[PATCH] Makefiles: Disable suffix rules and implicit rules

[Simon Marchi <simon.marchi@polymtl.ca>]

Since we don't use suffix rules nor implicit rules in gdb, we can
disable them.  The advantage is a slightly faster make [1].

Here are some numbers about the speedup.  I ran this on my trusty old
Intel Q6600, so the time numbers are probably higher than what you'd get
on any recent hardware.  I ran "make" in the gdb/ directory of an
already built repository (configured with --enable-targets=all).  I
recorded the time of execution (average of 5).  I then ran "make -d" and
recorded the number of printed lines, which gives a rough idea of the
number of operations done.

I compared the following configurations, to see the impact of both the
empty .SUFFIXES target and the empty pattern rules, as well as running
"make -r", which can be considered the "ideal" case.

 A - baseline
 B - baseline + .SUFFIXES
 C - baseline + pattern rules
 D - baseline + .SUFFIXES + pattern rules
 E - baseline + make -r

 config | time (s) | "make -d"
 -----------------------------
    A   |   5.74   |  2396643
    B   |   1.19   |   298469
    C   |   2.81   |  1266573
    D   |   1.13   |   245489
    E   |   1.01   |   163914

We can see that the empty .SUFFIXES target has a bigger impact than the
empty pattern rules, but still it doesn't hurt to disable the implicit
pattern rules as well.

There are still some mentions of implicit rules I can't get rid of in
the "make -d" output.  For example, it's trying to build .c files from
.w files:

  Looking for an implicit rule for '/home/simark/src/binutils-gdb/gdb/infrun.c'.
  Trying pattern rule with stem 'infrun'.
  Trying implicit prerequisite '/home/simark/src/binutils-gdb/gdb/infrun.w'.

and trying to build Makefile.in from a bunch of extensions:

  Looking for an implicit rule for 'Makefile.in'.
  Trying pattern rule with stem 'Makefile.in'.
  Trying implicit prerequisite 'Makefile.in.o'.
  Trying pattern rule with stem 'Makefile.in'.
  Trying implicit prerequisite 'Makefile.in.c'.
  Trying pattern rule with stem 'Makefile.in'.
  Trying implicit prerequisite 'Makefile.in.cc'.
  ... many more ...

If somebody knows how to disable them, we can do it, but at this point
the returns are minimal, so it is not that important.

I verified that both in-tree and out-of-tree builds work.

[1] Switching from explicit rules to pattern rules for files in
    subdirectories actually made it slower, so this is kind of a way to
    redeem myself.  But it the end it's faster than it was previously,
    so it was all worth it. :)

gdb/ChangeLog:

	* disable-implicit-rules.mk: New file.
	* Makefile.in: Include disable-implicit-rules.mk.
	* data-directory/Makefile.in: Likewise.
	* doc/Makefile.in: Likewise.

gdb/gdbserver/ChangeLog:

	* Makefile.in: Include disable-implicit-rules.mk.

gdb/testsuite/ChangeLog:

	* Makefile.in: Include disable-implicit-rules.mk.
---
 gdb/Makefile.in                |  3 +++
 gdb/data-directory/Makefile.in |  3 +++
 gdb/disable-implicit-rules.mk  | 10 ++++++++++
 gdb/doc/Makefile.in            |  3 +++
 gdb/gdbserver/Makefile.in      |  3 +++
 gdb/testsuite/Makefile.in      |  3 +++
 6 files changed, 25 insertions(+)
 create mode 100644 gdb/disable-implicit-rules.mk

diff --git a/gdb/Makefile.in b/gdb/Makefile.in
index 415cca3..2cf1b3d 100644
--- a/gdb/Makefile.in
+++ b/gdb/Makefile.in
@@ -2909,4 +2909,7 @@ endif
 # Dependencies.
 -include $(patsubst %.o, $(DEPDIR)/%.Po, $(all_object_files))
 
+# Disable implicit make rules.
+include $(srcdir)/disable-implicit-rules.mk
+
 ### end of the gdb Makefile.in.
diff --git a/gdb/data-directory/Makefile.in b/gdb/data-directory/Makefile.in
index 51b5c2a..d27765e 100644
--- a/gdb/data-directory/Makefile.in
+++ b/gdb/data-directory/Makefile.in
@@ -412,3 +412,6 @@ MAKEOVERRIDES=
 
 Makefile: $(srcdir)/Makefile.in $(top_builddir)/config.status
 	cd $(top_builddir) && $(MAKE) data-directory/Makefile
+
+# Disable implicit make rules.
+include $(srcdir)/../disable-implicit-rules.mk
diff --git a/gdb/disable-implicit-rules.mk b/gdb/disable-implicit-rules.mk
new file mode 100644
index 0000000..d797882
--- /dev/null
+++ b/gdb/disable-implicit-rules.mk
@@ -0,0 +1,10 @@
+# An empty .SUFFIXES target clears the suffixes list, which will skip lookups
+# of many implicit rules, speeding up the execution of make.
+.SUFFIXES:
+
+# Disable some implicit pattern rules.
+%: %,v
+%: RCS/%,v
+%: RCS/%
+%: s.%
+%: SCCS/s.%
diff --git a/gdb/doc/Makefile.in b/gdb/doc/Makefile.in
index 70af0fe..be63c87 100644
--- a/gdb/doc/Makefile.in
+++ b/gdb/doc/Makefile.in
@@ -676,3 +676,6 @@ maintainer-clean realclean: distclean
 install: install-info install-man
 
 uninstall: uninstall-info uninstall-man
+
+# Disable implicit make rules.
+include $(srcdir)/../disable-implicit-rules.mk
diff --git a/gdb/gdbserver/Makefile.in b/gdb/gdbserver/Makefile.in
index 5316c95..e17cf70 100644
--- a/gdb/gdbserver/Makefile.in
+++ b/gdb/gdbserver/Makefile.in
@@ -1079,4 +1079,7 @@ endif
 # Dependencies.
 -include $(patsubst %.o, $(DEPDIR)/%.Po, $(all_object_files))
 
+# Disable implicit make rules.
+include $(srcdir)/../disable-implicit-rules.mk
+
 # This is the end of "Makefile.in".
diff --git a/gdb/testsuite/Makefile.in b/gdb/testsuite/Makefile.in
index 29897ab..ad1dab2 100644
--- a/gdb/testsuite/Makefile.in
+++ b/gdb/testsuite/Makefile.in
@@ -372,3 +372,6 @@ read1.so: lib/read1.c
 # Build the read1 machinery.
 .PHONY: read1
 read1: read1.so expect-read1
+
+# Disable implicit make rules.
+include $(srcdir)/../disable-implicit-rules.mk
-- 
2.10.0