Build llvm with ccache if package is present

[sgundapa]

Summary

This patch has the following changes

The CMake variable LLVM_CCACHE_BUILD is set to OFF by default.

Set this to ON for a ccache enabled build

CCACHE_CPP2 is required to compile the source file directly instead

of compiling the preprocessed file. This will help WERROR is turned ON
for a host clang compiler

The below two options makes more sense in the context of a buildbot

CCACHE_HASHDIR is required to maintain the separate cached data across

builders. This will also help the debuggers to point to the correct source
location

CCACHE_SIZE is important in the perspective of buildbot to increase the

limit on the amount of data to hold in cache for faster compilation

CCACHE_DIR is used to save the cached data to a specific directory.

[beanz]

A few small comments below, but mostly good here.

CMakeLists.txt

83

I think the CMAKE_SYSTEM_NAME check here is unnecessary. ccache works on FreeBSD, OS X, and probably under MingW32, so I suspect if the program is present it should just be used.

I'm not sure how ccache behaves on Windows, so you might need to ensure that if you're on windows you're also on MingW32, which would be something like:

if(CCACHE_PROGRAM AND LLVM_CCACHE_BUILD AND (NOT WINDOWS OR MINGW32 OR CYGWIN))

I also think it would be good to log an error if LLVM_CCACHE_BUILD=On and CCACHE_PROGRAM is not found.

[sgundapa]

Agree. Will fix the patch and push a new one in a while

[sgundapa]

Also, we might not need "NOT WINDOWS OR MINGW32 OR CYGWIN" and find_program is going to take care of it

[sgundapa]

ping!

[sgundapa]

Ping!

[beanz]

LGTM.

[sgundapa]

I have merged this change as r277389.

CMakeLists.txt

if(LLVM_PARALLEL_COMPILE_JOBS)
   if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
     message(WARNING "Job pooling is only available with Ninja generators.")
   else()
     set_property(GLOBAL APPEND PROPERTY JOB_POOLS compile_job_pool=${LLVM_PARALLEL_COMPILE_JOBS})
     set(CMAKE_JOB_POOL_COMPILE compile_job_pool)
   endif()
 endif()
 
+# Build llvm with ccache if the package is present
+set(LLVM_CCACHE_BUILD OFF CACHE BOOL "Set to ON for a ccache enabled build")
+if(LLVM_CCACHE_BUILD)
+  find_program(CCACHE_PROGRAM ccache)
+  if(CCACHE_PROGRAM)
+      set(LLVM_CCACHE_SIZE "" CACHE STRING "Size of ccache")
+      set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
+      set(CCACHE_PROGRAM "CCACHE_CPP2=yes CCACHE_HASHDIR=yes ${CCACHE_PROGRAM}")
+      if (LLVM_CCACHE_SIZE)
+        set(CCACHE_PROGRAM "CCACHE_SIZE=${LLVM_CCACHE_SIZE} ${CCACHE_PROGRAM}")
+      endif()
+      if (LLVM_CCACHE_DIR)
+        set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
+      endif()
+      set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
+  else()
+    message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
+  endif()
+endif()
+
 set(LLVM_BUILD_GLOBAL_ISEL OFF CACHE BOOL "Experimental: Build GlobalISel")
 if(LLVM_BUILD_GLOBAL_ISEL)
   add_definitions(-DLLVM_BUILD_GLOBAL_ISEL)
 endif()
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs.")
 if(LLVM_PARALLEL_LINK_JOBS)