toolchain: add link-time-optimization support

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 Kümmel <syntheticpp@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

diff --git a/package/binutils/Config.in.host b/package/binutils/Config.in.host
index d0ed84e0591da6425ae81b376080e6ba2c180e71..4049cf6a3e99d4ec1c43539925fccacfabb65d26 100644 (file)
--- 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 9e992533f3bf525dd3294c43f278728a117a6c13..ac62f1f0d20a1c3e0ff4d26bad7840645cec1ffc 100644 (file)
--- 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 e07d881444e8663859922004de5984e828079b2f..1a5281ca5be896cb99889c4fc47c8d00dec784d2 100644 (file)
--- 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 ffac15c7869f0d74f812b3c7143deb3aa6cf101b..b5d2ddba1ec862305dfb6753fa7f961939d902e4 100644 (file)
--- 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 78138d370ad0fd3a7b95d76bafbde2503a34ecc5..612c49d2e174f309010eb37669de99feba2319b8 100644 (file)
--- 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; \
                        ;; \