[Bug inline-asm/36048] New: wrong assumption about registers used in asm statements

[jlima at sim dot ul dot pt]

The code snippet below compiles correctly without optimization, but with the
optimization flag turned on (at least with -O2) the compiler wrongly assumes
that the register %edx was not modified by the inline assembly statement.
The bug can be reproduced by issuing the following command.

To reproduce the bug, compile the code snippet bellow with the command

gcc -S -O2 file.c

and look the file file.s in the lines between the to #APP sections.
See bellow, after the source file, the assembly code produced with and without
optimization.

/********* file.c *************************************/

#define rsmpl8b(addr, d1, d2) __asm__ __volatile__(\
                "outb   %%al, %%dx\n\t" \
                "movb   %%cl, %%al\n\t" \
                "addw   $2, %%dx\n\t" \
                "outb   %%al, %%dx\n\t" \
                : : "a" (d1), "c" (d2), "d" (addr))

void teste()
{
        int i;
        unsigned short base_addr = 0x378;
        for (i = 0; i < 0x1000; i++) {
                rsmpl8b(base_addr, 0xFF, 0xFE);
                rsmpl8b(base_addr, 0xFD, 0xFC);
        }
}


/*************** file.s compiled with gcc -S -O2 file.c *****/

        .file   "file.c"
        .text
.globl teste
        .type   teste, @function
teste:
        pushl   %ebp
        movl    $888, %edx
        movl    %esp, %ebp
        pushl   %edi
        movl    $255, %edi
        pushl   %esi
        movl    $254, %esi
        pushl   %ebx
        movl    $253, %ebx
        subl    $4, %esp
        movl    $0, -16(%ebp)
        .p2align 4,,15
.L2:
        movl    %edi, %eax
        movl    %esi, %ecx
#APP
        outb    %al, %dx
        movb    %cl, %al
        addw    $2, %dx
        outb    %al, %dx

#NO_APP
        movl    $252, %ecx
        movl    %ebx, %eax
#APP
        outb    %al, %dx
        movb    %cl, %al
        addw    $2, %dx
        outb    %al, %dx

#NO_APP
        incl    -16(%ebp)
        cmpl    $4096, -16(%ebp)
        jne     .L2
        popl    %eax
        popl    %ebx
        popl    %esi
        popl    %edi
        popl    %ebp
        ret
        .size   teste, .-teste
        .ident  "GCC: (GNU) 4.1.2 20070214 (  (gdc 0.24, using dmd
1.020)(Gentoo 4.1.2p1.0.2)"
        .section        .note.GNU-stack,"",@progbits

/*************** file.s compiled with gcc -S file.c *********/

        .file   "file.c"
        .text
.globl teste
        .type   teste, @function
teste:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $16, %esp
        movw    $888, -2(%ebp)
        movl    $0, -8(%ebp)
        jmp     .L2
.L3:
        movl    $255, %eax
        movl    $254, %ecx
        movzwl  -2(%ebp), %edx
#APP
        outb    %al, %dx
        movb    %cl, %al
        addw    $2, %dx
        outb    %al, %dx

#NO_APP
        movl    $253, %eax
        movl    $252, %ecx
        movzwl  -2(%ebp), %edx
#APP
        outb    %al, %dx
        movb    %cl, %al
        addw    $2, %dx
        outb    %al, %dx

#NO_APP
        incl    -8(%ebp)
.L2:
        cmpl    $4095, -8(%ebp)
        jle     .L3
        leave
        ret
        .size   teste, .-teste
        .ident  "GCC: (GNU) 4.1.2 20070214 ((gdc 0.24, using dmd 1.020))
(Gentoo 4.1.2p1.0.2)"
        .section        .note.GNU-stack,"",@progbits


-- 
           Summary: wrong assumption about registers used in asm statements
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: inline-asm
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jlima at sim dot ul dot pt
  GCC host triplet: i386-linux-gnu
GCC target triplet: i386-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36048