Optimize clang source-based code coverage build configuration.

I've been testing the changes on net_parse_cookie_line_fuzzer,
which is a fuzz target of average size written in C++.

Improvements step by step:

0) Original configuration, Coverage + ASan:
158 MB  1,000 exec/s

1) Prohibit ASan (and other sanitizers), use only Coverage instrumentation:
132 MB  same speed

The following change hasn't been applied, but let's keep it in the description FTR:
- 2) Disable sanitizer coverage (which is different from clang source-based coverage):
- 90 MB   1,088 exec/s (speed +8-10%)

3) Avoid optimize_for_fuzzing config (i.e. use -O3 instead of -O1 for coverage build):
Same size  1,773 exec/s (speed +60-65% on top of the previous change)

4) Disable coverage for libFuzzer source code:
88 MB  3,988 exec/s (speed +125% on top of previous changes)

5) Disable coverage for libc++ and libc++abi sources
(https://chromium-review.googlesource.com/#/c/chromium/buildtools/+/693570):
86 MB  4,110 exec/s (speed +3% on top of previous changes)

In total, for that particular target:
- build size reduced by ~45%
- execution speed increased by ~310%

I've also tested the changes with zlib_uncompress_fuzzer (a tiny fuzz target for C-library):
- build size reduced by ~83%
- execution speed increased by ~120%

I haven't measured impact on the other fuzz targets, so it may vary a lot,
but the result seems to be quite significant anyway.


Bug: 759794
Change-Id: Icf61c979e38d0f7849ab7281bd9e24cf2b7a7d02
Reviewed-on: https://chromium-review.googlesource.com/693564
Reviewed-by: Brett Wilson <brettw@chromium.org>
Reviewed-by: Oliver Chang <ochang@chromium.org>
Commit-Queue: Abhishek Arya <inferno@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#506454}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c8fee977deb6f47aaf602813b00428adfe7baa74

config/BUILDCONFIG.gn

@@ -527,6 +527,7 @@ default_compiler_configs = [
   "//build/config/compiler:default_symbols",
   "//build/config/compiler:no_rtti",
   "//build/config/compiler:runtime_library",
+  "//build/config/coverage:default_coverage",
   "//build/config/sanitizers:default_sanitizer_flags",
 ]
 if (is_win) {

config/compiler/BUILD.gn

@@ -265,8 +265,6 @@ config("compiler") {
     # and build system rebuild things when their commandline changes). Nothing
     # should ever read this define.
     defines += [ "CR_CLANG_REVISION=\"$clang_revision\"" ]
-
-    configs += [ "//build/config/coverage" ]
   }
 
   # Non-Mac Posix compiler flags setup.
@@ -1723,6 +1721,12 @@ config("default_optimization") {
     configs = [ ":no_optimize" ]
   } else if (optimize_for_fuzzing) {
     assert(!is_win, "Fuzzing optimize level not supported on Windows")
+
+    # Coverage build is quite slow. Using "optimize_for_fuzzing" makes it even
+    # slower as it uses "-O1" instead of "-O3". Prevent that from happening.
+    assert(!use_clang_coverage,
+           "optimize_for_fuzzing=true should not be used with " +
+               "use_clang_coverage=true.")
     configs = [ ":optimize_fuzzing" ]
   } else {
     configs = [ ":optimize" ]

config/coverage/BUILD.gn

@@ -2,14 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//build/toolchain/toolchain.gni")
+import("//build/config/coverage/coverage.gni")
 
-declare_args() {
-  # Enable Clang's source-based code coverage.
-  use_clang_coverage = false
-}
-
-config("coverage") {
+config("default_coverage") {
   if (use_clang_coverage) {
     cflags = [
       "-fprofile-instr-generate",

config/coverage/OWNERS

@@ -0,0 +1,3 @@
+inferno@chromium.org
+mmoroz@chromium.org
+ochang@chromium.org

config/coverage/coverage.gni

@@ -0,0 +1,13 @@
+# Copyright 2017 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/toolchain/toolchain.gni")
+
+declare_args() {
+  # Enable Clang's Source-based Code Coverage.
+  use_clang_coverage = false
+}
+
+assert(!use_clang_coverage || is_clang,
+       "Clang Source-based Code Coverage requires clang.")