Pass -no-canonical-prefixes to clang by default.

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/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
@@ skipping 419 matching lines @@
 
   # Makes builds independent of absolute file path.
   # clang-cl (used if is_win) doesn't expose this flag.
   # Currently disabled for nacl since its toolchain lacks this flag (too old).
   # TODO(zforman): Once nacl's toolchain is updated, remove check.
   if (is_clang && is_linux && strip_absolute_paths_from_debug_symbols) {
     absolute_path = rebase_path("//.")
     cflags += [ "-fdebug-prefix-map=$absolute_path=." ]
   }
 
+  # Tells the compiler not to pass the default paths to the tools it invokes.

[Nico, 2017/04/25 23:21:07]

No, it tells the compiler to pass the default paths as relative paths instead of
as absolute paths:

thakis@thakis:~/src/llvm-build-nolibcxx$ diff <(bin/clang -c test.cc -### 2>&1 |
tr " " "\n") <(bin/clang -c test.cc -### -no-canonical-prefixes 2>&1 | tr " "
"\n")
12c12
< /usr/local/google/home/thakis/src/llvm-build-nolibcxx/bin
---
14c14
< "/usr/local/google/home/thakis/src/llvm-build-nolibcxx/bin/clang-4.0"
---
40c40
< "/usr/local/google/home/thakis/src/llvm-build-nolibcxx/lib/clang/5.0.0"
---
52c52
<
"/usr/local/google/home/thakis/src/llvm-build-nolibcxx/lib/clang/5.0.0/include"
---

[Dirk Pranke, 2017/04/25 23:26:45]

Thanks. Updated.

+  # We don't want this because we don't really need it and it can mess up
+  # the goma cache entries. It would be nice if it was on by default in
+  # clang, but it isn't.

[Nico, 2017/04/25 23:29:20]

Can you add a

# TODO(thakis): Figure out if this could be the default behavior.
# TODO(thakis): Expose this in clang-cl if not.

type of comment here?

[Dirk Pranke, 2017/04/26 00:39:30]

Sure.

+  if (is_clang && !is_win) {
+    cflags += [ "-fno-canonical-prefixes" ]

[Nico, 2017/04/25 23:21:07]

It's spelled -no-canonical-prefixes without f I think

[Dirk Pranke, 2017/04/25 23:26:45]

Done.

[shinyak, 2017/04/26 00:52:00]

Yeah, -no-canonical-prefixes is correct.
(Could you upload patch set 2?)

+  }
+
   # 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 1281 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }