Phil,
On Tue, May 23, 2006 at 11:43:08AM +0200, Philip Mucci wrote:
> Hi Stephane,
>
> It sure can...in a number of ways...But I believe the SSE/SSE2 counting
> isn't as accurate as one might like...It was the Athlon which AMD blew
> it on...no FP counter!
>
> However, you have 3 choices.
> PNE_OPT_FP_ADD_PIPE
> PNE_OPT_FP_MULT_PIPE,
> PNE_OPT_FP_MULT_AND_ADD_PIPE,
>
> Event 0x100, 0x200 and 0x300
Well, there are things I don't get understand here. Let's take
this simple program:
#include <sys/types.h>
#include <stdio.h>
main(int argc, char **argv)
{
unsigned long i, n;
float f=4;
n = strtoul(argv[1], NULL, 0);
for(i=0; i < n; i++) {
f+=1.9;
}
printf("f=%g\n", f);
}
Compiled with: cc float.c -o float -O3 -mtune=opteron -mcpu=opteron
The loop generates the following code:
400530: cvtss2sd (%rsp),%xmm0
400535: dec %rax
400538: addsd %xmm1,%xmm0
40053c: cvtsd2ss %xmm0,%xmm2
400540: movss %xmm2,(%rsp)
400545: jne 400530 <main+0x30>
With pfmon, I do 100,000,000 iterations:
$ pfmon --trigger-code-start=main --trigger-code-stop=main --us-c -u -e
cpu_clk_unhalted,retired_instructions,DISPATCHED_FPU_OPS_ADD,DISPATCHED_FPU_OPS_MULTIPLY
float 100000000
2,308,816,413 CPU_CLK_UNHALTED
600,006,705 RETIRED_INSTRUCTIONS
150,002,866 DISPATCHED_FPU_OPS_ADD
50,000,979 DISPATCHED_FPU_OPS_MULTIPLY
I don't understand where those MULTIPLY come from. There are also 50,000,000
additions extra.
In constrast, if I use double (instead of float) and compile the same way. I
get the following code:
400530: movlpd (%rsp),%xmm1
400535: dec %rax
400538: addsd %xmm0,%xmm1
40053c: movsd %xmm1,(%rsp)
400541: jne 400530 <main+0x30>
And pfmon yields:
1,163,126,715 CPU_CLK_UNHALTED
500,005,961 RETIRED_INSTRUCTIONS
100,001,398 DISPATCHED_FPU_OPS_ADD
4 DISPATCHED_FPU_OPS_MULTIPLY
As such, I am inclined to believe that the cvt instructions are the cause of
this extra "noise". It may
be coming from the way they are actually implemented.
It seems difficult to compute FLOPS on Opteron. I do not quite understand the
PIPE versions of those
events.
Any clue?
--
-Stephane
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