clang: Makes builds with clang less dependent on absolute file path

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/ccache.gni")
 
@@ skipping 292 matching lines @@
   if (linux_use_bundled_binutils) {
     cflags += [ "-B$binutils_path" ]
   }
 
   # Clang-specific compiler flags setup.
   # ------------------------------------
   if (is_clang) {
     cflags += [ "-fcolor-diagnostics" ]
   }
 
+  # Makes builds independent of absolute file path.
+  # Currently disabled for nacl since its toolchain is old and doesn't have this flag.
+  # TODO(zforman): Once nacl's toolchain is updated, remove check.
+  if (is_clang && !is_nacl) {
+    absolute_path = rebase_path("../../..")

[Zachary Forman, 2016/01/05 03:21:29]

Is there a GN builtin or otherwise that allows us to get the root source
directory? I would use "//", but this has the unfortunate effect of giving a
rebased path that ends with a '/'. As some symbols include only the directory
name (no trailing slash), having a trailing slash in the prefix map misses some
cases.

[Nico, 2016/01/05 12:22:48]

^ brettw

[brettw, 2016/01/05 19:10:11]

I think you can do:
  rebase_path("//.")
The thing happening here is that GN tries to match whether the input ends in a
slash or not so that concatenating paths works the same before and after
rebasing, but for the root dir this is ambiguous.

[Zachary Forman, 2016/01/06 23:20:49]

This worked - thanks!

+    cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
+  }
+
   # C++11 compiler flags setup.
   # ---------------------------
   if (is_linux || is_android || (is_nacl && is_clang)) {
     # gnu++11 instead of c++11 is needed because some code uses typeof() (a
     # GNU extension).
     # TODO(thakis): Eventually switch this to c++11 instead,
     # http://crbug.com/427584
     cflags_cc += [ "-std=gnu++11" ]
   } else if (!is_win && !is_nacl) {
     # TODO(mcgrathr) - the NaCl GCC toolchain doesn't support either gnu++11
@@ skipping 984 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }