Re: optimization/5328: Optimisation causes code ordering to be incorrect on x86

Re: optimization/5328: Optimisation causes code ordering to be incorrect on x86

[jakub]

Synopsis: Optimisation causes code ordering to be incorrect on x86

State-Changed-From-To: open->closed
State-Changed-By: jakub
State-Changed-When: Wed Feb  6 08:05:07 2002
State-Changed-Why:
    This is not a bug. Compiler is free to do it.
    See info gcc on -fstrict-aliasing, either fix that
    code or use -fno-strict-aliasing.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5328

optimization/5328: Optimisation causes code ordering to be incorrect on x86

[iain.templeton]

>Number:         5328
>Category:       optimization
>Synopsis:       Optimisation causes code ordering to be incorrect on x86
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Jan 08 18:56:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Iain Templeton
>Release:        3.0.2
>Organization:
Canon Information Systems Research Australia
>Environment:
System: Linux yves.research.canon.com.au 2.4.2-2 #1 Sun Apr 8 20:41:30 EDT 2001 i686 unknown
Architecture: i686

    PIII-850/Linux 2.4.2/Redhat 7.1

host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ./configure --prefix=/opt/gcc-3.0.2
>Description:

A really simple bit of code shown below comes out with the comparison
instruction and the mov instruction the wrong way around when using -O2.

Using -O2 -fno-schedule-insns2 doesn't show this problem, and neither
does -O1 -fschedule-insns -fschedule-insns2

This problem also effects 2.96 (but we don't talk about that). 2.95.3 (on
FreeBSD though) doesn't do this, rather it does it using some other kind
of optimisation and never outputs the assert test.


Ie:

gcc test.c -S -o test.S -O2					    WRONG
gcc test.c -S -o test.S -O2 -fno-schedule-insns2		    OK
gcc test.c -S -o test.S -O1 -fschedule-insns -fschedule-insns2	    OK

gcc -v gives:
yves% gcc -v -save-temps -O2 -S simpletest.c -o simpletest.S 
Reading specs from /usr/lib/gcc-lib/i386-redhat-linux/2.96/specs
gcc version 2.96 20000731 (Red Hat Linux 7.1 2.96-85)
 /usr/lib/gcc-lib/i386-redhat-linux/2.96/cpp0 -lang-c -v -iprefix /opt/bin/../lib/gcc-lib/i386-redhat-linux/2.96/ -D__GNUC__=2 -D__GNUC_MINOR__=96 -D__GNUC_PATCHLEVEL__=0 -D__ELF__ -Dunix -Dlinux -D__ELF__ -D__unix__ -D__linux__ -D__unix -D__linux -Asystem(posix) -D__OPTIMIZE__ -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -D__tune_i386__ simpletest.c simpletest.i
GNU CPP version 2.96 20000731 (Red Hat Linux 7.1 2.96-85) (cpplib) (i386 Linux/ELF)
ignoring nonexistent directory "/opt/lib/gcc-lib/i386-redhat-linux/2.96/include"
ignoring nonexistent directory "/opt/i386-redhat-linux/include"
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/usr/i386-redhat-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc-lib/i386-redhat-linux/2.96/include
 /usr/include
End of search list.
 /usr/lib/gcc-lib/i386-redhat-linux/2.96/cc1 simpletest.i -quiet -dumpbase simpletest.c -O2 -version -o simpletest.S
GNU C version 2.96 20000731 (Red Hat Linux 7.1 2.96-85) (i386-redhat-linux) compiled by GNU C version 2.96 20000731 (Red Hat Linux 7.1 2.96-85).


The assembly below gives the output, the cmpl is before the movl.


	.file	"simpletest.c"
	.version	"01.01"
gcc2_compiled.:
		.section	.rodata
.LC0:
	.string	"main"
.LC1:
	.string	"simpletest.c"
	.align 32
.LC2:
	.string	"(f1 = -12345.6789F, *(unsigned int *)&f1 == 0xC640E6B7U)"
.text
	.align 4
.globl main
	.type	 main,@function
main:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$8, %esp
	cmpl	$-968825161, -4(%ebp)
	movl	$0xc640e6b7, -4(%ebp)
	je	.L4
	pushl	$.LC0
	pushl	$7
	pushl	$.LC1
	pushl	$.LC2
	call	__assert_fail
	.p2align 2
.L4:
	xorl	%eax, %eax
	leave
	ret
.Lfe1:
	.size	 main,.Lfe1-main
	.ident	"GCC: (GNU) 2.96 20000731 (Red Hat Linux 7.1 2.96-85)"


>How-To-Repeat:

Compile the C code and look at the assembler output.

-- Code Start

extern void __assert_fail (__const char *__assertion, __const char *__file,
                           unsigned int __line, __const char *__function)
             __attribute__ ((__noreturn__));


extern void __assert_perror_fail (int __errnum, __const char *__file,
                                  unsigned int __line,
                                  __const char *__function)
             __attribute__ ((__noreturn__));




extern void __assert (const char *__assertion, const char *__file, int __line)
             __attribute__ ((__noreturn__));

int main(int argc, char **argv)
{
    float f1;

    /* assert((f1 = -12345.6789F, *(unsigned int *)&f1 == 0xC640E6B7U)); */
    ((void) (((f1 = -12345.6789F, *(unsigned int *)&f1 == 0xC640E6B7U)) ? 0 : (__assert_fail ("(f1 = -12345.6789F, *(unsigned int *)&f1 == 0xC640E6B7U)", "simpletest.c", 7, __PRETTY_FUNCTION__), 0)));
    return 0;
}

-- Code End

>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: