From: s5muma AT rz DOT uni-jena DOT de (Marc Mueller)
Newsgroups: comp.os.msdos.djgpp
Subject: GCC optimization option "-O3" buggy ?
Date: 3 Mar 1997 20:03:56 GMT
Organization: Friedrich-Schiller-University Jena, Germany
Lines: 124
Message-ID: <5ffarc$l0t@fsuj19.rz.uni-jena.de>
NNTP-Posting-Host: fsuj80.rz.uni-jena.de
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

Hi,

I wrote a small program to test o2c, a converter for TASM-like .OBJ files.

Compiling the C-source with optimization flag "-O3" causes general
protection faults in runtime, otherwise all runs quite right. 

First the source :

--------------------------------------ASMTEST.ASM
(written for TASM, convert .OBJ file using o2c)

        .386
        .MODEL USE32 FLAT, C
        .CODE

        PUBLIC _asmtst   
_asmtst PROC		// should fill a array of int's
        ARG    intarray : PTR DWORD, arraysize : DWORD

        cld
        push   ds
        pop    es
        mov    edi, [intarray]
        mov    ecx, [arraysize]
        xor    eax,eax
@@LP:   stosd
        inc    eax
        loop   @@LP

        ret
_asmtst ENDP
END
----------------------------------------End of ASMTEST.ASM

----------------------------------------ASMTEST.C

#include <stdlib.h>
#include <stdio.h>
#include <malloc.h>

extern void _asmtst( int *intarray, int arraysize );

int main( void )
{
  int *p1, *p0, h, arsize = 15000000;

			// ----- Allocate array
  if ( !( p1 = p0 = malloc( arsize * sizeof(int) ) ) )
    { exit( -1 ); }
			// ----- Fill array asm
  printf( "fill array asm\n" );
  _asmtst( p0, arsize );
			// ----- Fill array C
  printf( "fill array C\n" );
  for ( h=0; ( h<arsize ); )
    *(p1++) = h++;
			// ----- display array
  printf( "output\n" );
  p1 = p0;
  for ( h=0; ( h<arsize ); h++)
  {
    if ( !( h % (arsize / 100) ) )	// show only 100 int's
      printf( "%x\n", *p1 );
    p1++;
  }

}

----------------------------------------End of ASMTEST.C

As described above, compiling it with "-O3" causes "general protection
faults" ("SIGSEGV") in runtime (before printf( "output\n" ); ). Without
the option "-O3" all runs perfectly.

If I delete output of the array, it runs even with "-O3" well.

So the following source works (optimization enabled) :

---------------------------------------ASMTEST2.C

[...]
			// ----- Fill array C
  printf( "fill array c\n" );
  for ( h=0; ( h<arsize ); )
    *(p1++) = h++;

			// ----- display array
  printf( "output\n" );

}
--------------------------------------End of ASMTEST2.C

And the following doesn't work (optimization enabled):

--------------------------------------ASMTEST3.C

[...]
			// ----- Fill array C
  printf( "fill array c\n" );
  for ( h=0; ( h<arsize ); )
    *(p1++) = h++;

			// ----- display array
  printf( "output\n" );
  p1 = p0;

}

------------------------------------End of ASMTEST3.C

So, only a simple "p1 = p0;" more let's occure an protection fault DURING
filling the array.

Is it a bug or my fault ? (I'm using GCC 2.7.2., I used only the options
"-Wall -O3")


TIA,



   Marc