[PATCH v3 20/23] Makefile: use $(file) I/O instead of "FORCE" when possible

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Provide an alternate implementation of the recently added
"TRACK_template" which uses GNU make 4.2 features. If the 'file'
function is available we don't need to shell out at all to check if
our tracked files change.

We need to use the intermediate TRACK_template calling a
TRACK_template_eval within the generated rule so that we don't eagerly
fleshen these when "make" reads the file.

This doesn't make the runtime faster on my system, but helps to cut
down on the noise of things we shell out for
unconditionally. I.e. running "make" with "SHELL_PATH='sh -x'" (twice,
so we pick up the setting) shows than a no-op run went from emitting:

    $ SHELL_PATH='sh -x' make -j8 >/dev/null 2>&1; SHELL_PATH='sh -x' make 2>&1 |wc -l
    124

To:

    $ SHELL_PATH='sh -x' make -j8 >/dev/null 2>&1; SHELL_PATH='sh -x' make 2>&1 |wc -l
    95

That 124 to 95 number is a rough approximation of how many times we
shell out. "strace -f -c" similarly shows that we went from 8798 to
8466 syscalls. So this brings us further along in the goal of making
"make" do as little as possible when it's got nothing to re-build (see
[2]).

1. https://lore.kernel.org/git/874kdn1j6i.fsf@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 shared.mak | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/shared.mak b/shared.mak
index 363138a5577..4ee0bb7a13d 100644
--- a/shared.mak
+++ b/shared.mak
@@ -8,6 +8,22 @@
 #    info make --index-search=.DELETE_ON_ERROR
 .DELETE_ON_ERROR:
 
+### GNU Make version detection
+# We don't care about "release" versions like the "90" in "3.99.90"
+MAKE_VERSION_MAJOR = $(word 1,$(subst ., ,$(MAKE_VERSION)))
+MAKE_VERSION_MINOR = $(word 2,$(subst ., ,$(MAKE_VERSION)))
+
+# The oldest supported version of GNU make is 3-something. So "not v3"
+# is a future-proof way to ask "is it modern?"
+ifneq ($(MAKE_VERSION_MAJOR),3)
+# $(file >[...]) and $(file >>[...]) is in 4.0...
+MAKE_HAVE_FILE_WRITE = Need version 4.0 or later (released in late 2013)
+# .. but we need 4.2 for $(file <[...])
+ifneq ($(filter-out 0 1,$(MAKE_VERSION_MINOR)),)
+MAKE_HAVE_FILE_READ = Need version 4.2 or later (released in mid-2016)
+endif
+endif
+
 ### Quoting helpers
 
 ## Quote a ' inside a '': FOO='$(call shq,$(BAR))'
@@ -99,6 +115,10 @@ endef
 
 ## Template for making a GIT-SOMETHING, which changes if a
 ## TRACK_SOMETHING variable changes.
+##
+## This is the slower version used on GNU make <4.2.
+ifndef MAKE_HAVE_FILE_READ
+
 define TRACK_template
 .PHONY: FORCE
 $(1): FORCE
@@ -111,3 +131,41 @@ $(1): FORCE
 		echo "$$$$FLAGS" >$(1); \
 	fi
 endef
+
+endif # !MAKE_HAVE_FILE_READ
+
+## A TRACK_template template compatible with the one above. Uses
+## features of GNU make >=4.2 to avoid shelling out for this "hot"
+## "FORCE" logic.
+##
+## Since version >=4.2 can do both "I" and "O" in I/O with using
+## $(file <)/$(file >) we read the GIT-SOMETHING file into a variable
+## with the former, and if it's different from our expected value
+## write it out with the latter.
+ifdef MAKE_HAVE_FILE_READ
+
+define TRACK_template_eval
+$(1)_WRITE =
+$(1)_EXISTS = $(wildcard $(1))
+ifeq ($$($(1)_EXISTS),)
+$(1)_WRITE = new
+else
+$(1)_CONTENT = $(file <$(1))
+ifeq ($$($(1)_CONTENT),$($(2)))
+$(1)_WRITE = same
+else
+$(1)_WRITE = changed
+endif
+endif
+ifneq ($$($(1)_WRITE),same)
+$$(info $$(wspfx) $(1) parameters ($$($(1)_WRITE)))
+$$(file >$(1),$($(2)))
+endif
+endef # TRACK_template_eval
+
+define TRACK_template
+$(1):
+	$$(eval $$(call TRACK_template_eval,$(1),$(2)))
+endef
+
+endif # MAKE_HAVE_FILE_READ
-- 
2.34.0.795.g1e9501ab396