Re: [PATCH 1/3] Makefile: make the "sparse" target non-.PHONY

From: Jeff King <peff@peff.net>
Date: Sep 22, 2021, 02:24 UTC
Message-ID: <YUqTyJQ8euArYeVn@coredump.intra.peff.net>
In-Reply-To: <patch-1.3-e03fde1b642-20210921T224944Z-avarab@gmail.com>

On Wed, Sep 22, 2021 at 12:55:13AM +0200, Ævar Arnfjörð Bjarmason wrote:

> On my box with -j8 it was fast before, or around 5 seconds, now it
> only takes that long the first time, and the common case is <100ms, or
> however long it takes GNU make to stat the *.sp file and see that all
> the corresponding *.c files are older.

Sort of side note, but I think the main culprit here is all of the $(shell) invocations, etc, we run. Try:

  make SHELL_PATH='sh -x'

to get a taste (do it twice, because the first one will actually rebuild everything due to the changed shell; the second one _should_ be a noop but still runs a bunch of stuff).

Though even that doesn't tell the whole story, as it doesn't make clear which shells are invoked by make directly. Try this:

	cat >/tmp/foo <<-\EOF
	#!/bin/sh
	echo >&2 "==> shell $*"
	exec sh "$@"
	EOF

	make SHELL_PATH=/tmp/foo

There's lots of low-hanging fruit like:

==> shell -c echo TEST_SHELL_PATH=\''/tmp/foo'\' >>GIT-BUILD-OPTIONS+ ==> shell -c echo PERL_PATH=\''/usr/bin/perl'\' >>GIT-BUILD-OPTIONS+ ==> shell -c echo DIFF=\''diff'\' >>GIT-BUILD-OPTIONS+ ==> shell -c echo PYTHON_PATH=\''/usr/bin/python'\' >>GIT-BUILD-OPTIONS+ [...and over a dozen more...]

Those could easily be a single shell invocation, rather than one per echo. Another culprit is GIT-VERSION-GEN, which we run whether it's needed or not, and takes something like 25ms.

It's probably not worth spending too much time micro-optimizing here (and certainly it's orthogonal to your series), but there may be some low-hanging fruit (although a hacky attempt at minimizing the shell calls for GIT-BUILD-OPTIONS didn't seem to show any speedup, so maybe it's not so low-hanging after all).

-Peff