On Sat, Jan 23, 2010 at 5:47 PM, Steve White <swh...@aip.de> wrote:
> Hi,
>
> I recently revised some speed tests of basic CPU operations.
> There were a few surprises, but one was that, a test of double-precision
> divide was a factor of ten slower when compiled with gcc than with the
> Intel compiler icc.
>
> This was with full optimization turned on, with an Intel Duo (Yonah)
> processor.
>
> I figured gcc was simply not using SSE2, and icc was.
>
> But that is not the case at all. While gcc produces apparently SSE2
> assembler, icc does something quite different.
>
> What's going on?
for( j = 0; j < ITERATIONS; j++ )
for( i = 0; i < size; i++ )
dvec1[i] /= dvec2[i];
it seems that icc performed loop interchange and computes
for (i = 0; i < size; i++)
{
double one_over_dv2 = 1.0/dvec2[i];
for (j = 0; j < ITERATIONS; j++)
dvec1[j] *= one_over_dv2;
}
Richard.
> Find the .c file attached. Assembler snippets follow.
>
> ------------
> gcc has this (gcc -std=c99 -O3 -msse2 -mfpmath=sse -lm -S dt.c)
> ------------
> .L27:
> movapd (%esi,%eax), %xmm3 ;move 2 dbls at *(esi+eax) to xmm3
> divpd 192(%esp,%eax), %xmm3 ;(192 is xmm2) *(esp+eax),
> result->xmm3
> movapd %xmm3, (%esi,%eax) ;move 2 dbls from xmm3 back
> addl $16, %eax ;add 16 (len of 2 doubles) to eax
> cmpl $16384, %eax ;compare eax to 1024 * 16
> jne .L27 ;if not equal, do it again
>
> ------------
> icc has this (icc -Wall -w2 -fast -c dt.c)
> ------------
> # LOE eax xmm2
> ..B1.69: # Preds ..B1.71 ..B1.68
> movsd 8336(%esp,%eax,8), %xmm1 #108.30
> movsd _2il0floatpacket.13, %xmm0 #108.2
> divsd 24720(%esp,%eax,8), %xmm0 #108.2
> unpcklpd %xmm2, %xmm1 #108.30
> xorl %edx, %edx #
> movddup %xmm0, %xmm0 #108.2
> movddup %xmm0, %xmm0 #108.2
> # LOE eax edx xmm0 xmm1 xmm2
> ..B1.70: # Preds ..B1.70 ..B1.69
> mulpd %xmm0, %xmm1 #108.2
> mulpd %xmm0, %xmm1 #108.2
> mulpd %xmm0, %xmm1 #108.2
> mulpd %xmm0, %xmm1 #108.2
> addl $8, %edx #
> cmpl $131072, %edx #108.2
> jb ..B1.70 # Prob 99% #108.2
>
>
> --
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> | Steve White +49(331)7499-202
> | e-Science / AstroGrid-D Zi. 35 Bg. 20
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> | An der Sternwarte 16, D-14482 Potsdam
> |
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> |
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>
