From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 661613858421; Tue, 26 Oct 2021 11:38:32 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 661613858421 From: "jakub at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/102876] GCC fails to use constant initialization even when it knows the value to initialize Date: Tue, 26 Oct 2021 11:38:32 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c++ X-Bugzilla-Version: 11.2.0 X-Bugzilla-Keywords: missed-optimization X-Bugzilla-Severity: enhancement X-Bugzilla-Who: jakub at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 26 Oct 2021 11:38:32 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102876 --- Comment #13 from Jakub Jelinek --- (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 wri= tten > > by foo (and, note, it doesn't have to be just the case of explicitly be= ing > > 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 u= se > > the variable and/or modify it, so by using 2 as static initializer bar = would > > be miscompiled. >=20 > 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 optimi= zer can still optimize b's initialization from dynamic to static b =3D 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 initializat= ion individually, rather than just are all initializations constant, then optim= ize, otherwise punt.=