[tree-ssa] RFC: Never gimplify ASM_EXPRs

[Andrew Pinski]

What about using the prefetch builtin?
That would solve the problems right?

Thanks,
Andrew Pinski


On Thursday, Jun 5, 2003, at 10:16 US/Eastern, Diego Novillo wrote:

>
> Jason,
>
> Andrew Haley found widespread breakage in libjava due to a
> miscompilation of boehm-gc.
>
>
> ----------------------------------------------------------------------- 
> ---------
>         From: Andrew Haley <aph@redhat.com>
>         Date: Fri, 16 May 2003 18:39:01 +0100
>
>         Consider this code.
>
>         void fubar (void *x)
>         {
>           __asm__ __volatile__ ("       prefetch        %0": :  
> "m"(*(char *)(x)));
>         }
>
>         With gcc 3.3 you get
>
>         fubar:
>         #APP
>                        prefetch        (%rdi)
>         #NO_APP
>                 ret
>
>         No problem there, everything is fine.
>
>         With tree-ssa you get
>
>         fubar:
>                 movzbl  (%rdi), %edx
>                 movb    %dl, -1(%rsp)
>         #APP
>                        prefetch        -1(%rsp)
>         #NO_APP
>                 ret
>
>         and this crashes the garbage collector.  Because of this, no  
> gcj
>         programs even start.
>
>         I can fix the problem with the simple patch appended, but it  
> might be
>         better to fix the compiler bug.  Assuming, of course, that we  
> agree it
>         really is a bug.
>
>         Andrew.
> ----------------------------------------------------------------------- 
> ---------
>
> The root of the problem is that the gimplifier is converting that
> ASM_EXPR into:
>
> fubar (void *x)
> {
>   char *x.1;
>   char T.2;
>
>   x.1 = (char *)x;
>   T.2 = *x.1;
>   __asm__ __volatile__("prefetch %0"::"m" T.2);
> }
>
> I think this is a real bug and neither the gimplifier nor the  
> optimizers
> should ever attempt to look into ASM_EXPRs.  The same way we never even
> try to do anything with MD builtins.
>
> You could argue that we may be able to deal with ASMs, but I don't  
> think
> we should.  ASM_EXPRs and MD builtins are very low-level and we can't
> pretend to understand them at the tree level.  I'm sure this bug will  
> be
> very popular when trying to compile things like kernels.
>
> I'm testing a patch to completely remove processing of ASM_EXPRs from
> the gimplifier and tree optimizers.  We are going to be marking
> ASM_EXPRs with TREE_NOT_GIMPLE.
>
> Ideas to fix the problem some other way?
>
>
> Diego.
>
>
>