GCC vectorization problem on X86

Robert Bernecky bernecky@snakeisland.com
Thu Jun 3 19:31:00 GMT 2010


Hi. I'm having a problem with GCC vectorization on an Opteron 165.

I have two codes, which are, unfortunately, machine-generated
and large, which differ, as far as I tell, only in the source
of the loop size, N, for a loop roughly of this form:

  for( i=0; i<N; i++) {
    vec[i] = i;
   }

In both cases, N comes from another function and is theoretically
not inlined. In the first case, N is generated by an identity
function that hides its value; this case vectorizes nicely,
if the presence of punpckldq instructions is suitable evidence.
(papiex confirms vectorization with high PAPI_VEC_INS counts.)

In the other case, N comes from a sscanf, and is very well hidden,
since it comes from the command line, ultimately. This case
does not vectorize, at present. It did vectorize some months ago...

This is on:  gcc version 4.3.2 (Ubuntu 4.3.2-1ubuntu12)
Neither the compiler nor the OS have changed in that time; the
code going into gcc has, of course, changed as the sac2c compiler
has evolved.

So, are there some subtle (or not subtle...) criteria that gcc has
for deciding when to emit vector ops, based on array size, perhaps?

Alternately, if someone can point me at the relevant gcc source code,
maybe I can get an idea as to what's going on. Or, if there is
a bugzilla site for it, I'll take a look there.

Thanks,
Robert









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