From 57e522065a9d4163ff9eb828546794d85dbcfd48 Mon Sep 17 00:00:00 2001
From: "Daniel J. Hofmann" <daniel@trvx.org>
Date: Sat, 5 Sep 2015 00:12:13 +0200
Subject: [PATCH] Add linker optimizations and dead code and data elimination.
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Linkers also have options we can configure! The most usefull feature is
to give every function its own section. This results in some bloat at
compile time, but at link time now the linker can do dead code and data
elimination by simply discarding appropriate sections.

This works by splitting the `.text` section in a way that makes it
possible to later only pull in sections that are actually referenced.

That is, the basic idea is to keep the matching between sections and
functions intact, so we can optimize based on it in the linking stage.

Note: there's still an issue with how `libOSRM.a` gets build. CMake
currently passes the linker flags on to ar, in order to create a static
library. But ar does not understand the linker's flags.

Referenes:

- https://sourceware.org/binutils/docs/ld/Options.html#Options
- http://elinux.org/images/2/2d/ELC2010-gc-sections_Denys_Vlasenko.pdfMCþ"
- http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_EXE_LINKER_FLAGS.html
- http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_MODULE_LINKER_FLAGS.html
- http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_SHARED_LINKER_FLAGS.html
- http://www.cmake.org/cmake/help/v3.0/variable/CMAKE_STATIC_LINKER_FLAGS.html
---
 CMakeLists.txt | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 17b57c15d..d54d46b8e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -190,6 +190,28 @@ elseif(${CMAKE_CXX_COMPILER_ID} STREQUAL "MSVC")
   target_link_libraries(osrm-extract wsock32 ws2_32)
 endif()
 
+# Configuring linker
+execute_process(COMMAND ${CMAKE_CXX_COMPILER} "-Wl,--version" ERROR_QUIET OUTPUT_VARIABLE LINKER_VERSION)
+# For ld.gold and ld.bfs (the GNU linkers) we optimize hard
+if("${LINKER_VERSION}" MATCHES "GNU gold" OR "${LINKER_VERSION}" MATCHES "GNU ld")
+  message(STATUS "Setting linker optimizations")
+  if(NOT ${CMAKE_CXX_COMPILER_ID} STREQUAL "MSVC")
+    # Tell compiler to put every function in separate section, linker can then match sections and functions
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -ffunction-sections -fdata-sections")
+    # Tell linker to do dead code and data eminination during link time discarding sections
+    set(LINKER_FLAGS "${LINKER_FLAGS} -Wl,--gc-sections")
+  endif()
+  # Default linker optimization flags
+  set(LINKER_FLAGS "${LINKER_FLAGS} -Wl,-O1 -Wl,--hash-style=gnu -Wl,--sort-common")
+else()
+  message(STATUS "Using unknown linker, not setting linker optimizations")
+endif ()
+set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${LINKER_FLAGS}")
+# XXX(daniel-j-h): building libosrm.a invokes AR passing on the linker flags, but AR does not understand them
+#set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} ${LINKER_FLAGS}")
+set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${LINKER_FLAGS}")
+set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${LINKER_FLAGS}")
+
 # Activate C++11
 if(NOT ${CMAKE_CXX_COMPILER_ID} STREQUAL "MSVC")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ")