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Re: [PATCH] Avoid invoking ranlib on libbackend.a


On Sat, 16 Jul 2016, Patrick Palka wrote:

> 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
> 
> 

Even better, why build libbackend.a at all?  Why not just pass the list
of object files directly to the linker instead of going through an
intermediate archive?  The following patch cuts rebuild time by 50%,
from 28s to 14s by making libbackend.a no longer get built.  (The
Make-lang.in hunk is apparently necessary to avoid duplicate-reference
link errors since incpath.o is already listed in $(OBJS)).

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 5ae6100..7b8ec16 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1591,7 +1591,7 @@ endif
 # compilation or not.
 ALL_HOST_OBJS = $(ALL_HOST_FRONTEND_OBJS) $(ALL_HOST_BACKEND_OBJS)
 
-BACKEND = libbackend.a main.o libcommon-target.a libcommon.a \
+BACKEND = $(OBJS) main.o libcommon-target.a libcommon.a \
 	$(CPPLIB) $(LIBDECNUMBER)
 
 # This is defined to "yes" if Tree checking is enabled, which roughly means
diff --git a/gcc/cp/Make-lang.in b/gcc/cp/Make-lang.in
index cc3337d..d09b449 100644
--- a/gcc/cp/Make-lang.in
+++ b/gcc/cp/Make-lang.in
@@ -67,7 +67,7 @@ g++-cross$(exeext): xg++$(exeext)
 
 # The compiler itself.
 # Shared with C front end:
-CXX_C_OBJS = attribs.o incpath.o \
+CXX_C_OBJS = attribs.o \
 	$(C_COMMON_OBJS) $(CXX_TARGET_OBJS)
 
 # Language-specific object files for C++ and Objective C++.


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