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From: "nathan at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/99456] [11 regression] ABI breakage with some static
 initialization
Date: Mon, 08 Mar 2021 12:47:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99456

--- Comment #4 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
It's an ABI issue, because all compilers must agree on which parts of an in=
line
object are dynamically initialized.  (gcc-11 does not agree with gcc-10).

consider:

inline Type Var =3D Expr;

That'll be emitted as comdat in every TU that needs it.  Assume some of tho=
se
TUs generate a static init and others generate a dynamic init.  Let's say i=
t's
a dynamic one that gets into the executable, but one of the static TUs acce=
sses
Var before that dyn init has run.  Boom, zero-initialized entity observed.

Or, let's say a static one wins, but then a dynmic initializer runs
concurrently to a (static TU's) access.  Oops, we could observe (weird) par=
tial
writes.  (this would require Var to be a function-scope static because glob=
al
inits run in a single-thread environment, and it's less likelu to be a
problem).

In case it's not clear, this example shows both the above happening, and po=
or
code generation with (a) unused inline vars emitting code and (b) guard
variables for inline vars with no dynamic init.

Hope that helps.=