Effectiveness of SSE on Athlon XP with gcc 3.2

[Andres Chavarria]

Hi,

I was wondering how effective the parallelization using SSE is, and I wrote 
a test program that sums the values of a vector up. The SSE free code for 
looks like this:

// non SSE, numbers is an array of size cNumber
  float sum1,sum2,sum3,sum4;
  float val;

  sum1=0.0f; sum2=0.0f;sum3=0.0f; sum4=0.0f;
  for(i=0;i<cNumber;i+=4){
    sum1+=*(numbers++);
    sum2+=*(numbers++);
    sum3+=*(numbers++);
    sum4+=*(numbers++);
  }
  val=(sum1+sum2)+(sum1+sum2);


And the one using SSE looks like this:

// with SSE, numbers is an array of size cNumber used 
__attrib__((aligned(16))) to aligne it
  register v4sf val1,val2;
  float __attrib__((aligned(16))) momVal[4];
  float val;

  val1=__builtin_ia32_loadups(numbers);

  for(i=4;i<cNumber;i+=4){
    val2=__builtin_ia32_loadaps(numbers+i);
    val1=__builtin_ia32_addps(val2,val1);
  }

  __builtin_ia32_storeups(momVal,val1);

  val=(momVal[0]+momVal[1])+(momVal[2]+momVal[3]);

I used gcc v. 3.2 on a linux machine with kernel 2.4.19 (distribution SusE 
8.1) an compiled the following way:

gcc -msse -march=athlon-xp -o ssetest main.cpp -lstdc++


If I compare the calculation time of the first code with the second one I 
get on an Athlon XP 2200+ (actually 1800 Mhz) only a speedup of about (array 
size 307200):

calc. time without SSE: 2.72 sec
calc. time with SSE:	2.05 sec
speedup factor 2.72/2.05 = 1.33

Theoretically the speedup factor should be somewhat smaller than 4. So I'm 
quiet dissapointed... The first time I compiled and ran a similar program I 
did it on a Pentium 4 mobility (1800 Mhz) processor (options -msse 
-march=pentium4). I got about a speedup factor of 3.9, which was superb.

I don't think it is because of the processor (Athlon XP wouldn't have a 
chance against Pentium 4, and a report of such a difference should be 
present somewhere in the net...) I suspect the SSE code I wrote is far from 
good or that gcc does something wrong for the Athlon XP.

I don't have access to the Pentium 4 machine so I cant compare the above 
code directly anymore. It would be great I someone could test it on a 
Pentium 4 or if someone has a hint what is going on.

Thanx in advance,

Andres



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