Re: optimization/6988: __asm__ __volatile__ disappears

Re: optimization/6988: __asm__ __volatile__ disappears

[rth]

Synopsis: __asm__ __volatile__ disappears

State-Changed-From-To: open->closed
State-Changed-By: rth
State-Changed-When: Fri Jun 14 16:21:48 2002
State-Changed-Why:
    Not a bug.  Re-read the bit in the manual
    concerning unreachable code.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6988

optimization/6988: __asm__ __volatile__ disappears

[savage]

>Number:         6988
>Category:       optimization
>Synopsis:       __asm__ __volatile__ disappears
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 10 19:06:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Jens Troeger
>Release:        gcc 2.96-85 (RedHat 7.1)
>Organization:
>Environment:
Linux i686, Kernel 2.4.9-31custom (RedHat)
>Description:
this particular version of (RH-homemade) gcc simply
"optimizes" away __asm__ __volatile__ instructions,
and emits code that is unable to handle &&Llabel 
arithmetic. further, the inline assembly is moved around to a wrong place. compile the code with 

  gcc -S -g3 -O0 test.c

and you will see. i attach both, an example c file,
as well as the emitted asm file.
>How-To-Repeat:
------------------ test.c

int main(int argc, char* argv[]) {
 
        int len = &&Lend - &&Lstart;
        return 0;
 
Lstart:
        __asm__ __volatile__ ("nop; nop");
Lend:
        (void)0;
}

---------------------- test.s
see .L3 and .L4 

main:
        .stabn 68,0,2,.LM1-main
.LM1:
.LBB2:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $4, %esp
        .stabn 68,0,4,.LM2-main
.LM2:
        movl    $.L3, %eax
        subl    $.L4, %eax
        movl    %eax, -4(%ebp)
        .stabn 68,0,5,.LM3-main
.LM3:
        movl    $0, %eax
.L4:
.L3:
        .stabn 68,0,11,.LM4-main
.LM4:
.LBE2:
        leave
        ret
>Fix:

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