------- Additional Comments From dank at kegel dot com 2005-06-18 22:45 -------
I asked the fellow who posted the original problem report to give
me the results of 'cat /proc/cpuinfo' on the affected machine.
Here it is:
vendor_id : GenuineIntel
cpu family : 6
model : 8
model name : Pentium III (Coppermine)
stepping : 10
cpu MHz : 896.153
This is the same as one of the two affected CPU types here.
The slow routine appears to be the buffer cleaning routine,
though I haven't verified this with oprofile yet.
Here's its loop:
static char cleanse_ctr;
...
while (len--) {
*(ptr++) = cleanse_ctr;
cleanse_ctr += (17 + (unsigned char) ((int) ptr & 0xF));
}
and the output of -O3 -fPIC for both gcc-2.95.3 and gcc-4.0.0:
--- gcc-2.95.3 ---
.L5:
movl [EMAIL PROTECTED](%ebx),%edi
movb (%edi),%al
movb %al,(%edx)
incl %edx
movb (%edi),%cl
addb $17,%cl
movb %dl,%al
andb $15,%al
addb %al,%cl
movb %cl,(%edi)
subl $1,%esi
jnc .L5
.L4:
--- gcc-4 ---
.L4:
movb (%esi), %al
movb %al, (%edx)
leal (%ecx,%edi), %eax
andl $15, %eax
incl %ecx
addb (%esi), %al
incl %edx
addl $17, %eax
cmpl %ecx, 12(%ebp)
movb %al, (%esi)
jne .L4
It's not obvious to me why the gcc-4.0.0 generated code
should be slower when run on some CPUs, if in fact it is.
Is it the fact that the loop condition is checked with
a cmp against memory instead of a flag being set by subtracting
1 from a register?
(And where's the best place to learn about how to predict
how long assembly snippets like this will take to run
on various modern CPUs, anyway?)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19923
