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

[Karl Berry]

Hi Bruno,

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

Ok. I just hope those weird-looking %:: rules do not cause trouble with
other makes. I guess we'll find out.

The fnoc test also failed due to the new rules. I fixed that, added a
few words of documentation and NEWS, and pushed it.

Thanks Bruno. -k


2023-07-20  Bruno Haible  <bruno@clisp.org>

automake: disable GNU make's internal pattern rules, for speed.

>From https://bugs.gnu.org/64743.

* 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 to avoid matching the new %:: rules.
* t/fnoc.sh: Update test to avoid matching the new %:: SCCS rule.

* NEWS: Mention this.
* doc/automake.texi (Suffixes): Likewise.
(Doc changes written by Karl.)
diff --git a/NEWS b/NEWS
index c64d4ef15..ced0fd640 100644
--- a/NEWS
+++ b/NEWS
@@ -21,6 +21,9 @@ New in 1.17:
   - Variables using escaped \# will trigger portability warnings, but be
     retained when appended.  GNU Make & BSD Makes are known to support it.

+  - GNU Make's default pattern rules are disabled, for speed and debugging.
+    (.SUFFIXES was already cleared.) (bug#64743)
+
   - For Texinfo documents, if a .texi.in file exists, but no .texi, the
     .texi.in will be read. Texinfo source files need not be present at
     all, and if present, need not contain @setfilename. Then the file name
diff --git a/doc/automake.texi b/doc/automake.texi
index 4561d1948..3b4f06e73 100644
--- a/doc/automake.texi
+++ b/doc/automake.texi
@@ -10977,6 +10977,14 @@ Automake generate the suffix list for 
@code{.SUFFIXES}.  Any given
 @code{SUFFIXES} go at the start of the generated suffixes list, followed
 by Automake generated suffixes not already in the list.

+@c https://bugs.gnu.org/64743.
+@cindex built-in suffix and pattern rules, disabled
+@cindex suffix rules of Make, disabled
+@cindex pattern rules of GNU Make, disabled
+Automake disables the Make program's built-in rules with a
+@code{.SUFFIXES:} rule, and then adds whatever suffixes are
+necessary.  Automake also disables GNU Make's built-in pattern rules.
+
 @node Include
 @chapter Include

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/fnoc.sh b/t/fnoc.sh
index 97b694c63..7d6b49096 100644
--- a/t/fnoc.sh
+++ b/t/fnoc.sh
@@ -35,5 +35,10 @@ END
 $ACLOCAL
 $AUTOMAKE

-grep '[^F]CC' Makefile.in | grep -v MKDEP && exit 1
+# In bug#64743, explicit rules were added to disable make's default
+# built-in rules, including one for SCCS:
+# %:: SCCS/s.%
+# So don't match that. Meanwhile, MKDEP does not occur in the output
+# any more, but leaving it in in case it comes back.
+grep '[^F]CC' Makefile.in | grep -v MKDEP | grep -v SCCS && exit 1
 exit 0
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

 :

compile finished at Thu Jul 20 18:00:35 2023