win: Reorganize some build flags.

- Move global win-only flags from //build/config/compiler to
  //build/config/win:compiler which the former already references if is_win

- Move /FC from the toolchain to //build/config/win:compiler too: The
  toolchain isn't where random compiler flags should be added.

- This allows moving msvc_use_absolute_paths from a gni file to a gn file,
  so do that as well.

No intended behavior change.

Bug: none
Change-Id: I1b35a4242649105e005b94df911849ebf44d7c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1541636
Commit-Queue: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644799}

build/config/compiler/BUILD.gn

@@ -276,14 +276,6 @@ config("compiler") {
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
   } else {
-    cflags += [
-      # Assume UTF-8 by default to avoid code page dependencies.
-      "/utf-8",
-    ]
-    if (is_clang) {
-      # Don't look for includes in %INCLUDE%.
-      cflags += [ "/X" ]
-    }
   }
 
   if (is_clang && is_debug) {
@@ -493,13 +485,6 @@ config("compiler") {
     ]
   }
 
-  # Print absolute paths in diagnostics. There is no precedent for doing this
-  # on Linux/Mac (GCC doesn't support it), but MSVC does this with /FC and
-  # Windows developers rely on it (crbug.com/636109) so only do this on Windows.
-  if (msvc_use_absolute_paths && is_clang && is_win) {
-    cflags += [ "-fdiagnostics-absolute-paths" ]
-  }
-
   # C11/C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang) || current_os == "aix") {

build/config/win/BUILD.gn

@@ -45,6 +45,12 @@ declare_args() {
   #  and with this switch, clang emits it like this:
   #    foo/bar.cc:12:34: error: something went wrong
   use_clang_diagnostics_format = false
+
+  # Use absolute file paths in the compiler diagnostics and (for non-clang)
+  # __FILE__ macro if needed. Note that enabling this makes your build
+  # dependent your checkout's path, which means you won't get to use goma's
+  # global cache, and without clang your builds won't be deterministic.
+  msvc_use_absolute_paths = false
 }
 
 # This is included by reference in the //build/config/compiler config that
@@ -66,6 +72,7 @@ config("compiler") {
     "/Gy",  # Enable function-level linking.
     "/FS",  # Preserve previous PDB behavior.
     "/bigobj",  # Some of our files are bigger than the regular limits.
+    "/utf-8",  # Assume UTF-8 by default to avoid code page dependencies.
   ]
 
   # Force C/C++ mode for the given GN detected file type. This is necessary
@@ -79,7 +86,28 @@ config("compiler") {
     "/Zc:sizedDealloc-",
   ]
 
+  if (msvc_use_absolute_paths) {
+    # Pass /FC flag to the compiler if needed.
+    cflags += "/FC "
+
+    # Print absolute paths in diagnostics. There is no precedent for doing this
+    # on Linux/Mac (GCC doesn't support it), but MSVC does this with /FC and
+    # Windows developers rely on it (crbug.com/636109) so only do this on
+    # Windows.
+    # TODO(thakis): This comment no longer really make sense after
+    # https://chromium-review.googlesource.com/c/chromium/src/+/558871/
+    # See if we can remove msvc_use_absolute_paths. See also discussion in
+    # https://reviews.llvm.org/D23816
+    if (is_clang) {
+      cflags += [ "-fdiagnostics-absolute-paths" ]
+    }
+  }
+
   if (is_clang) {
+    # Don't look for includes in %INCLUDE%.
+    cflags += [ "/X" ]
+
+    # Tell clang which version of MSVC to emulate.
     cflags += [ "-fmsc-version=1911" ]
 
     if (is_component_build) {

build/toolchain/toolchain.gni

@@ -24,10 +24,6 @@ declare_args() {
 
   # Used for binary size analysis.
   generate_linker_map = is_android && is_official_build
-
-  # Use absolute file paths in the compiler diagnostics and __FILE__ macro
-  # if needed.
-  msvc_use_absolute_paths = false
 }
 
 if (generate_linker_map) {

build/toolchain/win/BUILD.gn

@@ -165,13 +165,6 @@ template("msvc_toolchain") {
       sys_lib_flags = ""
     }
 
-    clflags = ""
-
-    # Pass /FC flag to the compiler if needed.
-    if (msvc_use_absolute_paths) {
-      clflags += "/FC "
-    }
-
     tool("cc") {
       precompiled_header_type = "msvc"
       pdbname = "{{target_out_dir}}/{{label_name}}_c.pdb"
@@ -185,7 +178,7 @@ template("msvc_toolchain") {
         "$object_subdir/{{source_name_part}}.obj",
       ]
 
-      command = "$env_wrapper$cl /nologo /showIncludes ${clflags} $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
+      command = "$env_wrapper$cl /nologo /showIncludes $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_c}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
     }
 
     tool("cxx") {
@@ -201,7 +194,7 @@ template("msvc_toolchain") {
         "$object_subdir/{{source_name_part}}.obj",
       ]
 
-      command = "$env_wrapper$cl /nologo /showIncludes ${clflags} $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
+      command = "$env_wrapper$cl /nologo /showIncludes $sys_include_flags{{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}} /c {{source}} /Fo{{output}} /Fd\"$pdbname\""
     }
 
     tool("rc") {