On 05/01/2010 09:58 PM, Brian Budge wrote:
Unless you modify your algorithm to perform many dot products
simultaneously, you're probably at your limit. Depending on what
you're trying to do, you might get close to a 4x speedup on the dot
products (conversely, you might not be able to do any better)
hi Brian
may I ask you what did you mean by "many dot products simultaneously"?
from my reading, there are only limited number of xmm registers,
in order to do a dot product, I need to at least do
_mm_loadu_ps(p1);
_mm_loadu_ps(p2);
_mm_dp_ps(p1,p2);
_mm_store_ss(result1);
if I do many dot products, did you mean by repeating the above
pseudo-code for different vectors? or try to run
as many mm_dp_ps as possible for a fixed set of vectors?
In the inner loop of my code, I need to perform 12 inner
products in a row using 14 vectors. Will the overhead for
loading/storing kill the speed-up?
thanks
Qianqian
Brian
On Fri, Apr 30, 2010 at 8:35 PM, Qianqian Fang<fangqq@xxxxxxxxx> wrote:
hi Marc
On 04/30/2010 06:31 AM, Marc Glisse wrote:
On Thu, 29 Apr 2010, Qianqian Fang wrote:
Shouldn't there be some magic here for alignment purposes?
thank you for pointing this out. I changed the definition to
typedef struct CPU_float4{
float x,y,z,w;
} float4 __attribute__ ((aligned(16)));
but the run-time using SSE3 remains the same.
Is my above change correct?
now I am trying to use SSE4.x DPPS, but gcc gave me
error. I don't know if I used it with a wrong format.
Did you try using the intrinsic _mm_dp_ps?
yes, I removed the asm and use mm_dp_ps, it works now.
the code now looks like this:
inline float vec_dot(float3 *a,float3 *b){
float dot;
__m128 na,nb,res;
na=_mm_loadu_ps((float*)a);
nb=_mm_loadu_ps((float*)b);
res=_mm_dp_ps(na,nb,0x7f);
_mm_store_ss(&dot,res);
return dot;
}
sadly, using SSE4 only gave me a few percent (2~5%)
speed-up over the original C code. My profiling result
indicated the inner product took about 30% of my total
run time. Does this speedup make sense?
"dpps %%xmm0, %%xmm1, 0xF1 \n\t"
Maybe the order of the arguments is reversed in asm and it likes a $
before a constant (and it prefers fewer parentheses on the next line).
with gcc -S, I can see that the assembly is in fact
dpps 127, xmm1, xmm0, so perhaps it was reversed
in my previous version.
In any case, you shouldn't get a factor 2 compared to the SSE3 version, so
that won't be enough for you.
well, as I mentioned earlier, using SSE3 made my code 2.5x slower, not
faster.
SSE4 is now 2~5% faster, but still not as significant as I thought.
I guess that's probably the best I can do with it. Right?
thanks
Qianqian
