Re: [PATCH] IPA: support -flto + -flive-patching=inline-clone

[Jan Hubicka <hubicka@ucw.cz>]

> > WPA is Whole Program Analysis?
> 
> Yes.
> 
> > Okay, then  It will promote all static function to extern functions. That’s reasonable.
> 
> No, all extern functions to static functions.
> 
> > Is it hard to preserve the original “static” visibility in the IR?
> 
> Probably not hard, and the IPA pass adjusting visbility could as well
> mark the functions
> as not to be inlined with -flive-patching=inline-only-static.
> 
> > >
> > > OTOH inline-only-static could disable WPA inlining and do all inlining early ...
> >
> > Inline-only-static ONLY inlines static functions, how can it disable WPA inlining? Don’t quite understand here.
> 
> it's a flag so it can be used to control other things

GCC has two inliners
 1) ealry inlininer which happens at compile time and is quite
 restricted only to obvious cases (always_inline, flatten and very small
 functions)
 2) IPA inlining happening at link-time (WPA) which is using greedy
 algorithm and makes more complicated code size/speed tradeoffs
Indeed betwen 1 and 2 previously global functions may become static by
resolution info (they won't currently with kernel since we do
incremental linking).  We could easily keep track of originally static
functions and promoted to static functions and make IPA inlining to
honnor the patch.

I however wonder how much LTO optimization would remain. If we disable
all inter-module inlining and with live patching we also disable most of
other optimization, I think basically only unreachable code removal will
remain and possibly some propagation of "coldness" across the code.

I can implement this incrementally.
Martin, if live patching is happy about some symbols being promoted
static, the patch is OK.
Honza