Re: [PATCH][RFC] Introduce TREE_AOREFWRAP to cache ao_ref in the IL

[Richard Sandiford <richard.sandiford@arm.com>]

Michael Matz via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> Hello,
>
> On Thu, 14 Oct 2021, Richard Biener wrote:
>
>> > So, at _this_ write-through of the email I think I like the above idea 
>> > best: make ao_ref be a tree (at least its storage, because it currently 
>> > is a one-member-function class), make ao_ref.volatile_p be 
>> > tree_base.volatile_flag (hence TREE_VOLATILE(ao_ref)) (this reduces 
>> > sizeof(ao_ref) by 8), increase all nr-of-operand of each tcc_reference by 
>> > 1, and make TREE_AO_REF(reftree) be "TREE_OPERAND(reftree, 
>> > TREE_CODE_LENGTH(reftree) - 1)", i.e. the last operand of such 
>> > tcc_reference tree.
>> 
>> Hmm.  I'm not sure that's really something I like - it's especially
>> quite some heavy lifting while at the same time lacking true boldness
>> as to changing the representation of memory refs ;)
>
> Well, it would at least enable such changes later in an orderly fashion.
>
>> That said - I've prototyped the TREE_ASM_WRITTEN way now because it's 
>> even simpler than the original TREE_AOREFWRAP approach, see below.
>> 
>> Note that I'm not embedding it into the tree structure, I'm merely
>> using the same allocation to store two objects, the outermost ref
>> and the ao_ref associated with it.  Quote:
>> 
>> +  size_t length = tree_code_size (TREE_CODE (lhs));
>> +  if (!TREE_ASM_WRITTEN (lhs))
>> +    {
>> +      tree alt_lhs
>> +       = ggc_alloc_cleared_tree_node_stat (length + sizeof (ao_ref));
>> +      memcpy (alt_lhs, lhs, length);
>> +      TREE_ASM_WRITTEN (alt_lhs) = 1;
>> +      *ref = new ((char *)alt_lhs + length) ao_ref;
>
> You need to ensure that alt_lhs+length is properly aligned for ao_ref, but 
> yeah, for a hack that works.  If you really want to go that way you need 
> good comments about this hack.  It's really somewhat worrisome that the 
> size of the allocation depends on a bit in tree_base.
>
> (It's a really cute hack that works as a micro optimization, the question 
> is, do we really need to go there already, are all other less hacky 
> approaches not bringing similar improvements?  The cuter the hacks the 
> less often they pay off in the long run of production software :) )

FWIW, having been guilty of adding a similar hack(?) to SYMBOL_REFs
for block_symbol, I like the approach of concatenating/combining structures
based on flags.  The main tree and rtl types have too much baggage and
so I think there are some things that are better represented outside
of them.

I suppose cselib VALUE rtxes are also similar, although they're more
of a special case, since cselib data doesn't survive between passes.

Thanks,
Richard