Bug in egcs1.1.1pre on i686/Linux with loop unrolling.

[Hans Dolfing]

Hi,

Just found an interesting bug: The combination of
-march=pentiumpro and loop unrolling produces
some bogus results while -march=pentium is fine.

Testcase + details + example output is included.

Best regards,

  Hans

Testcase: ----------------------------------------------------

#include <stdio.h>

int main(void)
{
  int i;

  for (i=0; i<16; i++)
  {
     int j = i/4;

     fprintf(stderr,"i=%d,j=%d,i/4=%d\n",i,j,i/4);
  }
  return 0;
}


Details ---------------------------------------------------

egcs version is :

Reading specs from
/usr/local/lib/gcc-lib/i686-pc-linux-gnulibc1/egcs-2.91.58/specs
gcc version egcs-2.91.58 19981101 (egcs-1.1.1 pre-release)

Compiler built from egcs-1.0.3a sources with [gcc/g++/g77] diffs
to egcs-1.1b and diffs to egcs-1.1.1 prerelease.

config.guess: i686-pc-linux-gnulibc1

Configured with --enabled-haifa --enable-shared.

Here is the story (and a testcase):

1. The following versions are ok:

gcc -o t -O2  t.c -lm 
gcc -o t -O2 -march=pentium  -funroll-all-loops t.c -lm
gcc -o t -O2 -march=pentium -freduce-all-givs    t.c -lm 

2. These compile jobs give completely bogus results (see appended
   output examples).

gcc -o t -O2 -march=pentiumpro -funroll-all-loops   t.c -lm 
gcc -o t -O2 -march=pentiumpro -funroll-loops       t.c -lm 
gcc -o t -O2 -march=pentiumpro -freduce-all-givs    t.c -lm 

Note: I don't know if it is related but there was
      a bootstrapping problem. In the file ./objdir/gcc/auto-host.h,
      the text 'TOP' was somehow inluced (which is not valid
      ANSI-C). Manual removing of only these 3 characters allows
      the bootstrapping to proceed. I just mention this for
      the sake of completeness because there is always a chance
      that one problem relates to another :(


Output examples:

----------------- 1 -----------------
Compile : gcc -o t -O2 t.c -lm
Run     : t
Output  : ( OK )

i=0,j=0,i/4=0
i=1,j=0,i/4=0
i=2,j=0,i/4=0
i=3,j=0,i/4=0
i=4,j=1,i/4=1
i=5,j=1,i/4=1
i=6,j=1,i/4=1
i=7,j=1,i/4=1
i=8,j=2,i/4=2
i=9,j=2,i/4=2
i=10,j=2,i/4=2
i=11,j=2,i/4=2
i=12,j=3,i/4=3
i=13,j=3,i/4=3
i=14,j=3,i/4=3
i=15,j=3,i/4=3

----------------- 2 -----------------

Compile : gcc -o t -O2 -march=pentiumpro -funroll-loops   t.c -lm 
Run     : t
Output  : (Bogus)

i=0,j=0,i/4=0
i=1,j=1,i/4=1
i=2,j=0,i/4=0
i=3,j=0,i/4=0
i=4,j=1,i/4=1
i=5,j=2,i/4=2
i=6,j=1,i/4=1
i=7,j=1,i/4=1
i=8,j=2,i/4=2
i=9,j=3,i/4=3
i=10,j=2,i/4=2
i=11,j=2,i/4=2
i=12,j=3,i/4=3
i=13,j=4,i/4=4
i=14,j=3,i/4=3
i=15,j=3,i/4=3

----------------- 3 -----------------

Compile : gcc -o t -O2 -march=pentiumpro -freduce-all-givs   t.c -lm 
Run     : t
Output  :

i=0,j=0,i/4=0
i=1,j=1,i/4=1
i=2,j=1,i/4=1
i=3,j=1,i/4=1
i=4,j=1,i/4=1
i=5,j=2,i/4=2
i=6,j=2,i/4=2
i=7,j=2,i/4=2
i=8,j=2,i/4=2
i=9,j=3,i/4=3
i=10,j=3,i/4=3
i=11,j=3,i/4=3
i=12,j=3,i/4=3
i=13,j=4,i/4=4
i=14,j=4,i/4=4
i=15,j=4,i/4=4