Re: Bug: gcc ignores ebp clobber

On Monday 17 August 2009 10:42:00 Bernhard Kauer wrote:
> > When fixing, we need to be aware that using push/pop within inline asm
> > can itself introduce potential silent bugs, if a potentially stack-based
> > value is used in a non-register constraint occurring after the push
> > instruction.
>
> There are currently no memory constraints in the asm statements that
> clobber ebp. But you have to keep in mind this when changing them or
> writing new ones...

Two thoughts:

1. Is it possible to use an output constraint that specifies %ebp?  I thought 
that we should be able to do something like

  unsigned int discard_ebp;
  __asm__ ( "..." : "=B" ( discard_ebp ) ... );

which provides an alternative way to tell gcc that the asm potentially 
overwrites %ebp, rather than using the clobber list.  However, when I try 
this I get an error message "Impossible constraint in asm".  I'm not sure 
that "B" is the right constraint letter, so it may just be that I have my 
syntax incorrect.

2. Commit 54fbd1 "[build] Keep gcc 4.4 happy" has the log message

    "gcc 4.4 adds another few warnings, and also seems to complain if we
    place %ebp in the clobber list for any inline asm."

Is it possible that gcc4.4 does check correctly for %ebp being live across 
asms?  It definitely *doesn't* complain about %ebp in all clobber lists, as 
the log message suggests, since there are other instances of %ebp in clobber 
lists that gcc4.4 did not complain about.

Michael

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