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From: "yann at droneaud dot fr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug preprocessor/109183] New: [regression?] since GCC 11.1, -MM
 -MMD generates "a-" prefixed dependency files
Date: Sat, 18 Mar 2023 10:36:15 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109183

            Bug ID: 109183
           Summary: [regression?] since GCC 11.1, -MM -MMD generates "a-"
                    prefixed dependency files
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yann at droneaud dot fr
  Target Milestone: ---

I've found a rather surprising behavior change when using GCC >=3D 11.1 to =
build
some project using custom build environment.

How to reproduce:

    : > test.c
    gcc -MM -MMD test.c
    test -e a-test.d && echo "unexpected dependency file name"

I've git-bisect-ed between GCC 11.1 and GCC 10.1 and the behavior change did
happen as the result of this commit:

    commit 1dedc12d186a110854537e1279b4e6c29f2df35a
    Author: Alexandre Oliva <oliva@adacore.com>
    Date:   Tue May 26 04:30:15 2020 -0300

        revamp dump and aux output names

=20=20=20
https://gcc.gnu.org/git/?p=3Dgcc.git;a=3Dcommit;h=3D1dedc12d186a110854537e1=
279b4e6c29f2df35a

I didn't find an explanation there for this behavior change.

It's not noted in the documentation either
https://gcc.gnu.org/onlinedocs/gcc-11.3.0/gcc/Preprocessor-Options.html

So this make me believe it's some kind of regression for a corner case.

I understand the correct usage of -MMD is to be associated with some
processing, -E, -S, -c, etc, while -MM imply -E, so perhaps -MM and -MMD are
mutually exclusive and should not be allowed together. I dunno.

But generating "a-" prefixed dependency files is rather unexpected.=