From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 6FF2E3851C37; Tue, 28 Apr 2020 12:43:51 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 6FF2E3851C37
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
 s=default; t=1588077831;
 bh=L5gAhHXi8haPzwjVfky/pCDeCHqFPRey06zuFYb8hoQ=;
 h=From:To:Subject:Date:From;
 b=d7Yu1q3y70VuZ5ocR7feAt1OrMRm/AZC0qb0yHsWMjnn23GwJWXhthyDYh80pAUhD
 Dmd6YEuZYecwUJ47/AdK2ffhHmmw5sBBCqypVFXmqPav3s+akItDHVOM/RpCgzHKO0
 PgFdatMgajSkoiqCO21IfPnTv3xGnUr286u8D19o=
From: "felix.von.s at posteo dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/94818] New: GCC emits dead bodies of
 functions whose all calls have been eliminated by optimisations
Date: Tue, 28 Apr 2020 12:43:51 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 10.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: felix.von.s at posteo dot de
X-Bugzilla-Status: UNCONFIRMED
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: bug_id short_desc product version bug_status
 keywords bug_severity priority component assigned_to reporter
 target_milestone cf_gcctarget
Message-ID: <bug-94818-4@http.gcc.gnu.org/bugzilla/>
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 <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <http://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <http://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 28 Apr 2020 12:43:51 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94818

            Bug ID: 94818
           Summary: GCC emits dead bodies of functions whose all calls
                    have been eliminated by optimisations
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.von.s at posteo dot de
  Target Milestone: ---
            Target: x86_64-linux-gnu

In this code:

    __attribute__((__warning__("argh")))
    static void foo(void) {
        __asm__ (
            ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;"
            ";;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;"
        );
    }

    void bar0(int x) {
        if (__builtin_constant_p(x))
            foo();
    }

    void bar1(int x) {
        if (__builtin_constant_p(x))
            foo();
    }

No calls to foo are emitted in the final output, and GCC knows this, since =
no
warnings are emitted. However, GCC will still emit a function body for foo,
unless either the __asm__ statement is removed or both conditions are chang=
ed
to integer constant expressions, so that the function is eliminated by one =
of
the inlining passes.

(I decided to demonstrate it this way instead of using
__attribute__((__noinline__)), just to make sure this is not related to
__attribute__((__noinline__)) somehow implying __attribute__((__used__)).)=