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Date: Mon, 22 Apr 2002 14:33:00 -0000
From: Michel LESPINASSE <walken@zoy.org>
To: gcc list <gcc@gcc.gnu.org>
Subject: GCC performance regression - its memset !
Message-ID: <20020422213222.GA21429@zoy.org>
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OK, so I worked more to find the cause of the slowdown, and I figured
out its all because of memset(). This function seems to be about twice
slower than in 2.95, and also for some reason the time spent in memset
does not show up in gprof.

Here is a test case:

--------------------------- foo.c ------------------------------
#include <string.h>

short table[64];

void bar (void);

int main (void)
{
    int i;

    bar ();

    for (i = 0; i < 100000000; i++)
        memset (table + 1, 0, 63 * sizeof(short));
    return 0;
}
----------------------------- end of foo.c ------------------------

----------------------------- bar.c -------------------------------
void bar (void)
{
}
----------------------------- end of bar.c ------------------------

# gcc-2.95 -g -O3 -p foo.c bar.c
# time ./a.out
./a.out  5.75s user 0.00s system 100% cpu 5.739 total
# gprof -bp ./a.out
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total           
 time   seconds   seconds    calls  Ts/call  Ts/call  name    
100.00      5.74     5.74                             main
  0.00      5.74     0.00        1     0.00     0.00  bar


# gcc-3.1 -g -O3 -p foo.c bar.c
# time ./a.out
./a.out  10.78s user 0.00s system 101% cpu 10.634 total
# gprof -bp ./a.out
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total           
 time   seconds   seconds    calls  Ts/call  Ts/call  name    
100.00      0.62     0.62                             main
  0.00      0.62     0.00        1     0.00     0.00  bar


gcc-3.1 snapshot is about twice slower than 2.95 on that test case,
and for some reason the gprof output is bogus (it does not account for
the time spent in memset), while it was not with 2.95.

I did not know my code spent that much time in memset, I'll see what I
can do about it.

Hope this helps,

-- 
Michel "Walken" LESPINASSE
Is this the best that god can do ? Then I'm not impressed.