[code coverage] Hook coverage wrapper to build toolchains

This CL hooks the Clang code coverage wrapper into the build
toolchains, and specifically, a new build flag:
coverage_instrumentation_input_file is added to allow turning on/off
the wrapper and pass the list-files-to-instrument around.

Bug: 898695
Change-Id: I405ccbfc1796afa44534794d711f2953fac78f6d
Reviewed-on: https://chromium-review.googlesource.com/c/1309999
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604858}

build/config/coverage/BUILD.gn

@@ -6,22 +6,9 @@ import("//build/config/coverage/coverage.gni")
 
 config("default_coverage") {
   if (use_clang_coverage) {
-    cflags = [
-      "-fprofile-instr-generate",
-      "-fcoverage-mapping",
-
-      # Following experimental flags removes unused header functions from the
-      # coverage mapping data embedded in the test binaries, and the reduction
-      # of binary size enables building Chrome's large unit test targets on
-      # MacOS. Please refer to crbug.com/796290 for more details.
-      "-mllvm",
-      "-limited-coverage-experimental=true",
-    ]
-
     ldflags = []
     if (!is_win) {
       ldflags += [ "-fprofile-instr-generate" ]
-      cflags += [ "-fno-use-cxa-atexit" ]
     } else {
       # Windows directly calls link.exe instead of the compiler driver when
       # linking.  Hence, pass the runtime libraries instead of
@@ -37,5 +24,26 @@ config("default_coverage") {
                "use_clang_coverage=true not supported yet for this target_cpu")
       }
     }
+
+    # Coverage flags are only on by default when instrument all source files.
+    # Otherwise, coverage flags are dynamically passed to the compile command
+    # via the //build/toolchain/clang_code_coverage_wrapper.py script.
+    if (coverage_instrumentation_input_file == "") {
+      cflags = [
+        "-fprofile-instr-generate",
+        "-fcoverage-mapping",
+
+        # Following experimental flags removes unused header functions from the
+        # coverage mapping data embedded in the test binaries, and the reduction
+        # of binary size enables building Chrome's large unit test targets on
+        # MacOS. Please refer to crbug.com/796290 for more details.
+        "-mllvm",
+        "-limited-coverage-experimental=true",
+      ]
+
+      if (!is_win) {
+        cflags += [ "-fno-use-cxa-atexit" ]
+      }
+    }
   }
 }

build/config/coverage/coverage.gni

@@ -4,10 +4,26 @@
 
 import("//build/toolchain/toolchain.gni")
 
+# There are two ways to enable code coverage instrumentation:
+# 1. When |use_clang_coverage| is true and |coverage_instrumentation_input_file|
+#    is empty, all source files are instrumented.
+# 2. When |use_clang_coverage| is true and |coverage_instrumentation_input_file|
+#    is NOT empty and points to a text file on the file system, ONLY source
+#    files specified in the input file are instrumented.
 declare_args() {
   # Enable Clang's Source-based Code Coverage.
   use_clang_coverage = false
+
+  # The path to the coverage instrumentation input file should be a source root
+  # absolute path (e.g. //out/Release/coverage_instrumentation_input.txt), and
+  # the file consists of multiple lines where each line represents a path to a
+  # source file, and the paths must be relative to the root build directory.
+  # e.g. ../../base/task/post_task.cc for build directory 'out/Release'.
+  coverage_instrumentation_input_file = ""
 }
 
 assert(!use_clang_coverage || is_clang,
        "Clang Source-based Code Coverage requires clang.")
+
+assert(coverage_instrumentation_input_file == "" || use_clang_coverage,
+       "Instrument a subset of source files requires enabling clang coverage.")

build/toolchain/gcc_toolchain.gni

@@ -4,6 +4,7 @@
 
 import("//build/config/clang/clang.gni")
 import("//build/config/compiler/compiler.gni")
+import("//build/config/coverage/coverage.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/v8_target_cpu.gni")
 import("//build/toolchain/cc_wrapper.gni")
@@ -193,6 +194,28 @@ template("gcc_toolchain") {
       compiler_prefix = "${analyzer_wrapper} " + compiler_prefix
     }
 
+    if (defined(toolchain_args.coverage_instrumentation_input_file)) {
+      toolchain_coverage_instrumentation_input_file =
+          toolchain_args.coverage_instrumentation_input_file
+    } else {
+      toolchain_coverage_instrumentation_input_file =
+          coverage_instrumentation_input_file
+    }
+    _use_clang_coverage_wrapper =
+        toolchain_coverage_instrumentation_input_file != ""
+    if (_use_clang_coverage_wrapper) {
+      assert(!use_clang_static_analyzer,
+             "Clang static analyzer wrapper and Clang code coverage wrapper " +
+                 "cannot be used together.")
+
+      _coverage_wrapper =
+          rebase_path("//build/toolchain/clang_code_coverage_wrapper.py",
+                      root_build_dir) + " --files-to-instrument=" +
+          rebase_path(toolchain_coverage_instrumentation_input_file,
+                      root_build_dir)
+      compiler_prefix = "${_coverage_wrapper} " + compiler_prefix
+    }
+
     cc = compiler_prefix + invoker.cc
     cxx = compiler_prefix + invoker.cxx
     asm = asm_prefix + invoker.cc

build/toolchain/mac/BUILD.gn

@@ -8,6 +8,7 @@
 
 import("../goma.gni")
 import("//build/config/clang/clang.gni")
+import("//build/config/coverage/coverage.gni")
 if (is_ios) {
   import("//build/config/ios/ios_sdk.gni")
 }
@@ -135,6 +136,29 @@ template("mac_toolchain") {
       ld = cxx
     }
 
+    if (defined(toolchain_args.coverage_instrumentation_input_file)) {
+      toolchain_coverage_instrumentation_input_file =
+          toolchain_args.coverage_instrumentation_input_file
+    } else {
+      toolchain_coverage_instrumentation_input_file =
+          coverage_instrumentation_input_file
+    }
+    _use_clang_coverage_wrapper =
+        toolchain_coverage_instrumentation_input_file != ""
+    if (_use_clang_coverage_wrapper) {
+      assert(!use_clang_static_analyzer,
+             "Clang static analyzer wrapper and Clang code coverage wrapper " +
+                 "cannot be used together.")
+
+      _coverage_wrapper =
+          rebase_path("//build/toolchain/clang_code_coverage_wrapper.py",
+                      root_build_dir) + " --files-to-instrument=" +
+          rebase_path(toolchain_coverage_instrumentation_input_file,
+                      root_build_dir)
+      cc = _coverage_wrapper + " ${cc}"
+      cxx = _coverage_wrapper + " ${cxx}"
+    }
+
     linker_driver =
         "TOOL_VERSION=${tool_versions.linker_driver} " +
         rebase_path("//build/toolchain/mac/linker_driver.py", root_build_dir)

build/toolchain/nacl_toolchain.gni

@@ -54,6 +54,7 @@ template("nacl_toolchain") {
 
       # We do not support clang code coverage in the NaCl toolchains.
       use_clang_coverage = false
+      coverage_instrumentation_input_file = ""
     }
   }
 }

docs/clang_code_coverage_wrapper.md

@@ -30,15 +30,15 @@ everything is huge, while it's negligible for instrument what-is-needed.
 
 ## How to use the coverage wrapper?
 To get the coverage wrapper hook into your build, add the following flags to
-your `args.gn` file.
+your `args.gn` file, assuming build directory is `out/Release`.
 
 ```
 use_clang_coverage = true
-coverage_instrumentation_input_file = "coverage_instrumentation_input.txt"
+coverage_instrumentation_input_file = "//out/Release/coverage_instrumentation_input.txt"
 ```
 
-The path to the coverage instrumentation input file should be relative to the
-root build directory, and the file consists of multiple lines where each line
+The path to the coverage instrumentation input file should be a source root
+absolute path, and the file consists of multiple lines where each line
 represents a path to a source file, and the specified paths must be relative to
 the root build directory. e.g. `../../base/task/post_task.cc` for build
 directory `out/Release`.