On Thu, 26 Jan 2012, Richard Guenther wrote:
> On Thu, 26 Jan 2012, Eric Botcazou wrote:
>
> > > Now that Richi has fixed up SRA not to pessimize code by changing non-BLK
> > > mode arguments into their BLKmode subparts, I think it would be nice
> > > to fix up also the expansion of the BLKmode MEM_REFs that have non-BLKmode
> > > non-addressable base decl. While this doesn't happen for this testcase
> > > anymore, it rarely still occurs and I think it is wrong to expand
> > > a BLKmode MEM_REF into a non-BLKmode rtx. While the callers sometimes
> > > can cope with it, often they can't.
> >
> > Another example is PR middle-end/51959, the BLKmode MEM_REF with
> > non-BLKmode
> > base is created by inlining, expanded to a DImode REG in expand_assignment
> > and
> > then sent to store_field, which attemps to spill it to memory but runs into
> > an
> > alias set conflict. I'll try to redirect it to the regular MEM_REF
> > expander
> > instead, so your patch might be a prerequisite.
>
> Hmm, how can store_field run into an "alias set conflict"? ...
I think the bug is that
if (mode == BLKmode
&& (REG_P (target) || GET_CODE (target) == SUBREG))
{
rtx object = assign_temp (type, 0, 1, 1);
rtx blk_object = adjust_address (object, BLKmode, 0);
assign_temp here uses TYPE which is not compatible with the
requested ALIAS_SET. expand_assignment calls it like
result = store_field (to_rtx, bitsize, bitpos,
bitregion_start, bitregion_end,
mode1, from,
TREE_TYPE (tem), get_alias_set (to),
nontemporal);
but that of course does not work for view-converted to (thus,
a MEM_REF on a placement-new accessed storage for example).
We need to simply force ALIAS_SET to be set on the assign_temp
result - but it looks like that's not easily possible without
hitting that very same assert. Ugh.
Of course get_inner_reference looks through these kind of
conversions when returning the ultimate decl in MEM [&foo],
see the jumps in tree-ssa-alias.c we perform to re-discover
the view-converting cases ... (at some point I realized that
this probably wasn't the best design decision). So maybe
if the passed type isn't used in any other way we can
get around and discover the view-convert and use the alias-type
of the MEM_REF ...
Richard.
