Merge pull request #1625 from evelikov/gc-sections

Instrument gcc/clang to discard unused code during link

1 files changed, 25 insertions, 19 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index c5c22a24..1e1ff575 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,8 @@
 #
 CMAKE_MINIMUM_REQUIRED(VERSION 2.8.12 FATAL_ERROR)
+if(POLICY CMP0065)
+  cmake_policy(SET CMP0065 NEW) #3.4 don't use `-rdynamic` with executables
+endif()
 if(POLICY CMP0074)
   cmake_policy(SET CMP0074 NEW) #3.12.0 `find_package()`` uses ``<PackageName>_ROOT`` variables.
 endif()
@@ -96,24 +99,8 @@ endif ()
 # Especially for early development, we want to be a little
 # aggressive about diagnosing build problems; this can get
 # relaxed somewhat in final shipping versions.
-IF (CMAKE_C_COMPILER_ID MATCHES "^GNU$")
-  SET(CMAKE_REQUIRED_FLAGS "-Wall -Wformat -Wformat-security")
-  #################################################################
-  # Set compile flags for all build types.
-  SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -Wformat -Wformat-security")
-  if (ENABLE_WERROR)
-    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Werror")
-  endif ()
-  #################################################################
-  # Set compile flags for debug build.
-  # This is added into CMAKE_C_FLAGS when CMAKE_BUILD_TYPE is "Debug"
-  SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wextra")
-  SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wunused")
-  SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wshadow")
-  SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wmissing-prototypes")
-  SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wcast-qual")
-ENDIF (CMAKE_C_COMPILER_ID MATCHES "^GNU$")
-IF (CMAKE_C_COMPILER_ID MATCHES "^Clang$")
+IF (CMAKE_C_COMPILER_ID MATCHES "^GNU$" OR
+    CMAKE_C_COMPILER_ID MATCHES "^Clang$")
   SET(CMAKE_REQUIRED_FLAGS "-Wall -Wformat -Wformat-security")
   #################################################################
   # Set compile flags for all build types.
@@ -130,7 +117,26 @@ IF (CMAKE_C_COMPILER_ID MATCHES "^Clang$")
   SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wshadow")
   SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wmissing-prototypes")
   SET(CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG} -Wcast-qual")
-ENDIF (CMAKE_C_COMPILER_ID MATCHES "^Clang$")
+  # Ideally this will be a compile/link time check, yet there's no obvious way
+  # how considering how old our minimum required cmake version is. The official
+  # cmake.org side does not host the manual pages even. Normally we can use
+  # either of the following two, yet neither is supported as of 3.0.2
+  # - check_linker_flag - does not exist
+  # - try_compile - does not support linker flags
+  #
+  # The CI fails with this on MacOS
+  IF(NOT CMAKE_SYSTEM_NAME MATCHES "Darwin")
+    # Place the functions and data into separate sections, allowing the linker
+    # to garbage collect the unused ones.
+    SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -ffunction-sections -fdata-sections")
+    SET(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -Wl,--gc-sections")
+    SET(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} -Wl,--gc-sections")
+    # Printing the discarded section is "too much", so enable on demand.
+    #SET(CMAKE_EXE_LINKER_FLAGS_DEBUG "${CMAKE_EXE_LINKER_FLAGS_DEBUG} -Wl,--print-gc-sections")
+    #SET(CMAKE_SHARED_LINKER_FLAGS_DEBUG "${CMAKE_SHARED_LINKER_FLAGS_DEBUG} -Wl,--print-gc-sections")
+  ENDIF(NOT CMAKE_SYSTEM_NAME MATCHES "Darwin")
+ENDIF (CMAKE_C_COMPILER_ID MATCHES "^GNU$" OR
+       CMAKE_C_COMPILER_ID MATCHES "^Clang$")
 IF (CMAKE_C_COMPILER_ID MATCHES "^XL$")
   SET(CMAKE_C_COMPILER "xlc_r")
   SET(CMAKE_REQUIRED_FLAGS "-qflag=e:e -qformat=sec")