Re: [PATCH] Fix alias.c ICE on inline-asm "=m" incomplete operand (PR inline-asm/85022)

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Mar 23, 2018 at 09:53:53AM +0100, Richard Biener wrote:
> > Something I wasn't really aware, apparently we allow extern vars with
> > incomplete types in "m" and "=m" constrained asm.  The problem is that
> > incomplete vars have VOIDmode mode and so their MEMs do as well, apparently
> > everything works with it except a 2013-ish assert in alias.c.
> > 
> > The following patch just makes sure x_mode is not VOIDmode if x doesn't have
> > VOIDmode, if x has VOIDmode, then it is fine for x_mode to be VOIDmode too.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> OK.
> 
> The question now is whether we handle dependences correctly for
> VOIDmode MEMs?  Do they have !MEM_SIZE_KNOWN_P?

They have MEM_SIZE_KNOWN_P true unfortunately.
set_mem_attributes_minus_bitpos starts with:
      defattrs = mode_mem_attrs[(int) GET_MODE (ref)];
      gcc_assert (!defattrs->expr);
      gcc_assert (!defattrs->offset_known_p);

      /* Respect mode size.  */
      attrs.size_known_p = defattrs->size_known_p;
      attrs.size = defattrs->size;
and because both TYPE_SIZE_UNIT and DECL_SIZE_UNIT are NULL, nothing updates
the size_known_p.
Wonder if we just shouldn't make mode_mem_attrs[VOIDmode]->size_known_p
false and mode_mem_attrs[VOIDmode]->size NULL, like we do for BLKmode?

   for (i = 0; i < (int) MAX_MACHINE_MODE; i++)
     {
       mode = (machine_mode) i;
       attrs = ggc_cleared_alloc<mem_attrs> ();
       attrs->align = BITS_PER_UNIT;
       attrs->addrspace = ADDR_SPACE_GENERIC;
-      if (mode != BLKmode)
+      if (mode != BLKmode && mode != VOIDmode)
         {
           attrs->size_known_p = true;
           attrs->size = GET_MODE_SIZE (mode);
           if (STRICT_ALIGNMENT)
             attrs->align = GET_MODE_ALIGNMENT (mode);
         }

?

	Jakub