[Bug c++/102876] GCC fails to use constant initialization even when it knows the value to initialize

["jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102876

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Mathias Stearn from comment #12)
> (In reply to Jakub Jelinek from comment #10)
> > So we'd just punt at optimizing that, we don't know if b is read or written
> > by foo (and, note, it doesn't have to be just the case of explicitly being
> > passed address of some var, it can get the address through other means).
> > On the other side, we can't optimize b to b: .long 2, because bar can use
> > the variable and/or modify it, so by using 2 as static initializer bar would
> > be miscompiled.
> 
> I'm pretty sure that that is explicitly allowed by
> https://eel.is/c++draft/basic.start.static#3, so it should *not* be
> considered a miscompilation. The example only shows reading from another
> static-duration variable's initializer, but I believe writing would also be
> covered.

You are clearly talking about a different case than I was.
Once a variable is initialized, dynamic initializers which are ordered after
that certainly can't see a different value of the var than it should.
What you refer to is that because of the foo call in my testcase the optimizer
can still optimize b's initialization from dynamic to static b = 1
initialization.  But, when bar is called, b needs to be 1, not 2.

>From what you wrote, I bet llvm implements what I was talking about in #c8
before
"But I'm worried", i.e. if the static initialization function is optimized into
just a series of constant stores, all the vars in it can be optimized.
With the markers I've mentioned, we could consider each dynamic initialization
individually, rather than just are all initializations constant, then optimize,
otherwise punt.