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Re: [PATCH] Avoid some unnecessary set_cfun calls


Jakub Jelinek <jakub@redhat.com> writes:
> On Wed, Nov 13, 2013 at 11:27:10AM +0100, Richard Biener wrote:
>> > Also, I wonder if we couldn't defer the expensive ira_init, if the info
>> > computed by it is used only during RTL optimization passes (haven't verified
>> > it yet), then supposedly we could just remember using some target hook
>> > what the last state was when we did ira_init last time, and call ira_init
>> > again at the start of expansion or so if it is different from the last time.
>> > For i?86/x86_64/ppc* this would be whether the current function's
>> > DECL_FUNCTION_SPECIFIC_TARGET is the same as one for which ira_init has been
>> > called, for rx whether interrupt attribute is the same and for mips whatever
>> > is needed.
>> 
>> I wonder why we cannot move all the stuff we re-init to a member
>> of struct function (or rather have a pointer to that info there
>> to cache it across functions with the same options).  That is,
>> get rid of more global state?  That would make switching back
>> and forth cheaper.
>
> Isn't that what the SWITCHABLE_TARGET stuff is all about?
> So, perhaps we should just define SWITCHABLE_TARGET on i?86/x86_64/powerpc*
> (and rx if maintainer cares) and tweak it to attach somehow
> struct target_globals * to TARGET_OPTION_NODE somehow.
> A problem might be that lots of the save_target_globals
> allocated structures are heap allocated rather than GC, so we might leak
> memory.  Wonder if save_target_globals couldn't just compute the
> aggregate size of all the structures it allocates with XCNEW right now
> (plus required alignment if needed) and just allocate them together
> with the ggc_alloc_target_globals after the target_globals structure
> itself.

Yeah, that might be worth doing.  I think the only non-GCed structures
with subpointers are target_ira_int and target_lra_int, but we could
probably convert them to GCed structures.  (And perhaps use the same
technique recursively.  E.g. LRA could work out the maximum number of
operand_alternative structures needed and allocate them in one go.)

Thanks,
Richard


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