Re: [PATCH 3/5] IPA ICF pass

[Jan Hubicka <hubicka@ucw.cz>]

> On Fri, 20 Jun 2014, Trevor Saunders wrote:
> >> +@item -fipa-icf
> >> +@opindex fipa-icf
> >> +Perform Identical Code Folding for functions and read-only variables.

I would perhaps explicitly say that the optimizations reduce code size
and may disturb unwind stacks by replacing a function by equivalent
one with different name.
> >> +Behavior is similar to Gold Linker ICF optimization. Symbols proved

Perhaps tell a bit more here. The optimization works more effectively with link
time optimization enabled and that the Gold and GCC ICF works on different
levels and thus are not equivalent optimizations - there are equivallences that
are found only by GCC and equivalences found only by Gold.

>> +as semantically equivalent are redirected to corresponding symbol. The pass
> >> +sensitively decides for usage of alias, thunk or local redirection.
> >> +This flag is enabled by default at @option{-O2}.
Probably at -Os too.
> 
> I found this a bit hard to read/understand.
> 
> Perhaps first describe what it does and then, before "This flag is
> enabled..." note that "This is similar to the ICF optimization performed
> by the Gold linker".

> 
> "Symbols proved" (plural) vs "to corresponding symbol" seems to miss
> an an "a" as in "a corresponding symbol".  Alas, how is that one 
> determined?  Is this more "...are merged into one", from the user's
> perspective?
> 
> What does it mean to "sensitively decide for usage of alias, thunk,
> or local redirection"?

I think this is just a technical detail of the implementation.  I would not put that
into user manual.  It means that for some functions you can make alias, for others
you need thunk (so addresses stay different)
> 
> Gerald