build: add option to enable LTO

This patch adds an option to enable link time optimization.  In addition
to LTO option itself (-flto) fat-lto-objects are being used.  This is
because during the build pmdinfogen scans the generated ELF objects to
find this_pmd_name* symbol in symbol table.  Without fat-lto-objects gcc
produces ELF only with extra symbols for internal use during linking.

Signed-off-by: Andrzej Ostruszka <aostruszka@marvell.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>

.travis.yml

@@ -105,6 +105,15 @@ matrix:
       apt:
         packages:
           - *extra_packages
+  - env: DEF_LIB="static" OPTS="-Db_lto=true" EXTRA_PACKAGES=1 CC=gcc-7
+    compiler: gcc
+    addons:
+      apt:
+        sources:
+            - ubuntu-toolchain-r-test
+        packages:
+          - *extra_packages
+          - gcc-7
 
 
 script: ./.ci/${TRAVIS_OS_NAME}-build.sh

config/common_base

@@ -49,6 +49,11 @@ CONFIG_RTE_FORCE_INTRINSICS=n
 #
 CONFIG_RTE_ARCH_STRICT_ALIGN=n
 
+#
+# Enable link time optimization
+#
+CONFIG_RTE_ENABLE_LTO=n
+
 #
 # Compile to share library
 #

config/meson.build

@@ -225,3 +225,16 @@ add_project_arguments('-D_GNU_SOURCE', language: 'c')
 if is_freebsd
 	add_project_arguments('-D__BSD_VISIBLE', language: 'c')
 endif
+
+if get_option('b_lto')
+	if cc.has_argument('-ffat-lto-objects')
+		add_project_arguments('-ffat-lto-objects', language: 'c')
+	else
+		error('compiler does not support fat LTO objects - please turn LTO off')
+	endif
+	# workaround for gcc bug 81440
+	if cc.get_id() == 'gcc' and cc.version().version_compare('<8.0')
+		add_project_arguments('-Wno-lto-type-mismatch', language: 'c')
+		add_project_link_arguments('-Wno-lto-type-mismatch', language: 'c')
+	endif
+endif

doc/guides/prog_guide/index.rst

@@ -65,5 +65,6 @@ Programmer's Guide
     ext_app_lib_make_help
     perf_opt_guidelines
     writing_efficient_code
+    lto
     profile_app
     glossary

doc/guides/prog_guide/lto.rst

@@ -0,0 +1,43 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2019 Marvell International Ltd.
+
+Link Time Optimization
+======================
+
+The DPDK supports compilation with link time optimization turned on.
+This depends obviously on the ability of the compiler to do "whole
+program" optimization at link time and is available only for compilers
+that support that feature.
+To be more specific, compiler (in addition to performing LTO) have to
+support creation of ELF objects containing both normal code and internal
+representation (called fat-lto-objects in gcc and icc).
+This is required since during build some code is generated by parsing
+produced ELF objects (pmdinfogen).
+
+The amount of performance gain that one can get from LTO depends on the
+compiler and the code that is being compiled.
+However LTO is also useful for additional code analysis done by the
+compiler.
+In particular due to interprocedural analysis compiler can produce
+additional warnings about variables that might be used uninitialized.
+Some of these warnings might be "false positives" though and you might
+need to explicitly initialize variable in order to silence the compiler.
+
+Please note that turning LTO on causes considerable extension of
+build time.
+
+When using make based build, link time optimization can be enabled for
+the whole DPDK by setting:
+
+.. code-block:: console
+
+    CONFIG_ENABLE_LTO=y
+
+in config file.
+
+For the meson based build it can be enabled by setting meson built-in
+'b_lto' option:
+
+.. code-block:: console
+
+    meson build -Db_lto=true

doc/guides/rel_notes/release_19_11.rst

@@ -248,6 +248,15 @@ New Features
   traditional l2fwd example. It demonstrates usage of poll and event mode IO
   mechanism under a single application.
 
+* **Added build support for Link Time Optimization.**
+
+  LTO is an optimization technique used by the compiler to perform whole
+  program analysis and optimization at link time.  In order to do that
+  compilers store their internal representation of the source code that
+  the linker uses at the final stage of compilation process.
+
+  See :doc:`../prog_guide/lto` for more information:
+
 
 Removed Items
 -------------

mk/toolchain/gcc/rte.toolchain-compat.mk

@@ -88,6 +88,10 @@ else
 		MACHINE_CFLAGS := $(filter-out -march% -mtune% -msse%,$(MACHINE_CFLAGS))
 	endif
 
+	ifeq ($(shell test $(GCC_VERSION) -lt 74 && echo 1), 1)
+		CONFIG_RTE_ENABLE_LTO=n
+	endif
+
 	# Disable thunderx PMD for gcc < 4.7
 	ifeq ($(shell test $(GCC_VERSION) -lt 47 && echo 1), 1)
 		CONFIG_RTE_LIBRTE_THUNDERX_NICVF_PMD=d

mk/toolchain/gcc/rte.vars.mk

@@ -62,6 +62,18 @@ endif
 # process cpu flags
 include $(RTE_SDK)/mk/toolchain/$(RTE_TOOLCHAIN)/rte.toolchain-compat.mk
 
+ifeq ($(CONFIG_RTE_ENABLE_LTO),y)
+# 'fat-lto' is used since pmdinfogen needs to have 'this_pmd_nameX'
+# exported in symbol table and without this option only internal
+# representation is present.
+TOOLCHAIN_CFLAGS += -flto -ffat-lto-objects
+TOOLCHAIN_LDFLAGS += -flto
+# workaround for GCC bug 81440
+ifeq ($(shell test $(GCC_VERSION) -lt 80 && echo 1), 1)
+WERROR_FLAGS += -Wno-lto-type-mismatch
+endif
+endif
+
 # workaround GCC bug with warning "missing initializer" for "= {0}"
 ifeq ($(shell test $(GCC_VERSION) -lt 47 && echo 1), 1)
 WERROR_FLAGS += -Wno-missing-field-initializers

mk/toolchain/icc/rte.vars.mk

@@ -54,5 +54,13 @@ endif
 # process cpu flags
 include $(RTE_SDK)/mk/toolchain/$(RTE_TOOLCHAIN)/rte.toolchain-compat.mk
 
+ifeq ($(CONFIG_RTE_ENABLE_LTO),y)
+# 'fat-lto' is used since pmdinfogen needs to have 'this_pmd_nameX'
+# exported in symbol table and without this option only internal
+# representation is present.
+TOOLCHAIN_CFLAGS += -flto -ffat-lto-objects
+TOOLCHAIN_LDFLAGS += -flto
+endif
+
 export CC AS AR LD OBJCOPY OBJDUMP STRIP READELF
 export TOOLCHAIN_CFLAGS TOOLCHAIN_LDFLAGS TOOLCHAIN_ASFLAGS