From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 783EA38C11F7; Tue, 28 Jun 2022 14:20:32 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 783EA38C11F7 From: "david at westcontrol dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug ipa/101279] Function attributes often block inlining Date: Tue, 28 Jun 2022 14:20:32 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: ipa X-Bugzilla-Version: 12.0 X-Bugzilla-Keywords: documentation, missed-optimization X-Bugzilla-Severity: normal X-Bugzilla-Who: david at westcontrol 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, 28 Jun 2022 14:20:32 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101279 --- Comment #7 from David Brown --- (In reply to rguenther@suse.de from comment #6) > > Am 28.06.2022 um 14:53 schrieb david at westcontrol dot com : > >=20 > > =EF=BB=BFhttps://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101279 > >=20 > > --- Comment #5 from David Brown --- > > (In reply to Richard Biener from comment #4) > >> (In reply to frankhb1989 from comment #3) > >>> There is a more specific instance here: can_inline_edge_by_limits_p in > >>> ipa-inline.cc treats flags and "optimize" attributes differently. > >>=20 > >> A bit up there's a blacklist we maintain where inlining is not OK beca= use it > >> results in semantic differences. > >>=20 > >> Generally we it's hard to second-guess the users intention when looking > >> at an inline edge with different optimization settings of caller and c= allee. > >> For C++ comdats there might be even multiple variants with different > >> optimization level (but we only keep one, special-casing this a bit). > >=20 > > I appreciate the "err on the side of caution" attitude. Perhaps there = could be > > an extra "I know what I'm doing" attribute that lets you override the > > blacklisting in a particular case. This would only really make sense i= n cases > > where the attribute can be attached to the expressions and statements w= ithin > > the function (I think "-fwrapv" would be in this category). In cases w= here > > this is not possible, an error or warning message would be in order as = the > > compiler can't do what the programmer is asking. >=20 > Can you provide a specific example that you would allow this way? >=20 >=20 I'd go back to my original example : __attribute__((optimize("-fwrapv"))) static inline int wrapped_add(int a, int b) { return a + b; } int foo(int x, int y, int z) { return wrapped_add(wrapped_add(x, y), z); } If you want to disable inlining of "wrapped_add" due to the change in the semantics of integer arithmetic, that's fair enough. But if I /really/ want inlining, and write something like : __attribute__((optimize("-fwrapv"), always_inline)) static inline int wrapped_add(int a, int b) { return a + b; } then I want the code treated as : return (__wrapping_int) a + (__wrapping_int) b; or return __wrapping_add_int(a, b); If the way gcc marks and handles "-fwrapv" arithmetic does not support something like that, which would allow inline mixing with other code, then = that would result in a compiler warning or error message. It might be best to have a new attribute name here rather than using "always_inline" - I have not thought through the consequences. It might also be that if the compiler can be changed to support inlining of= a given optimisation attribute, then the attribute in question can be whiteli= sted for inlining for everyone. I suppose what I am saying is that if the compi= ler can't be sure that inlining is safe, then it could be cautious by default w= hile letting the programmer override that caution explicitly.=