From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id E25D63858413; Tue, 26 Oct 2021 11:04:20 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org E25D63858413 From: "redbeard0531 at gmail dot com" 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 11:04:20 +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: redbeard0531 at gmail dot com 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 11:04:21 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102876 --- Comment #12 from Mathias Stearn --- (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 writt= en > 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 wo= uld > 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 conside= red a miscompilation. The example only shows reading from another static-durati= on variable's initializer, but I believe writing would also be covered. I took a look at how other compilers handle this, and it is somewhat interesting: https://godbolt.org/z/9YvcbEeax int foo(int&); inline int bar() { return 7; } extern int b; int z =3D bar(); int a =3D foo(b); // comment this out and watch clang change... int b =3D bar(); int c =3D bar(); GCC: always does dynamic init for everything. MSVC: always does static init for z, b, and c. always dynamic init for a. Clang: it seems like if it does any dynamic init in the TU, it doesn't prom= ote any dynamic init to static. So with that code all four variables are dynamicially initialized, but if you comment out a, the remaining 3 become static. If you add an unrelated variable that requires dynamic init, those 3 become dynamically initialized again. I don't understand why clang does what it does. I don't think it is require= d to do that by the standard, and it clearly seems suboptimal. So I would rather= GCC behave like MSVC in this case than like clang. Also note what happens if we provide a definition for foo like `inline int foo(int& x) { return x +=3D 6; }`: https://godbolt.org/z/sWd6chsnP. Now bot= h MSVC and Clang will static initialize z, b, and c to 7 and *static* initialize a= to 6. GCC gets the same result dynamically, but for some reason tries to load b prior to adding 6, even though it has to be 0 (barring a UB write to b from another TU).=