Re: [C PATCH] Discard P - (P + CST) optimization in pointer_diff (PR c/61240)

[Jan Hubicka <hubicka at ucw dot cz>]

> It's only important to not inline the static constructor into the
> one function calling all of them.  And only not doing that during
> early inlining (but still inline into those constructors).  That's
> because we eventually want to eliminate the body of the static
> constructor in favor of a DECL_INITIAL in that new pass.

I see, that indeed makes sense (and I was thinking of that).
Disabling early inlining of static ctors seems OK, but we may
not even need to do that given that we process topologically - so
we will identify simple ctor before we inline it.
(assuming we do not want to do this at LTO that would also make
sense)
> 
> So we need to be able to identify
> 
> struct X { int i; X() { i = 1; } } a;
> 
> _constructor_of_a ()
> {
>   a.i = 1;
> }
> 
> and place 'a' into .data with the appropriate initialization.
> And then remove a.i = 1

Yeah, I think annotating cgraph nodes with info "this is static
ctor of this variable" would be easy and we don't neven need to
mess with the rest of C++ FE finalization code (i.e. one unifying
it to static construction functions).
Early inliner can skip inlining these if desirable and the new
pass can easily identify them.

Being able to turn stores into ctor is something that may be
shareable with Martin's ipa-prop analysis.

He is basically doing that IMO.
> 
> Of course similar thing would be possible from the function
> with all those constructors inlined but I guess it's much
> harder if you may end up seeing calls inside (stuff may get
> overwritten...).
> 
> OTOH, I wonder if
> 
> struct X { int i; X() { i = 1; } } a;
> struct X { int i; X() { i = 1; a.i = 2; } } b;
> 
> is well-defined (with proper constructor priority attribute
> to init a last or without).  That is, may we write to 'a'
> from the constructor of b before it has been constructed?

Hehe, not a question for me :) 

Honza
> 
> Richard.