Re: RFC: PATCH to avoid linking multiple front ends at once with parallel make

[Jason Merrill <jason at redhat dot com>]

On 05/15/2013 03:51 AM, Alexandre Oliva wrote:
> I borrowed the general approach from libtool's shell locking machinery.
> I suppose it might be the case that lock files (in libtool's case) get
> removed twice, but I'm pretty sure there was broad portability testing
> in the process that led to that construct.

It looks like libtool doesn't trap "signal" 0, which is why there isn't 
the redundancy there.  autoconf has separate traps for 0 and other signals.

In my testing, bash seems to consistently run the 0 trap when it exits 
due to a signal; are there other shells that don't?

> > The extra exit here also seems unnecessary.
>
> Wouldn't the exit status of an interrupted or failed compilation be that
> of the successful rmdir rather than what you'd get without the wrapper?

It shouldn't be: POSIX says "The value of "$?" after the trap action 
completes shall be the value it had before trap was invoked."

> the script is ran with $(SHELL), which IIRC is typically /bin/sh, not
> necessarily bash.

I thought that 'trap' was specific to bash; now that I see it's part of 
POSIX, I'll make the rest of the script more portable.

How does this look?

commit bb484cdad5e0d79176134a4eef8e81092448739a
Author: Jason Merrill <jason@redhat.com>
Date:   Thu Apr 25 17:54:17 2013 -0400

    	* Makefile.in (LLINKER): New variable.
    	(mostlyclean): Remove link mutex.
    	* configure.ac: Handle --enable-link-mutex.
    	* lock-and-run.sh: New script.

diff --git a/gcc/lock-and-run.sh b/gcc/lock-and-run.sh
new file mode 100644
index 0000000..012a35f
--- /dev/null
+++ b/gcc/lock-and-run.sh
@@ -0,0 +1,29 @@
+#! /bin/sh
+# Shell-based mutex using mkdir.
+
+lockdir=$1 prog=$2; shift 2 || exit 1
+count=0
+# Remember when we started trying to acquire the lock.
+touch lock-stamp.$$
+trap 'rm -r "$lockdir" lock-stamp.$$' 0
+until mkdir "$lockdir" 2>/dev/null; do
+    # Say something periodically so the user knows what's up.
+    if [ `expr $count % 30` = 0 ]; then
+	# Reset if the lock has been renewed.
+	if [ -n "`find \"$lockdir\" -newer lock-stamp.$$`" ]; then
+	    touch lock-stamp.$$
+	    count=1
+	# Steal the lock after 5 minutes.
+	elif [ $count == 300 ]; then
+	    echo removing stale $lockdir >&2
+	    rm -r "$lockdir"
+	else
+	    echo waiting to acquire $lockdir >&2
+	fi
+    fi
+    sleep 1
+    count=`expr $count + 1`
+done
+echo $prog "$@"
+$prog "$@"
+# The trap runs on exit.
diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 903125e..23e2926 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -235,6 +235,13 @@ LINKER = $(CC)
 LINKER_FLAGS = $(CFLAGS)
 endif
 
+# Like LINKER, but use a mutex for serializing front end links.
+ifeq (@DO_LINK_MUTEX@,true)
+LLINKER = $(SHELL) $(srcdir)/lock-and-run.sh linkfe.lck $(LINKER)
+else
+LLINKER = $(LINKER)
+endif
+
 # -------------------------------------------
 # Programs which operate on the build machine
 # -------------------------------------------
@@ -4529,6 +4536,8 @@ mostlyclean: lang.mostlyclean
 	-rm -f gtype.state
 # Delete genchecksum outputs
 	-rm -f *-checksum.c
+# Delete lock-and-run bits
+	-rm -rf linkfe.lck lock-stamp.*
 
 # Delete all files made by compilation
 # that don't exist in the distribution.
diff --git a/gcc/ada/gcc-interface/Make-lang.in b/gcc/ada/gcc-interface/Make-lang.in
index ef12b4b..4fed34f 100644
--- a/gcc/ada/gcc-interface/Make-lang.in
+++ b/gcc/ada/gcc-interface/Make-lang.in
@@ -185,6 +185,7 @@ endif
 GCC_LINKERFLAGS = $(filter-out -Werror, $(ALL_LINKERFLAGS))
 
 GCC_LINK=$(LINKER) $(GCC_LINKERFLAGS) $(LDFLAGS)
+GCC_LLINK=$(LLINKER) $(GCC_LINKERFLAGS) $(LDFLAGS)
 
 # Lists of files for various purposes.
 
@@ -562,7 +563,8 @@ TARGET_ADA_SRCS =
 # Since the RTL should be built with the latest compiler, remove the
 #  stamp target in the parent directory whenever gnat1 is rebuilt
 gnat1$(exeext): $(TARGET_ADA_SRCS) $(GNAT1_OBJS) $(ADA_BACKEND) libcommon-target.a $(LIBDEPS)
-	+$(GCC_LINK) -o $@ $(GNAT1_OBJS) $(ADA_BACKEND) libcommon-target.a $(LIBS) $(SYSLIBS) $(BACKENDLIBS) $(CFLAGS)
+	+$(GCC_LLINK) -o $@ $(GNAT1_OBJS) $(ADA_BACKEND) \
+	  libcommon-target.a $(LIBS) $(SYSLIBS) $(BACKENDLIBS) $(CFLAGS)
 	$(RM) stamp-gnatlib2-rts stamp-tools
 
 gnatbind$(exeext): ada/b_gnatb.o $(CONFIG_H) $(GNATBIND_OBJS) ggc-none.o libcommon-target.a $(LIBDEPS)
diff --git a/gcc/c/Make-lang.in b/gcc/c/Make-lang.in
index 8310e0a..47aa4cb 100644
--- a/gcc/c/Make-lang.in
+++ b/gcc/c/Make-lang.in
@@ -75,7 +75,7 @@ cc1-checksum.c : build/genchecksum$(build_exeext) checksum-options \
 cc1-checksum.o : cc1-checksum.c $(CONFIG_H) $(SYSTEM_H)
 
 cc1$(exeext): $(C_OBJS) cc1-checksum.o $(BACKEND) $(LIBDEPS)
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ $(C_OBJS) \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ $(C_OBJS) \
 	  cc1-checksum.o $(BACKEND) $(LIBS) $(BACKENDLIBS)
 #
 # Build hooks:
diff --git a/gcc/configure b/gcc/configure
index 39e911c..1f03eac 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -670,6 +670,7 @@ subdirs
 dollar
 gcc_tooldir
 enable_lto
+DO_LINK_MUTEX
 MAINT
 zlibinc
 zlibdir
@@ -916,6 +917,7 @@ with_long_double_128
 with_gc
 with_system_zlib
 enable_maintainer_mode
+enable_link_mutex
 enable_version_specific_runtime_libs
 enable_plugin
 enable_libquadmath_support
@@ -1627,6 +1629,8 @@ Optional Features:
   --enable-maintainer-mode
                           enable make rules and dependencies not useful (and
                           sometimes confusing) to the casual installer
+  --enable-link-mutex     avoid linking multiple front-ends at once to avoid
+                          thrashing on the build machine
   --enable-version-specific-runtime-libs
                           specify that runtime libraries should be installed
                           in a compiler-specific directory
@@ -17830,7 +17834,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 17833 "configure"
+#line 17837 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -17936,7 +17940,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 17939 "configure"
+#line 17943 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -27061,6 +27065,26 @@ else
   MAINT='#'
 fi
 
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether to avoid linking multiple front-ends at once" >&5
+$as_echo_n "checking whether to avoid linking multiple front-ends at once... " >&6; }
+  # Check whether --enable-link-mutex was given.
+if test "${enable_link_mutex+set}" = set; then :
+  enableval=$enable_link_mutex; do_link_mutex=$enableval
+else
+  do_link_mutex=no
+fi
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $do_link_mutex" >&5
+$as_echo "$do_link_mutex" >&6; }
+
+if test "$do_link_mutex" = "yes"; then
+   DO_LINK_MUTEX=true
+else
+   DO_LINK_MUTEX=false
+fi
+
+
 # --------------
 # Language hooks
 # --------------
diff --git a/gcc/configure.ac b/gcc/configure.ac
index a6cdf24..855affd 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -4958,6 +4958,24 @@ else
 fi
 AC_SUBST(MAINT)dnl
 
+dnl Whether to prevent multiple front-ends from linking at the same time
+
+AC_MSG_CHECKING([whether to avoid linking multiple front-ends at once])
+  AC_ARG_ENABLE(link-mutex,
+[AS_HELP_STRING([--enable-link-mutex],
+		[avoid linking multiple front-ends at once to avoid thrashing
+		 on the build machine])],
+      do_link_mutex=$enableval,
+      do_link_mutex=no)
+AC_MSG_RESULT($do_link_mutex)
+
+if test "$do_link_mutex" = "yes"; then
+   DO_LINK_MUTEX=true
+else
+   DO_LINK_MUTEX=false
+fi
+AC_SUBST(DO_LINK_MUTEX)
+
 # --------------
 # Language hooks
 # --------------
diff --git a/gcc/cp/Make-lang.in b/gcc/cp/Make-lang.in
index cda4897..df8ed3e 100644
--- a/gcc/cp/Make-lang.in
+++ b/gcc/cp/Make-lang.in
@@ -100,7 +100,7 @@ cc1plus-checksum.c : build/genchecksum$(build_exeext) checksum-options \
 cc1plus-checksum.o : cc1plus-checksum.c $(CONFIG_H) $(SYSTEM_H)
 
 cc1plus$(exeext): $(CXX_OBJS) cc1plus-checksum.o $(BACKEND) $(LIBDEPS)
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 	      $(CXX_OBJS) cc1plus-checksum.o $(BACKEND) $(LIBS) $(BACKENDLIBS)
 
 ifeq ($(ENABLE_MAINTAINER_RULES), true)
diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
index 1265129..03fedd1 100644
--- a/gcc/doc/install.texi
+++ b/gcc/doc/install.texi
@@ -1293,6 +1293,11 @@ opposite effect.  If neither option is specified, the configure script
 will try to guess whether the @code{.init_array} and
 @code{.fini_array} sections are supported and, if they are, use them.
 
+@item --enable-link-mutex
+When building GCC, use a mutex to avoid linking the compilers for
+multiple languages at the same time, to avoid thrashing on build
+systems with limited free memory.  The default is not to use such a mutex.
+
 @item --enable-maintainer-mode
 The build rules that regenerate the Autoconf and Automake output files as
 well as the GCC master message catalog @file{gcc.pot} are normally
diff --git a/gcc/fortran/Make-lang.in b/gcc/fortran/Make-lang.in
index c3f826c..ee70423 100644
--- a/gcc/fortran/Make-lang.in
+++ b/gcc/fortran/Make-lang.in
@@ -98,7 +98,7 @@ gfortran-cross$(exeext): gfortran$(exeext)
 # The compiler itself is called f951.
 f951$(exeext): $(F95_OBJS) \
 		$(BACKEND) $(LIBDEPS) attribs.o
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 		$(F95_OBJS) $(BACKEND) $(ZLIB) $(LIBS) attribs.o \
 		$(BACKENDLIBS)
 
diff --git a/gcc/go/Make-lang.in b/gcc/go/Make-lang.in
index dd98080..5fe4083 100644
--- a/gcc/go/Make-lang.in
+++ b/gcc/go/Make-lang.in
@@ -76,7 +76,7 @@ GO_OBJS = \
 	go/unsafe.o
 
 go1$(exeext): $(GO_OBJS) attribs.o $(BACKEND) $(LIBDEPS)
-	+$(CXX) $(ALL_CXXFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 	      $(GO_OBJS) attribs.o $(BACKEND) $(LIBS) $(BACKENDLIBS)
 
 # Documentation.
diff --git a/gcc/java/Make-lang.in b/gcc/java/Make-lang.in
index 7ed3220..8a6210f 100644
--- a/gcc/java/Make-lang.in
+++ b/gcc/java/Make-lang.in
@@ -99,7 +99,7 @@ jvspec.o-warn = -Wno-error
 
 jc1$(exeext): $(JAVA_OBJS) $(BACKEND) $(LIBDEPS) attribs.o
 	rm -f $@
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 		$(JAVA_OBJS) $(BACKEND) $(ZLIB) $(LIBICONV) $(LIBS) attribs.o $(BACKENDLIBS)
 
 jcf-dump$(exeext): $(JCFDUMP_OBJS) $(LIBDEPS)
diff --git a/gcc/lto/Make-lang.in b/gcc/lto/Make-lang.in
index a0bdc26..22df32c 100644
--- a/gcc/lto/Make-lang.in
+++ b/gcc/lto/Make-lang.in
@@ -71,7 +71,7 @@ lto.stagefeedback:
 lto-warn = $(STRICT_WARN)
 
 $(LTO_EXE): $(LTO_OBJS) $(BACKEND) $(LIBDEPS)
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 		$(LTO_OBJS) $(BACKEND) $(BACKENDLIBS) $(LIBS)
 
 # Dependencies
diff --git a/gcc/objc/Make-lang.in b/gcc/objc/Make-lang.in
index 9e005c1..f04d606 100644
--- a/gcc/objc/Make-lang.in
+++ b/gcc/objc/Make-lang.in
@@ -68,7 +68,7 @@ cc1obj-checksum.c : build/genchecksum$(build_exeext) checksum-options \
 cc1obj-checksum.o : cc1obj-checksum.c $(CONFIG_H) $(SYSTEM_H)
 
 cc1obj$(exeext): $(OBJC_OBJS) $(C_AND_OBJC_OBJS) cc1obj-checksum.o $(BACKEND) $(LIBDEPS)
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 	      $(OBJC_OBJS) $(C_AND_OBJC_OBJS) cc1obj-checksum.o \
 	      $(BACKEND) $(LIBS) $(BACKENDLIBS)
 
diff --git a/gcc/objcp/Make-lang.in b/gcc/objcp/Make-lang.in
index 731ca9e..ec10fc8 100644
--- a/gcc/objcp/Make-lang.in
+++ b/gcc/objcp/Make-lang.in
@@ -72,7 +72,7 @@ cc1objplus-checksum.c : build/genchecksum$(build_exeext) checksum-options \
 cc1objplus-checksum.o : cc1objplus-checksum.c $(CONFIG_H) $(SYSTEM_H)
 
 cc1objplus$(exeext): $(OBJCXX_OBJS) cc1objplus-checksum.o $(BACKEND) $(LIBDEPS)
-	+$(LINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
+	+$(LLINKER) $(ALL_LINKERFLAGS) $(LDFLAGS) -o $@ \
 		$(OBJCXX_OBJS) cc1objplus-checksum.o $(BACKEND) $(LIBS) $(BACKENDLIBS)
 
 # Objective C++ language specific files.