Re: [PATCH] Fix PR56888

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Feb 23, 2016 at 12:32 PM, Richard Biener <rguenther@suse.de> wrote:
> On Tue, 23 Feb 2016, Jan Hubicka wrote:
>
>> >
>> > Ok, so maybe a better question to symtab would be if there is an
>> > actual definition for what __builtin_FOO will call.  Not really
>> > whether that definition is cfun.  Of course all the fortify
>> > always-inline wrappers should not count as such (just in case
>> > the symtab code is confused about those).
>>
>> Also GNU extern inlines that are often used to deal special cases.
>> >
>> > So,
>> >
>> > bool symbol_table::have_definition (enum built_in_fn);
>> >
>> > ?  Not sure how to best implement that either.  asmname lookups are
>> > expensive ...
>>
>> I am back from China trip, so i can handle you patch if you want.

Honza - ping.  Can you please think of a symtab predicate that tells me
whether a cgraph node is an implementation for BUILT_IN_X?  (see original
patch in this thread).

It's before another GCC release and we're trying to improve QOI wise for
almost 3 releases now...

Thanks a lot,
Richard.

>> I see that by stopping the optimization on whole translation unit that
>> defines memcpy/memset will solve the reachability issue I mentioned
>> in previous mail, but also when LTOing stuff like Linux kernel, it will
>> prevent the optimization on the whole program.
>
> Yes, but I think it's reasonable to disable such transform if the
> memcpy implementation is being optimized.
>
>> I am not quite sure how to deal with the alwaysinline wrappers however,
>> because there theoretically may contain memcpy/memset loops themselves.
>
> It might be a non-issue as we are doing the transforms only after
> inlining when those bodies should be gone and thus symtab shouldn't see
> such implementation.
>
> Better to double-check, of course.  We'd want
>
> #include <string.h>
>
> int main()
> {
>   int s[1204];
>   for (int i = 0; i < 1204; ++i)
>    s[i] = 0;
>   memset (s, 0, sizeof (s));
> }
>
> still be optimized as memset.
>
> Richard.
>
>> Honza
>> >
>> > Richard.
>>
>>
>
> --
> Richard Biener <rguenther@suse.de>
> SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nuernberg)