gcc does everything correct, as the Eigen source (w/o SSE3) 
says:

template<> EIGEN_STRONG_INLINE float predux<Packet4f>(const Packet4f& a)
{
  Packet4f tmp = _mm_add_ps(a, _mm_movehl_ps(a,a));
  return pfirst(_mm_add_ss(tmp, _mm_shuffle_ps(tmp,tmp, 1)));
}

So _mm_movehl_ps(a,a) actually requires that the upper half is copied 
from a (i.e. xmm1). And I guess it can make a difference, because if the 
upper half of xmm2 happens to contain a denormalized number, the addps 
instruction might be slower on some hardware (not sure about that, though).

Christoph


--
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Dipl.-Inf. Christoph Hertzberg
Cartesium 0.051
Universität Bremen
Enrique-Schmidt-Straße 5
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