Re-introduce ccache support (#863)

Recent change 9a4ba81 removed ccache
support from the build process. This commit re-introduces ccache support
and updates GN files accordingly. Since ccache only supports
compilation, updates have been made to the compiler prefixes.
Additionally, assertions have been added to ensure ccache is not used on
Windows or in conjunction with RBE.

Fixes flutter/flutter#149100

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Author: bc-lee

build/toolchain/android/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sysroot.gni")  # Imports android/config.gni.
+import("//build/toolchain/ccache.gni")
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 import("//build/toolchain/rbe.gni")
@@ -56,6 +57,11 @@ template("android_toolchain") {
       assembler_prefix = ""
       compiler_prefix = string_join(" ", compiler_args)
       link_prefix = ""
+    } else if (use_ccache) {
+      # ccache only supports compilation, not linking.
+      assembler_prefix = "ccache "
+      compiler_prefix = ""
+      link_prefix = ""
     } else {
       compiler_prefix = ""
       link_prefix = ""

build/toolchain/ccache.gni

@@ -19,7 +19,12 @@
 # speed, you can do:
 # export CCACHE_CPP2=yes
 
+import("//build/toolchain/rbe.gni") # for use_rbe
+
 declare_args() {
   # Set to true to enable ccache.  Probably doesn't work on windows.
   use_ccache = false
 }
+
+assert(!is_win || !use_ccache, "ccache is not supported on Windows")
+assert(!use_ccache || !use_rbe, "ccache is not supported with RBE")

build/toolchain/fuchsia/BUILD.gn

@@ -4,13 +4,18 @@
 
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/rbe.gni")
+import("//build/toolchain/ccache.gni")
 import("//build/toolchain/toolchain.gni")
 
 if (use_rbe) {
   compiler_args =
       rewrapper_command + [ "--labels=type=compile,compiler=clang,lang=cpp " ]
   compiler_prefix = string_join(" ", compiler_args)
   link_prefix = ""
+} else if (use_ccache) {
+  # ccache only supports compilation, not linking.
+  compiler_prefix = "ccache"
+  link_prefix = ""
 } else {
   compiler_prefix = ""
   link_prefix = ""

build/toolchain/linux/BUILD.gn

@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/sysroot.gni")
+import("//build/toolchain/ccache.gni")
 import("//build/toolchain/gcc_toolchain.gni")
 import("//build/toolchain/rbe.gni")
 import("//build/toolchain/toolchain.gni")
@@ -23,6 +24,10 @@ if (use_rbe) {
   ]
   compiler_prefix = string_join(" ", compiler_args)
   link_prefix = ""
+} else if (use_ccache) {
+  # ccache only supports compilation, not linking.
+  compiler_prefix = "ccache "
+  link_prefix = ""
 } else {
   compiler_prefix = ""
   link_prefix = ""

build/toolchain/mac/BUILD.gn

@@ -10,6 +10,7 @@ import("//build/config/sysroot.gni")
 import("//build/toolchain/clang.gni")
 import("//build/toolchain/clang_static_analyzer.gni")
 import("//build/toolchain/rbe.gni")
+import("//build/toolchain/ccache.gni")
 import("//build/toolchain/toolchain.gni")
 
 if (host_cpu == "arm64") {
@@ -20,7 +21,12 @@ if (host_cpu == "arm64") {
       rebase_path("$buildtools_path/mac-x64/clang/bin", root_build_dir)
 }
 
-if (use_rbe) {
+if (use_ccache) {
+  # ccache only supports compilation, not linking.
+  cxx_prefix = "ccache "
+  objc_prefix = "ccache "
+  link_prefix = ""
+} else if (use_rbe) {
   remote_wrapper =
       rebase_path("//flutter/build/rbe/remote_wrapper.sh", root_build_dir)
   local_wrapper =

build/toolchain/wasm.gni

@@ -17,6 +17,13 @@ declare_args() {
 
 em_config_path = "$emsdk_dir/.emscripten"
 
+if (use_ccache) {
+  # ccache only supports compilation, not linking.
+  compiler_prefix = "ccache "
+} else {
+  compiler_prefix = ""
+}
+
 template("wasm_toolchain") {
   gcc_toolchain(target_name) {
     extra_toolchain_args = {
@@ -27,9 +34,9 @@ template("wasm_toolchain") {
 
     # emsdk_dir and em_config are defined in wasm.gni.
     ar = "$emsdk_dir/upstream/emscripten/emar --em-config $em_config_path"
-    cc = "$emsdk_dir/upstream/emscripten/emcc --em-config $em_config_path"
-    cxx = "$emsdk_dir/upstream/emscripten/em++ --em-config $em_config_path"
-    asm = cc
+    cc = "$compiler_prefix$emsdk_dir/upstream/emscripten/emcc --em-config $em_config_path"
+    cxx = "$compiler_prefix$emsdk_dir/upstream/emscripten/em++ --em-config $em_config_path"
+    asm = "$emsdk_dir/upstream/emscripten/emcc --em-config $em_config_path"
 
     # emscripten emits this .worker.js file conditionally depending on whether
     # pthreads are on or not. Unfortunately, there is no way to conditionally
@@ -39,7 +46,7 @@ template("wasm_toolchain") {
     # dummy file. If the target does use pthreads, this dummy file will be
     # overwritten. If the target does not, the dummy file will satisfy the
     # toolchain's requirement that it has this as an output.
-    ld = "$cxx"
+    ld = "$emsdk_dir/upstream/emscripten/em++ --em-config $em_config_path"
     readelf = "readelf"
     nm = "nm"