[Buildroot] [git commit] toolchain: add link-time-optimization support

[Buildroot] [git commit] toolchain: add link-time-optimization support

[Thomas Petazzoni]

commit: http://git.buildroot.net/buildroot/commit/?id=814f63ec32585f281855587fe0a287cee32e4232
branch: http://git.buildroot.net/buildroot/commit/?id=refs/heads/master

Add a new option BR2_GCC_ENABLE_LTO that builds gcc and binutils with
LTO support.

Individual packages still have to enable LTO explicitly by passing '-flto' to
GCC, which passes it on to the linker. This option does not add that flag
globally. Some packages detect if the compiler supports LTO and enable the flag
if it does.

To support LTO, ar and ranlib must be called with an argument which triggers the
usage of the LTO plugin. Since GCC doesn't call these tools itself, it instead
provides wrappers for ar and ranlib that pass the LTO arguments. This way
existing Makefiles don't need to be changed for LTO support. However, these
wrappers are called <tuple>-gcc-ar which matches the pattern to link to the
buildroot wrapper in the external toolchain logic. So the external toolchain
logic is updated to provide the correct symlink.

[Thomas:
  - Add a separate BR2_BINUTILS_ENABLE_LTO option to enable LTO
    support in binutils. This is a blind option, selected by
    BR2_GCC_ENABLE_LTO. It just avoids having binutils.mk poke
    directly into gcc Config.in options.
  - Remove the check on the AVR32 special gcc version, which we don't
    support anymore.
  - Adapt the help text of the LTO Config.in option to no longer
    mention "Since version 4.5", since we only support gcc >= 4.5 in
    Buildroot anyway.
  - Fix typo in toolchain-external.mk comment.]

Signed-off-by: Peter K??mmel <syntheticpp@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---
 package/binutils/Config.in.host                    |    3 +++
 package/binutils/binutils.mk                       |    4 ++++
 package/gcc/Config.in.host                         |    7 +++++++
 package/gcc/gcc.mk                                 |    4 ++++
 toolchain/toolchain-external/toolchain-external.mk |    8 ++++++++
 5 files changed, 26 insertions(+), 0 deletions(-)

diff --git a/package/binutils/Config.in.host b/package/binutils/Config.in.host
index d0ed84e..4049cf6 100644
--- a/package/binutils/Config.in.host
+++ b/package/binutils/Config.in.host
@@ -38,6 +38,9 @@ config BR2_BINUTILS_VERSION
 	default "2.24"		if BR2_BINUTILS_VERSION_2_24
 	default "2.25"		if BR2_BINUTILS_VERSION_2_25
 
+config BR2_BINUTILS_ENABLE_LTO
+	bool
+
 config BR2_BINUTILS_EXTRA_CONFIG_OPTIONS
 	string "Additional binutils options"
 	default ""
diff --git a/package/binutils/binutils.mk b/package/binutils/binutils.mk
index 9e99253..ac62f1f 100644
--- a/package/binutils/binutils.mk
+++ b/package/binutils/binutils.mk
@@ -96,5 +96,9 @@ BINUTILS_PRE_PATCH_HOOKS += BINUTILS_XTENSA_PRE_PATCH
 HOST_BINUTILS_PRE_PATCH_HOOKS += BINUTILS_XTENSA_PRE_PATCH
 endif
 
+ifeq ($(BR2_BINUTILS_ENABLE_LTO),y)
+HOST_BINUTILS_CONF_OPTS += --enable-plugins --enable-lto
+endif
+
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))
diff --git a/package/gcc/Config.in.host b/package/gcc/Config.in.host
index e07d881..1a5281c 100644
--- a/package/gcc/Config.in.host
+++ b/package/gcc/Config.in.host
@@ -116,6 +116,13 @@ config BR2_GCC_ENABLE_TLS
 	  Enable the compiler to generate code for accessing
 	  thread local storage variables
 
+config BR2_GCC_ENABLE_LTO
+	bool "Enable compiler link-time-optimization support"
+	select BR2_BINUTILS_ENABLE_LTO
+	help
+	  This option enables link-time optimization (LTO) support in
+	  gcc.
+
 config BR2_GCC_ENABLE_OPENMP
 	bool "Enable compiler OpenMP support"
 	depends on !BR2_PTHREADS_NONE && !BR2_arc && !BR2_microblaze
diff --git a/package/gcc/gcc.mk b/package/gcc/gcc.mk
index ffac15c..b5d2ddb 100644
--- a/package/gcc/gcc.mk
+++ b/package/gcc/gcc.mk
@@ -132,6 +132,10 @@ else
 HOST_GCC_COMMON_CONF_OPTS += --disable-tls
 endif
 
+ifeq ($(BR2_GCC_ENABLE_LTO),y)
+HOST_GCC_COMMON_CONF_OPTS += --enable-plugins --enable-lto
+endif
+
 ifeq ($(BR2_GCC_ENABLE_LIBMUDFLAP),y)
 HOST_GCC_COMMON_CONF_OPTS += --enable-libmudflap
 else
diff --git a/toolchain/toolchain-external/toolchain-external.mk b/toolchain/toolchain-external/toolchain-external.mk
index 78138d3..612c49d 100644
--- a/toolchain/toolchain-external/toolchain-external.mk
+++ b/toolchain/toolchain-external/toolchain-external.mk
@@ -647,12 +647,20 @@ endif
 # Build toolchain wrapper for preprocessor, C and C++ compiler and setup
 # symlinks for everything else. Skip gdb symlink when we are building our
 # own gdb to prevent two gdb's in output/host/usr/bin.
+# When the link-time-optimazation flag '-flto' is used, then the compiler
+# and binutils have to support lto. ar/ranlib need to be called with the
+# lto plugin. The wrappers *-gcc-ar and *-gcc-ranlib provided by GCC could
+# be used as drop-ins for ar/runlib when Makefiles are used which do not
+# pass the lto arguments.
 define TOOLCHAIN_EXTERNAL_INSTALL_WRAPPER
 	$(Q)$(call MESSAGE,"Building ext-toolchain wrapper")
 	mkdir -p $(HOST_DIR)/usr/bin; cd $(HOST_DIR)/usr/bin; \
 	for i in $(TOOLCHAIN_EXTERNAL_CROSS)*; do \
 		base=$${i##*/}; \
 		case "$$base" in \
+		*-ar|*-ranlib|*-nm) \
+			ln -sf $$(echo $$i | sed 's%^$(HOST_DIR)%../..%') .; \
+			;; \
 		*cc|*cc-*|*++|*++-*|*cpp) \
 			ln -sf ext-toolchain-wrapper $$base; \
 			;; \

[Buildroot] [git commit] toolchain: add link-time-optimization support

[Arnout Vandecappelle]

On 07/03/15 15:01, Thomas Petazzoni wrote:
> Individual packages still have to enable LTO explicitly by passing '-flto' to
> GCC, which passes it on to the linker. This option does not add that flag
> globally. Some packages detect if the compiler supports LTO and enable the flag
> if it does.

 Given that this new option (probably) changes how some packages are built, I
think there should be a few autobuilder configs that enable it. Any volunteers
to take care of that?

 Regards,
 Arnout
-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7CB5 E4CC 6C2E EFD4 6E3D A754 F963 ECAB 2450 2F1F

[Buildroot] [git commit] toolchain: add link-time-optimization support

[Thomas Petazzoni]

Dear Arnout Vandecappelle,

On Sat, 07 Mar 2015 17:14:29 +0100, Arnout Vandecappelle wrote:

>  Given that this new option (probably) changes how some packages are built, I
> think there should be a few autobuilder configs that enable it. Any volunteers
> to take care of that?

No: the commit only allows to enable LTO support in the compiler. It
does not start building packages with -flto, unless of course some
packages automatically detect the availability of the LTO feature in
the compiler and make use of it.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

[Buildroot] [git commit] toolchain: add link-time-optimization support

[Arnout Vandecappelle]

On 07/03/15 17:24, Thomas Petazzoni wrote:
> Dear Arnout Vandecappelle,
> 
> On Sat, 07 Mar 2015 17:14:29 +0100, Arnout Vandecappelle wrote:
> 
>>  Given that this new option (probably) changes how some packages are built, I
>> think there should be a few autobuilder configs that enable it. Any volunteers
>> to take care of that?
> 
> No: the commit only allows to enable LTO support in the compiler. It
> does not start building packages with -flto, unless of course some
> packages automatically detect the availability of the LTO feature in
> the compiler and make use of it.

 That's precisely what I mean. As far as I know, there isn't a single package
where we pass something like --disable-lto. Now, I'm not sure if there is any
package at all that will automatically turn on lto, but if there are then I
think we should build it.

 However, I just realized that we are already building with external sourcery
and linaro toolchains that support lto. So we already have the tests that I wanted.

 Regards,
 Arnout


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle
GPG fingerprint:  7CB5 E4CC 6C2E EFD4 6E3D A754 F963 ECAB 2450 2F1F

[Buildroot] [git commit] toolchain: add link-time-optimization support

[Thomas Petazzoni]

Dear Arnout Vandecappelle,

On Sat, 07 Mar 2015 17:35:17 +0100, Arnout Vandecappelle wrote:

>  That's precisely what I mean. As far as I know, there isn't a single package
> where we pass something like --disable-lto. Now, I'm not sure if there is any
> package at all that will automatically turn on lto, but if there are then I
> think we should build it.

Ok.

>  However, I just realized that we are already building with external sourcery
> and linaro toolchains that support lto. So we already have the tests that I wanted.

Right. But that shouldn't prevent us from enabling LTO once a while in
the autobuilders for internal toolchain builds, so your original idea
is still useful :)

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com