[PING PATCH, PR 57748] Set mode of structures with zero sized arrays to be BLK

Martin Jambor mjambor@suse.cz
Wed Aug 28 09:34:00 GMT 2013


On Wed, Aug 28, 2013 at 10:17:52AM +0200, Richard Biener wrote:
> On Wed, 28 Aug 2013, Richard Biener wrote:
> > Eh ... :/
> > 
> > I'm extremely nervous about this change.  I also believe the change
> > is unrelated to the issue in the bugreport (even if it happens to
> > fix the ICE).
> > 
> > Let me have a (short) look.
> 
> Everything looks ok up until
> 
>       if (offset != 0)
>         {
>           enum machine_mode address_mode;
> 
> where we are supposed to factor in offset into the memory address.
> This code doesn't handle the movmisalign path which has the memory
> in 'mem' instead of in to_rtx.
> 
> And indeed a hack like
> 
> Index: gcc/expr.c
> ===================================================================
> --- gcc/expr.c  (revision 202043)
> +++ gcc/expr.c  (working copy)
> @@ -4753,6 +4753,9 @@ expand_assignment (tree to, tree from, b
>         {
>           enum machine_mode address_mode;
>           rtx offset_rtx;
> +         rtx saved_to_rtx = to_rtx;
> +         if (misalignp)
> +           to_rtx = mem;
>  
>           if (!MEM_P (to_rtx))
>             {
> @@ -4785,6 +4788,11 @@ expand_assignment (tree to, tree from, b
>           to_rtx = offset_address (to_rtx, offset_rtx,
>                                    highest_pow2_factor_for_target (to,
>                                                                    
> offset));
> +         if (misalignp)
> +           {
> +             mem = to_rtx;
> +             to_rtx = saved_to_rtx;
> +           }
>         }
>  
>        /* No action is needed if the target is not a memory and the field
> 
> fixes the testcase and results in (at -O)
> 
> foo:
> .LFB1:
>         .cfi_startproc
>         pushq   %rbx
>         .cfi_def_cfa_offset 16
>         .cfi_offset 3, -16
>         movq    %rdi, %rbx
>         call    get_i
>         cltq
>         addq    $1, %rax
>         salq    $4, %rax
>         movdqa  .LC0(%rip), %xmm0
>         movdqu  %xmm0, (%rbx,%rax)
>         popq    %rbx
>         .cfi_def_cfa_offset 8
>         ret
> 
> exactly what is expected.
> 
> Martin, do you want to audit the (few) places we do the movmisalign
> game for the same problem or shall I put it on my TODO?  The audit
> should look for all MEM_P (to_rtx) tests which really should look
> at 'mem' for the unaligned move case (I see a volatile bitfiled
> case for example ...).

And I thought I had an easy way out.  Well, let me see what I can do.

Thanks,

Martin



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