[ccache] Add Windows support

[NathanielMcVicar]

Summary

Windows builds can't receive environment variables on the command line or make use of RULE_LAUNCH_COMPILE with ccache.

[Ericson2314]

I know nothing about ccache on Windows, but if it didn't work before, I think it is pretty clear that this at least won't cause any sort of regression.

[Ericson2314]

So ideally someone else reviews this, but if not, +1 from me as at least no worse than before.

[stella.stamenova]

LGTM

llvm/CMakeLists.txt

195

Can you add a comment that we can't use RULE_LAUNCH_COMPILE on Windows even though in theory it should be able to work?

[NathanielMcVicar]

Add comment for RULE_LAUNCH_COMPILE on Windows

[NathanielMcVicar]

Fix merge base

llvm/CMakeLists.txt

 endif()
 unset(SHOULD_ENABLE_PROJECT)
 
 # 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_MAXSIZE "" CACHE STRING "Size of ccache")
-      set(LLVM_CCACHE_DIR "" CACHE STRING "Directory to keep ccached data")
-      set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes"
-          CACHE STRING "Parameters to pass through to ccache")
 
+    if(NOT CMAKE_SYSTEM_NAME MATCHES "Windows")
       set(CCACHE_PROGRAM "${LLVM_CCACHE_PARAMS} ${CCACHE_PROGRAM}")
       if (LLVM_CCACHE_MAXSIZE)
         set(CCACHE_PROGRAM "CCACHE_MAXSIZE=${LLVM_CCACHE_MAXSIZE} ${CCACHE_PROGRAM}")
       endif()
       if (LLVM_CCACHE_DIR)
         set(CCACHE_PROGRAM "CCACHE_DIR=${LLVM_CCACHE_DIR} ${CCACHE_PROGRAM}")
       endif()
       set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ${CCACHE_PROGRAM})
     else()
+      if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
+         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes")
+        message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
+      endif()
+      # RULE_LAUNCH_COMPILE should work with Ninja but currently has issues
+      # with cmd.exe and some MSVC tools other than cl.exe
+      set(CMAKE_C_COMPILER_LAUNCHER ${CCACHE_PROGRAM})
+      set(CMAKE_CXX_COMPILER_LAUNCHER ${CCACHE_PROGRAM})     
+    endif()
+  else()
     message(FATAL_ERROR "Unable to find the program ccache. Set LLVM_CCACHE_BUILD to OFF")
   endif()
 endif()
 
 set(LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS "" CACHE STRING
   "Optional arguments for the native tool used in CMake --build invocations for external projects.")
 mark_as_advanced(LLVM_EXTERNAL_PROJECT_BUILD_TOOL_ARGS)