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 BAE113858D28; Wed, 12 Apr 2023 10:13:51 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org BAE113858D28
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1681294431;
	bh=Tn2vZnzMPQY0R66CFO0V/AEkpmiEcopcZ+JRBg6Yszg=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=QoinDlhQn/KHYpV4lQ5ZqTaf9tDmc1IzM1A1AJTkwqcmTe3UcOW7Vn4vgBUbdE3ss
	 +P0a34FjbXlQbe9+NwjWgTCijjyXVNMafBw9kSctcv7F6/TPy2OlrWCapU9aicIj+a
	 nkmftIeYHBz0AAUMr6mdS+rTm2QabMAfZ6VyCOOk=
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/109442] Dead local copy of std::vector not removed
 from function
Date: Wed, 12 Apr 2023 10:13:51 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 13.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: REOPENED
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: <bug-109442-4-Trpa4nSYGI@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-109442-4@http.gcc.gnu.org/bugzilla/>
References: <bug-109442-4@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
List-Id: <gcc-bugs.sourceware.org>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109442

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #7)
> I think it's valid in theory. I don't know if it's possible for GCC to do=
 it
> in practice. There doesn't seem to be anything the library can do to help,
> so WONTFIX.

I think it's valid in theory because the implementation of std::vector is
opaque. It's not required to actually call operator new and operator delete=
 to
obtain its storage, and so if the library and compiler collaborate to provi=
de
storage some other way (e.g. using a stack buffer) then that's allowed.

But in practice it's hard to do that.

Maybe it could be done with a new __builtin_elidable_operator_new and
__builtin_elidable_operator_delete pair that the library could use, or
attributes like you suggested in comment 4. That would allow the library to=
 say
"I'm calling operator new and operator delete because I need to obtain memo=
ry,
but **these particular invocations** are not required to be visible to the
user, and therefore can be elided in the same way as new expressions and de=
lete
expressions".

In fact Clang's __builtin_operator_new already has exactly those semantics:

https://clang.llvm.org/docs/LanguageExtensions.html#builtin-operator-new-an=
d-builtin-operator-delete

And we already use those builtins if available, see PR 94295.

So maybe what we need is to implement those.=