[PATCH 2/8] Makefile: disable GNU make built-in wildcard rules

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Dec 17, 2021, 01:39 UTC
Message-ID: <patch-2.8-b0c63abe091-20211217T012902Z-avarab@gmail.com>
In-Reply-To: <cover-0.8-00000000000-20211217T012902Z-avarab@gmail.com>

Override built-in rules of GNU make that use a wildcard target. This can speeds things up significantly as we don't need to stat() so many files. GNU make does that by default to see if it can retrieve their contents from RCS or SCCS. See [1] for an old mailing list discussion about how to disable these.

The speed-up may wary. I've seen 1-10% depending on the speed of the local disk, caches, -jN etc. Running:

    strace -f -c -S calls make -j1 NO_TCLTK=Y

Shows that we reduce the number of syscalls we make, mostly in "stat" calls.

We could also invoke make with "-r" by setting "MAKEFLAGS = -r" early. Doing so might make us a bit faster still. But doing so is a much bigger hammer, since it will disable all built-in rules, some (all?) of which can be seen with:

    make -f/dev/null -p | grep -v -e ^# -e ^$

We may have something that relies on them, so let's go for the more isolated optimization here that gives us most or all of the wins.

1. https://lists.gnu.org/archive/html/help-make/2002-11/msg00063.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 shared.mak | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/shared.mak b/shared.mak
index 0170bb397ae..29f0e69ecb9 100644
--- a/shared.mak
+++ b/shared.mak
@@ -1,3 +1,14 @@
+### Remove GNU make implicit rules
+
+## This speeds things up since we don't need to look for and stat() a
+## "foo.c,v" every time a rule referring to "foo.c" is in play. See
+## "make -p -f/dev/null | grep ^%::'".
+%:: %,v
+%:: RCS/%,v
+%:: RCS/%
+%:: s.%
+%:: SCCS/s.%
+
 ### Flags affecting all rules
 
 # A GNU make extension since gmake 3.72 (released in late 1994) to

-- 
2.34.1.1119.g7a3fc8778ee