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[PATCH] Avoid invoking ranlib on libbackend.a


This patch makes the 0.5GB libbackend.a get built via "ar rcs" instead
of via "ar rc" + "ranlib", if possible.  The problem with the latter is
that it's about twice as slow as the former and it makes libbackend.a
get written twice which is wasteful.  On my machine this optimization
reduces rebuild time in a --disable-bootstrap tree after touching tree.c
by around 25%, from ~28s to ~21s.  This optimization is only performed
when the current ar is GNU ar where we can be reasonably sure that the
"rcs" directive is supported.

Does this change look reasonable?  I am currently bootstrapping it on
x86_64-pc-linux-gnu.

gcc/ChangeLog:

	* configure.ac (ar_is_gnu): New variable.  AC_SUBST it.
	* configure: Regenerate.
	* Makefile.in (AR_IS_GNU): New variable.
	(USE_AR_RCS): New variable.
	(libbackend.a): When building this archive, if USE_AR_RCS then
	just invoke "ar rcs" instead of invoking "ar rc" followed by
	"ranlib".
---
 gcc/Makefile.in  | 18 ++++++++++++++++++
 gcc/configure    | 15 +++++++++++++--
 gcc/configure.ac |  8 ++++++++
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 0786fa3..5ae6100 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -237,10 +237,22 @@ FLEX = @FLEX@
 FLEXFLAGS =
 AR = @AR@
 AR_FLAGS = rc
+AR_IS_GNU = @ar_is_gnu@
 NM = @NM@
 RANLIB = @RANLIB@
 RANLIB_FLAGS = @ranlib_flags@
 
+# Whether invocations of ar + ranlib can be replaced by a single
+# invocation of "ar rcs".
+USE_AR_RCS = no
+ifeq ($(AR_IS_GNU),yes)
+ifeq ($(AR_FLAGS),rc)
+ifeq ($(RANLIB_FLAGS),)
+USE_AR_RCS = yes
+endif
+endif
+endif
+
 # Libraries to use on the host.
 HOST_LIBS = @HOST_LIBS@
 
@@ -1882,8 +1894,14 @@ compilations: $(BACKEND)
 # This archive is strictly for the host.
 libbackend.a: $(OBJS)
 	-rm -rf libbackend.a
+	@# Elide the invocation of ranlib if possible, as doing so
+	@# significantly reduces the build time of this archive.
+ifeq ($(USE_AR_RCS),yes)
+	$(AR) $(AR_FLAGS)s libbackend.a $(OBJS)
+else
 	$(AR) $(AR_FLAGS) libbackend.a $(OBJS)
 	-$(RANLIB) $(RANLIB_FLAGS) libbackend.a
+endif
 
 libcommon-target.a: $(OBJS-libcommon-target)
 	-rm -rf libcommon-target.a
diff --git a/gcc/configure b/gcc/configure
index ed44472..db761c8 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -749,6 +749,7 @@ CXXDEPMODE
 DEPDIR
 am__leading_dot
 doc_build_sys
+ar_is_gnu
 AR
 NM
 BISON
@@ -8459,6 +8460,16 @@ fi
 
 fi
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ar is GNU ar" >&5
+$as_echo_n "checking whether ar is GNU ar... " >&6; }
+ar_is_gnu=no
+if $AR --version 2>/dev/null | sed 1q | grep "GNU ar" > /dev/null; then
+  ar_is_gnu=yes
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ar_is_gnu" >&5
+$as_echo "$ar_is_gnu" >&6; }
+
+
 # The jit documentation looks better if built with sphinx, but can be
 # built with texinfo if sphinx is not available.
 # Set "doc_build_sys" to "sphinx" or "texinfo" accordingly.
@@ -18475,7 +18486,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 18478 "configure"
+#line 18489 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -18581,7 +18592,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 18584 "configure"
+#line 18595 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 086d0fc..3749b21 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -1081,6 +1081,14 @@ else
   AC_CHECK_PROG(AR, ar, ar, ${CONFIG_SHELL-/bin/sh} ${srcdir}/../missing ar)
 fi
 
+AC_MSG_CHECKING(whether ar is GNU ar)
+ar_is_gnu=no
+if $AR --version 2>/dev/null | sed 1q | grep "GNU ar" > /dev/null; then
+  ar_is_gnu=yes
+fi
+AC_MSG_RESULT($ar_is_gnu)
+AC_SUBST(ar_is_gnu)
+
 # The jit documentation looks better if built with sphinx, but can be
 # built with texinfo if sphinx is not available.
 # Set "doc_build_sys" to "sphinx" or "texinfo" accordingly.
-- 
2.9.2.321.g3e7e728


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