[llvm] [cmake] Pass arguments to ccache on Windows (PR #218791)

[Martin Storsjö via llvm-commits]

================
@@ -335,48 +335,42 @@ if(LLVM_CCACHE_BUILD)
     set(LLVM_CCACHE_PARAMS "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros"
         CACHE STRING "Parameters to pass through to ccache")
 
-    if(NOT CMAKE_HOST_WIN32)
-      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 ${CCACHE_PROGRAM})
-
-      if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-        # ccache with PCH can lead to false-positives when only a macro
-        # definition changes with non-Clang compilers, because macro definitions
-        # are not compared in preprocessed mode.
-        # See: https://github.com/ccache/ccache/issues/1668
-        if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
-          message(NOTICE "Using ccache with precompiled headers with non-Clang "
+    if (LLVM_CCACHE_MAXSIZE)
+      set(LLVM_CCACHE_PARAMS "CCACHE_MAXSIZE=${LLVM_CCACHE_MAXSIZE} ${LLVM_CCACHE_PARAMS}")
+    endif()
+    if (LLVM_CCACHE_DIR)
+      set(LLVM_CCACHE_PARAMS "CCACHE_DIR=${LLVM_CCACHE_DIR} ${LLVM_CCACHE_PARAMS}")
+    endif()
+
+    if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Clang")
+      # ccache with PCH can lead to false-positives when only a macro
+      # definition changes with non-Clang compilers, because macro definitions
+      # are not compared in preprocessed mode.
+      # See: https://github.com/ccache/ccache/issues/1668
+      if(NOT DEFINED CMAKE_DISABLE_PRECOMPILE_HEADERS)
+        message(NOTICE "Using ccache with precompiled headers with non-Clang "
             "compilers is not supported. CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON. "
             "Pass -DCMAKE_DISABLE_PRECOMPILE_HEADERS=OFF to override this.")
-          set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
-        elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-          message(WARNING "Using ccache with precompiled headers with non-Clang "
+        set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
+      elseif(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
+        message(WARNING "Using ccache with precompiled headers with non-Clang "
             "compilers is not supported.")
-        endif()
       endif()
+    endif()
+
+    if (NOT CMAKE_HOST_WIN32)
+      # On platforms other than Windows pass the arguments as an env var to support older ccache versions
+      set(CCACHE_PROGRAM "${LLVM_CCACHE_PARAMS} ${CCACHE_PROGRAM}")
     else()
-      # Until a way to reliably configure ccache on Windows is found,
-      # disable precompiled headers for Windows + ccache builds
-      if(NOT CMAKE_DISABLE_PRECOMPILE_HEADERS)
-        message(WARNING "Using ccache with precompiled headers on Windows is currently not supported.
-          CMAKE_DISABLE_PRECOMPILE_HEADERS will be set to ON.")
-        set(CMAKE_DISABLE_PRECOMPILE_HEADERS "ON")
-      endif()
-      if(LLVM_CCACHE_MAXSIZE OR LLVM_CCACHE_DIR OR
-         NOT LLVM_CCACHE_PARAMS MATCHES "CCACHE_CPP2=yes CCACHE_HASHDIR=yes CCACHE_SLOPPINESS=pch_defines,time_macros")
-        message(FATAL_ERROR "Ccache configuration through CMake is not supported on Windows. Please use environment variables.")
-      endif()
+      # Since ccache 4.8 it is possible to pass options as arguments
+      set(CCACHE_PROGRAM "${CCACHE_PROGRAM} ${LLVM_CCACHE_PARAMS}")
----------------
mstorsjo wrote:

If you have mismatched options, they are just silently ignored. And in order to observe it with a PCH build, you'd have to rebuild and see if it actually ends up speeding anything up.

You can observe the issue like this:
```
$ CCACHE_DEBUG=1 ccache gcc test/hello.c -o hello; ls hello.*.ccache-log; rm -f hello.*.ccache-log
hello.20260827_113131_789033.ccache-log
$ ccache CCACHE_DEBUG=1 gcc test/hello.c -o hello; ls hello.*.ccache-log; rm -f hello.*.ccache-log
ls: cannot access 'hello.*.ccache-log': No such file or directory
$ ccache debug=true gcc test/hello.c -o hello; ls hello.*.ccache-log; rm -f hello.*.ccache-log
hello.20260827_113140_264271.ccache-log
```

So here the env variable `CCACHE_DEBUG=1` does have effect, likewise does passing `debug=true` as an argument, but passing `CCACHE_DEBUG=1` as an argument is silently ignored and has no observeable effect.

https://github.com/llvm/llvm-project/pull/218791