Re: Bug: gcc ignores ebp clobber

On Saturday 15 August 2009 08:41:53 Stefan Hajnoczi wrote:
> With gcc (Debian 4.3.3-14) 4.3.3 and the latest gpxe.git tree:
> ./arch/i386/drivers/net/undiload.c:                            :
> "ebx", "ecx", "edx", "esi", "edi", "ebp" );
> %ebp not live across inline asm.
>
> ./arch/i386/drivers/net/undinet.c:                             :
> "ecx", "edx", "esi", "ebp" );
> %ebp not live across inline asm.
>
> ./arch/i386/interface/pxe/pxe_call.c:                          :
> "esi", "ebp", "memory" );
> %ebp not live across inline asm.
>
> ./arch/i386/firmware/pcbios/bios_console.c:
> : "ebp" );
>
> %ebp not live across inline asm.
>
> ./arch/i386/image/nbi.c:                : "ecx", "edx", "ebp" );
> ./arch/i386/image/nbi.c:                : "ecx", "edx", "ebp", "memory" );
> %ebp not live across inline asm.
>
> ./arch/i386/image/elfboot.c:                           : "eax", "ebx",
> "ecx", "edx", "esi", "ebp",
> %ebp not live across inline asm.
>
> ./arch/i386/image/bootsector.c:                        : "eax", "ecx",
> "esi", "ebp" );
> %ebp not live across inline asm.
>
> We should still fix this since a new version of GCC may come along and
> decide to use %ebp in one of these functions.

Following on from the e-mail I just wrote, talking about using "=B" as an 
alternative to the clobber list:

Most of those are calls to real-mode code.

If we can use "=B" (or whatever the correct constraint is) to mark %ebp as an 
output constraint, then that would be the best solution.

If we *can't* use such an output constraint, then we can never usefully return 
a value via %ebp anyway, so we may as well cause real_call() to preserve %ebp 
automatically, in which case we can just remove %ebp from the clobber list in 
those cases.

Does someone familiar with gcc internals want to comment on what the correct 
output constraint (if any) should be for %ebp?

Michael

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