aarch64: Restrict aarch64-tune.md regeneration to --enable-maintainer-mode [PR105144]

aarch64: Restrict aarch64-tune.md regeneration to --enable-maintainer-mode [PR105144]

Normally updates to the source directory files are guarded with
--enable-maintainer-mode, e.g. we don't regenerate configure, config.h,
Makefile.in in directories that use automake etc. unless gcc is configured
that way.  Otherwise the source tree can't be e.g. stored on a read-only
filesystem etc.
In gcc/Makefile.in we use @MAINT@ for that but that works because
gcc/Makefile is generated by configure.  In config/*/t-* files we need to
check $(ENABLE_MAINTAINER_RULES):
 # The following provides the variable ENABLE_MAINTAINER_RULES that can
 # be used in language Make-lang.in makefile fragments to enable
 # maintainer rules.  So, ENABLE_MAINTAINER_RULES is 'true' in
 # maintainer mode, and '' otherwise.
 @MAINT@ ENABLE_MAINTAINER_RULES = true

On Mon, Apr 04, 2022 at 11:10:14AM +0100, Richard Sandiford wrote:
> I guess the risk is that it will become even easier to forget
> to commit an updated aarch64-tune.md.  Perhaps we should have a
> non-maintainer rule to build aarch64-tune.md locally and check it
> against the source-directory version, and fail the build if there's
> a mismatch.  Or maybe we should just generate aarch64-tune.md in the
> build directory and remove the source directory version.

I've tried if aarch64-tune.md will be read from the build dir, but it is
not.  The gen* files can use -I options to add additional directories, but
they don't use them.

Here is a variant patch which will complain and fail if there is a change
and --enable-maintainer-mode is not enabled.

2022-04-04  Jakub Jelinek  <jakub@redhat.com>

	PR target/105144
	* config/aarch64/t-aarch64 (s-aarch64-tune-md): Do move-if-change
	only if configured with --enable-maintainer-mode, otherwise compare
	tmp-aarch64-tune.md with $(srcdir)/config/aarch64/aarch64-tune.md and
	if they differ, emit a message and fail.