Merge pull request #1625 from evelikov/gc-sections

Instrument gcc/clang to discard unused code during link

3 files changed, 45 insertions, 23 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")
diff --git a/Makefile.am b/Makefile.am
index cc8ae29c..9469cf69 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -284,7 +284,7 @@ endif
 
 # -no-undefined marks that libarchive doesn't rely on symbols
 # defined in the application.  This is mandatory for cygwin.
-libarchive_la_LDFLAGS= -no-undefined -version-info $(ARCHIVE_LIBTOOL_VERSION)
+libarchive_la_LDFLAGS= -no-undefined -version-info $(ARCHIVE_LIBTOOL_VERSION) $(GC_SECTIONS)
 libarchive_la_LIBADD= $(LTLIBICONV)
 
 # Manpages to install
@@ -1024,7 +1024,7 @@ endif
 
 bsdtar_LDADD= libarchive.la libarchive_fe.la $(LTLIBICONV)
 bsdtar_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdtar_ccstatic) $(PLATFORMCPPFLAGS)
-bsdtar_LDFLAGS= $(bsdtar_ldstatic)
+bsdtar_LDFLAGS= $(bsdtar_ldstatic) $(GC_SECTIONS)
 
 bsdtar_EXTRA_DIST= \
 	tar/bsdtar.1 \
@@ -1190,7 +1190,7 @@ endif
 
 bsdcpio_LDADD= libarchive_fe.la libarchive.la $(LTLIBICONV)
 bsdcpio_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdcpio_ccstatic) $(PLATFORMCPPFLAGS)
-bsdcpio_LDFLAGS= $(bsdcpio_ldstatic)
+bsdcpio_LDFLAGS= $(bsdcpio_ldstatic) $(GC_SECTIONS)
 
 bsdcpio_EXTRA_DIST= \
 	cpio/bsdcpio.1 \
@@ -1344,7 +1344,7 @@ endif
 
 bsdcat_LDADD= libarchive_fe.la libarchive.la $(LTLIBICONV)
 bsdcat_CPPFLAGS= -I$(top_srcdir)/libarchive -I$(top_srcdir)/libarchive_fe $(bsdcat_ccstatic) $(PLATFORMCPPFLAGS)
-bsdcat_LDFLAGS= $(bsdcat_ldstatic)
+bsdcat_LDFLAGS= $(bsdcat_ldstatic) $(GC_SECTIONS)
 
 bsdcat_EXTRA_DIST= \
 	cat/bsdcat.1 \
diff --git a/configure.ac b/configure.ac
index 85868ade..f9fe624e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -542,6 +542,22 @@ fi
 # Checks for supported compiler flags
 AX_APPEND_COMPILE_FLAGS([-Wall -Wformat -Wformat-security])
 
+# Place the functions and data into separate sections, allowing the linker
+# to garbage collect the unused ones.
+save_LDFLAGS=$LDFLAGS
+LDFLAGS="$LDFLAGS -Wl,--gc-sections"
+AC_MSG_CHECKING([whether ld supports --gc-sections])
+AC_LINK_IFELSE(
+    [AC_LANG_SOURCE([static char UnusedFunc() { return 5; } int main() { return 0;}])],
+    [AC_MSG_RESULT([yes])
+        GC_SECTIONS="-Wl,--gc-sections";
+        AX_APPEND_COMPILE_FLAGS([-ffunction-sections -fdata-sections])],
+    [AC_MSG_RESULT([no])
+        GC_SECTIONS="";])
+LDFLAGS=$save_LDFLAGS
+
+AC_SUBST(GC_SECTIONS)
+
 # Checks for typedefs, structures, and compiler characteristics.
 AC_C_CONST
 # la_TYPE_UID_T defaults to "int", which is incorrect for MinGW