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 AC3CF3858D35; Thu, 13 Apr 2023 19:27:59 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org AC3CF3858D35
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org;
	s=default; t=1681414079;
	bh=Kc1o+citI7vKM3NhlUNy1YP4AlrVlfjtfc4WD+ykht8=;
	h=From:To:Subject:Date:In-Reply-To:References:From;
	b=P7JX11FcyKI7mB9U/W2bl+r6CaoXa2yk6vPD1zoF3DGwlaLm1c7c8Qu0XSU4UqtmP
	 XHYO2qChXfAQtK8taE4dPCX3someuKwjKo430cgBbhprje1M7PkEinLNB4baaYG/3h
	 rZ1ZEeErdjS3+1ScSP/gt8dW4muOq+Ia3roxBOo4=
From: "allan.w.macdonald at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/109183] [regression?] since GCC 11.1, -MM -MMD
 generates "a-" prefixed dependency files
Date: Thu, 13 Apr 2023 19:27:59 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: preprocessor
X-Bugzilla-Version: unknown
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: allan.w.macdonald at gmail dot com
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: 
Message-ID: <bug-109183-4-o9RKmDSsfJ@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-109183-4@http.gcc.gnu.org/bugzilla/>
References: <bug-109183-4@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=3D109183

--- Comment #11 from Allan W. Macdonald <allan.w.macdonald at gmail dot com=
> ---
The makefiles I've been maintaining contain a mechanism to make sure that a=
ny
change in a locally-included file will cause the c file that includes it to=
 be
compiled again, like so:

### Extract of makefile:
%.d: %.c
        gcc -MM -MD $<

-include $(OBJECTS:.o=3D.d)
###

So, if I understand this correctly (tell me if I'm wrong), preprocessing is
performed to generate the .d files before any file is compiled to an object=
 in
this situation.  These .d files are then included and turned into rules usi=
ng a
substitution reference.

The 'a-' prefix breaks this mechanism.

The workaround I mentioned,

        gcc -E -MMD $< > /dev/null

seems to achieve the same behaviour but there's probably a better way to do=
 it.=