Automatically compile with CCache and link using LLD if possible (#842)

* Automatically compile with CCache and link using LLD if possible

Author: vector-of-bool

CMakeLists.txt

@@ -41,12 +41,12 @@ option (ENABLE_RDTSCP
    "Fast performance counters on Intel using the RDTSCP instruction"
    OFF
 )
-option (ENABLE_APPLE_FRAMEWORK "Build libraries as frameworks on darwin platforms" OFF) 
+option (ENABLE_APPLE_FRAMEWORK "Build libraries as frameworks on darwin platforms" OFF)
 set (ENABLE_ICU AUTO CACHE STRING "Enable ICU support, necessary to use non-ASCII usernames or passwords, default AUTO.")
 option (ENABLE_UNINSTALL "Enable creation of uninstall script and associated uninstall build target." ON)
 set (ENABLE_CLIENT_SIDE_ENCRYPTION AUTO CACHE STRING "Enable Client-Side Field Level Encryption support. Requires libmongocrypt. Set to ON/AUTO/OFF, default AUTO.")
 set (ENABLE_MONGODB_AWS_AUTH AUTO CACHE STRING "Enable support for MONGODB-AWS authentication mechanism. Set to ON/AUTO/OFF, default AUTO. (also requires ENABLE_SSL not set to OFF)")
-option (ENABLE_PIC 
+option (ENABLE_PIC
    "Enables building of position independent code for static library components."
    ON
 )
@@ -70,6 +70,11 @@ set (CMAKE_MODULE_PATH
 include (InstallRequiredSystemLibraries)
 include (GNUInstallDirs)
 
+# Enable CCache, if possible
+include (CCache)
+# Link with LLD, if possible
+include (LLDLinker)
+
 set (BUILD_VERSION "0.0.0" CACHE STRING "Library version (for both libbson and libmongoc)")
 
 include (ParseVersion)

build/cmake/CCache.cmake

@@ -0,0 +1,22 @@
+#[[
+    This module enables CCache support by inserting a ccache executable as
+    the compiler launcher for C and C++ if there is a ccache executable availbale
+    on the system.
+
+    CCache support will be automatically enabled if it is found on the system.
+    CCache can be forced on or off by setting the MONGO_USE_CCACHE CMake option to
+    ON or OFF.
+]]
+
+# Find and enable ccache for compiling
+find_program (CCACHE_EXECUTABLE ccache)
+if (CCACHE_EXECUTABLE)
+    message (STATUS "Found ccache: ${CCACHE_EXECUTABLE}")
+    option (MONGO_USE_CCACHE "Use CCache when compiling" ON)
+endif ()
+
+if (MONGO_USE_CCACHE)
+    message (STATUS "Compiling with CCache enabled. Disable by setting MONGO_USE_CCACHE to OFF")
+    set (CMAKE_CXX_COMPILER_LAUNCHER "${CCACHE_EXECUTABLE}")
+    set (CMAKE_C_COMPILER_LAUNCHER "${CCACHE_EXECUTABLE}")
+endif ()

build/cmake/CMakeLists.txt

@@ -11,6 +11,8 @@ set (build_cmake_MODULES
    MongoCPackage.cmake
    ParseVersion.cmake
    SphinxBuild.cmake
+   CCache.cmake
+   LLDLinker.cmake
 )
 
 set_local_dist (build_cmake_DIST_local

build/cmake/LLDLinker.cmake

@@ -0,0 +1,60 @@
+#[[
+    This module conditionally enables -fuse-ld=lld if it is supported by the compiler.
+    This is purely for build performance, and has no apparent effect on the generated
+    code.
+
+    LLD is *significantly* faster to link and produces significantly better link-time
+    error diagnostics.
+
+    LLD linking will be automatically enabled if it is supported, but can be forced
+    on or off by setting the MONGO_USE_LLD CMake option to ON or OFF.
+]]
+
+if (NOT COMMAND add_link_options)
+    # This only works on new-enough versions of CMake that support LINK_OPTIONS as
+    # a separate configuration entity
+    return ()
+endif ()
+
+include (CMakePushCheckState)
+include (CheckCSourceCompiles)
+cmake_push_check_state (RESET)
+    # Newer GNU compilers support lld with the '-fuse-ld=lld' flag.
+    set (CMAKE_REQUIRED_FLAGS "-fuse-ld=lld")
+    set (CMAKE_REQUIRED_LINK_OPTIONS "-fuse-ld=lld")
+    check_c_source_compiles ([[
+        #include <stdio.h>
+
+        int main (void) {
+            puts ("Hello, world!");
+            return 0;
+        }
+    ]] HAVE_LLD_LINKER_SUPPORT)
+cmake_pop_check_state ()
+
+
+if (HAVE_LLD_LINKER_SUPPORT)
+    # Expose an option to toggle usage of lld
+    option (MONGO_USE_LLD "Link runtime binaries using LLVM's lld linker" ON)
+elseif (NOT DEFINED _MONGO_LD_LLD_LINKER)
+    # We don't have -fuse-lld support, but that might be because of a misconfig in
+    # the environment. Issue a *one-time* diagnostic telling the user if they *almost*
+    # have LLD support. This branch is only for diagnostic purposes.
+    find_program (_MONGO_LD_LLD_LINKER
+        NAMES ld.lld ld.lld-13 ld.lld-12 ld.lld-11 ld.lld-10 ld.lld-9 ld.lld-8 ld.lld-7 ld.lld-6 ld.lld-5
+        )
+    mark_as_advanced (_MONGO_LD_LLD_LINKER)
+    # If we found one, we are compiling with GCC, *and* the found lld has a version suffix, issue a message
+    # telling the user how they might be able to get lld work.
+    if (_MONGO_LD_LLD_LINKER AND CMAKE_C_COMPILER_ID STREQUAL "GNU" AND NOT _MONGO_LD_LLD_LINKER MATCHES "ld.lld$")
+        message (STATUS "NOTE: A GNU-compatible lld linker was found (${_MONGO_LD_LLD_LINKER}), but support from GCC requires that")
+        message (STATUS "  the 'ld.lld' binary be named *exactly* 'ld.lld' (without a version suffix!)")
+        message (STATUS "  To enable 'lld' support, try creating a symlink of 'ld.lld' somewhere on your PATH that points")
+        message (STATUS "  to '${_MONGO_LD_LLD_LINKER}'")
+    endif ()
+endif ()
+
+if (MONGO_USE_LLD)
+    message (STATUS "Linking using LLVM lld. Disable by setting MONGO_USE_LLD to OFF")
+    add_link_options (-fuse-ld=lld)
+endif ()