Re: stop IPA wrapping 'main'

[Jan Hubicka <hubicka@ucw.cz>]

> gcc.dg/20031102-1.c now causes some 'surprising' optimization
> behaviour.  It is essentially
> 
> int FooBar(void)
> {
>  ... stuff
>   return 0;
> }
> 
> int main(void)
> {
>   return FooBar();
> }
> 
> 
> What happens is  that FooBar gets inlined into main, and then
> ipa-icf notices FooBar and main have identical bodies.  It chooses
> to have FooBar tail call main, which results in a surprising  call
> of 'main'.   On PTX this is particularly unfortunate because we have
> to emit a single prototype for main with the regular argc and argv
> arguments (the backend gets around 'int main (void)' by faking the
> additional 2 args).  But that fails here because the tail call
> doesn't match the prototype.
> 
> Anyway, picking 'main' as the source function struck me as a poor
> choice, hence the attached patch.  It picks the second function of a
> congruent set, if the first is 'main'.  Note that even on, say
> x86-linux, we emit a tail call rather than an alias for the included
> testcase.
> 
> I removed the gcc_assert, as the vector indexing operator already
> checks the subscript is within range.
> 
> Alternatively I could probably just fixup the testcase to make
> FooBar uninlinable, as I suspect that might have been the original
> intent.
> 
> tested on x86_64-linux and ptx-none.
> 
> nathan

> 2015-12-17  Nathan Sidwell  <nathan@acm.org>
> 
> 	gcc/
> 	* ipa-icf.c (sem_item_optimizer::merge): Don't pick 'main' as the
> 	source function.
> 
> 	gcc/testsuite/
> 	* gcc.dg/ipa/ipa-icf-merge-1.c: New.

OK, thanks. Indeed we should not introduce new calls to main :)
It contains some magic stuff on x86 targets, too.

Honza