Date: Fri, 11 May 2001 15:29:09 +0400 (MSD)
From: "Igor Yu. Zhbanov" <bsg AT uniyar DOT ac DOT ru>
To: pgcc AT delorie DOT com
Subject: Assembly code generation bug in pgcc-1.1.3.
Message-ID: <Pine.GSO.3.96.SK.1010511152850.5788B-100000@univ.uniyar.ac.ru>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Reply-To: pgcc AT delorie DOT com

Hello!

I have found a bug in pgcc-1.1.3 compiler.
"pgcc -v" reports:
Reading specs from /usr/lib/gcc-lib/i586-pc-linux-gnu/pgcc-2.91.66/specs
gcc version pgcc-2.91.66 19990314 (egcs-1.1.2 release)

My system is: linux-2.2.19; CPU: AMD-K6-II 450MHz; glibc-2.2

Here is the minimal test example exp.c:
_____________________________________________________
#include<stdio.h>

int multiply_by2(int arg)
{
 int result=arg*2;

/* Uncomment the line below to get correct result. */
/* asm(""); */

 if(result==0) result=arg; /* Why not? :) */
 return result;
}

int main(void)
{
 printf("5 x 2 = %d\n",multiply_by2(5));
 return 0;
}
-----------------------------------------------------
How do you think what is the result of multiply_by2(5)? Ten? No, five!

The compilation command is 'pgcc -O exp.c'. But you may also compile this file
with '-O1' or with '-O2'.

Here is the fragment of assembly output of 'pgcc -O -S exp.c'.

______'asm("");' line is commented_________'asm("");' line is not commented ___
					|
        .file   "exp.c"			|	        .file   "exp.c"
        .version        "01.01"		|	        .version        "01.01"
gcc2_compiled.:				|	gcc2_compiled.:
.text					|	.text
.globl multiply_by2			|	.globl multiply_by2
        .type    multiply_by2,@function	|	.type    multiply_by2,@function
multiply_by2:				|	multiply_by2:
        pushl %ebp			|	        pushl %ebp
        movl %esp,%ebp			|	        movl %esp,%ebp
        movl 8(%ebp),%edx		|	        movl 8(%ebp),%edx
        leal 0(,%edx,2),%eax		|	        leal 0(,%edx,2),%eax
					|	#APP
					|
					|	#NO_APP
					|	        testl %eax,%eax
        jne .L17			|	        jne .L17
        movl %edx,%eax			|	        movl %edx,%eax
.L17:					|	.L17:
        movl %ebp,%esp			|	        movl %ebp,%esp
        popl %ebp			|	        popl %ebp
        ret				|	        ret
------------------------------------------------------------------------------
As you may see assembly text of function 'multiply_by2' in case of pgcc's bug,
instruction 'testl %eax,%eax' is missed and since 'lea' instruction does not
affect to any procesor's flags on Intel platforms, conditional jump 'jne .L17'
can not be executed, so '5 x 2 = 5'. I have used 'asm("");' operator since it
least affects the assembly output.

P.S. I have found this bug during compilation of XFree86-3.3.6. When I have
installed KDE-2.2.1  X server crashes  after start. During debuging of Xserver
with my favourite method (by inserting fprintf()) I have found out that after
inserting fprintf() in one function of libfont.a X server starts normally.
I was surprised. Cutting off all unnecessary I have written that small file.

Thank you.