Re: Bug: gcc ignores ebp clobber

Thanks for pointing this out!

Summary of the bug:
GCC does not cope with the frame pointer being clobbered by inline
asm.  It produces no error during compilation but makes no effort to
preserve the frame pointer as the user has indicated in the clobber
list.

>From the bug report:
"If I compile the test case from that bug with -fomit-frame-pointer,
gcc still generates invalid code. (We
don't use frame pointers in our system at all)."

gPXE builds with -fomit-frame-pointer.  This actually reduces the
chance of this bug being noticed since %ebp may not be live.  When
used as a frame pointer you *will* get hit by this bug :).  In fact,
when I look at the emitted code, I do not see a single instance where
broken code is being produced.

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.

Stefan

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