Subject: bug#64743: Speed up GNU make's internal processing

From: Bruno Haible <[email-masked]>
Date: Thu, 20 Jul 2023 16:07:53 +0200

Hi,

Paul Smith, the GNU make maintainer, gives this recommendation how
to disable built-in rules and thus speed up make's processing [1]:

 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
My recommendation has been to disable the built-in rules directly, if
you don't need them.  For example you can use:

  .SUFFIXES:

to disable most of the built in rules (this is a POSIX standard
facility so it's helpful even for other versions of make).

Unfortunately that doesn't fix all problems because GNU Make also has a
few built-in rules that are defined using pattern rules (because suffix
rules are not powerful enough).  So a full list of "turn it all off"
would be this:

  .SUFFIXES:
  %:: %,v
  %:: RCS/%,v
  %:: RCS/%
  %:: s.%
  %:: SCCS/s.%
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

GNU Automake already emits the '.SUFFIXES:' line. To optimize things
for GNU make, it should also emit the remaining part.

It has no effect with non-GNU make [2].

The effect, for example in gettext's gettext-runtime/src directory, is
that
  - The output of "make -n -d" shrinks from 11028 lines to 4928 lines.
  - The number of 'stat()' calls made by "make -n" shrinks from 188 to 178.
    (make no longer tests whether various directories have an 'RCS' or 'SCCS'
    subdirectory.)

Patch is attached.

Bruno

[1] https://lists.gnu.org/archive/html/bug-make/2023-07/msg00063.html
[2] https://lists.gnu.org/archive/html/bug-make/2023-07/msg00067.html
>From c8ca5b95a0c322177ba0fb55ddabc1c92fa04f10 Mon Sep 17 00:00:00 2001
From: Bruno Haible <[email-masked]>
Date: Thu, 20 Jul 2023 11:50:51 +0200
Subject: [PATCH] Speed up GNU make's internal processing.

Based on a recommendation by Paul Smith in
<https://lists.gnu.org/archive/html/bug-make/2023-07/msg00063.html>.

* lib/am/footer.am: Disable GNU make's internal pattern rules.
* lib/Automake/Rule.pm (_conditionals_for_rule): Add special handling
for these pattern rules from footer.am.
(define): Likewise.
* t/nodep.sh: Update test.
---
 lib/Automake/Rule.pm | 39 +++++++++++++++++++++++----------------
 lib/am/footer.am     | 11 +++++++++++
 t/nodep.sh           |  2 +-
 3 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/lib/Automake/Rule.pm b/lib/Automake/Rule.pm
index 9f72d2728..52ee3be4a 100644
--- a/lib/Automake/Rule.pm
+++ b/lib/Automake/Rule.pm
@@ -695,6 +695,9 @@ sub _conditionals_for_rule ($$$$)
 
   return $cond if !$message; # No ambiguity.
 
+  # Don't coalesce the several pattern rules from footer.am into a single one.
+  return $cond if $target eq "%:" && $where->get =~ /\/am\/footer\.am$/;
+
   if ($owner == RULE_USER)
     {
       # For user rules, just diagnose the ambiguity.
@@ -764,23 +767,27 @@ sub define ($$$$$)
 
   my $tdef = _rule_defn_with_exeext_awareness ($target, $cond, $where);
 
-  # A GNU make-style pattern rule has a single "%" in the target name.
-  msg ('portability', $where,
-       "'%'-style pattern rules are a GNU make extension")
-    if $target =~ /^[^%]*%[^%]*$/;
-
-  # See whether this is a duplicated target declaration.
-  if ($tdef)
+  # The pattern rules in footer.am look like duplicates, but really aren't.
+  if ($source !~ /\/am\/footer\.am$/)
     {
-      # Diagnose invalid target redefinitions, if any.  Note that some
-      # target redefinitions are valid (e.g., for multiple-targets
-      # pattern rules).
-      _maybe_warn_about_duplicated_target ($target, $tdef, $source,
-                                           $owner, $cond, $where);
-      # Return so we don't redefine the rule in our tables, don't check
-      # for ambiguous condition, etc.  The rule will be output anyway
-      # because '&read_am_file' ignores the return code.
-      return ();
+      # A GNU make-style pattern rule has a single "%" in the target name.
+      msg ('portability', $where,
+           "'%'-style pattern rules are a GNU make extension")
+        if $target =~ /^[^%]*%[^%]*$/;
+
+      # See whether this is a duplicated target declaration.
+      if ($tdef)
+        {
+          # Diagnose invalid target redefinitions, if any.  Note that some
+          # target redefinitions are valid (e.g., for multiple-targets
+          # pattern rules).
+          _maybe_warn_about_duplicated_target ($target, $tdef, $source,
+                                               $owner, $cond, $where);
+          # Return so we don't redefine the rule in our tables, don't check
+          # for ambiguous condition, etc.  The rule will be output anyway
+          # because '&read_am_file' ignores the return code.
+          return ();
+        }
     }
 
   my $rule = _crule $target;
diff --git a/lib/am/footer.am b/lib/am/footer.am
index 9715c826c..388defb14 100644
--- a/lib/am/footer.am
+++ b/lib/am/footer.am
@@ -17,3 +17,14 @@
 # Tell versions [3.59,3.63) of GNU make to not export all variables.
 # Otherwise a system limit (for SysV at least) may be exceeded.
 .NOEXPORT:
+
+# Tell GNU make to disable its built-in pattern rules.
+## This reduces make's internal processing.
+## Recommended by Paul Smith in
+## <https://lists.gnu.org/archive/html/bug-make/2023-07/msg00063.html>.
+## These rules have no effect on non-GNU make implementations.
+%:: %,v
+%:: RCS/%,v
+%:: RCS/%
+%:: s.%
+%:: SCCS/s.%
diff --git a/t/nodep.sh b/t/nodep.sh
index 4201c8ead..0174b838e 100644
--- a/t/nodep.sh
+++ b/t/nodep.sh
@@ -37,6 +37,6 @@ $ACLOCAL
 $AUTOMAKE
 
 sed 's/printf .*%s//' Makefile.in > Makefile.tmp
-grep '%' Makefile.tmp && exit 1
+grep -v '^%::' Makefile.tmp | grep '%' && exit 1
 
 :
-- 
2.34.1