Native Coverage support in Soong (gcov)

This is configured the same as make -- a global NATIVE_COVERAGE=true
flag to allow native coverage, then COVERAGE_PATHS=path1,path2,... to
turn it on for certain paths.

There are .gcnodir files exported to Make and saved in $OUT/coverage/...
files which are `ar` archives containing all of the compiler-produced
.gcno files for a particular executable / shared library.

Unlike the Make implementation, this only passes links the helper
library (automatically through --coverage) when one of the object files
or static libraries being used actually has coverage enabled.

Host support is currently disabled, since we set -nodefaultlibs, which
prevents libclang_rt.profile-*.a from being picked up automatically.

Bug: 32749731
Test: NATIVE_COVERAGE=true COVERAGE_PATHS=system/core/libcutils m -j libbacktrace libutils tombstoned
      $OUT/coverage/system/lib*/libcutils.gcnodir looks correct (self)
      $OUT/coverage/system/lib*/libbacktrace.gcnodir looks correct (static)
      $OUT/coverage/system/lib*/libutils.gcnodir doesn't exist (shared)
      $OUT/coverage/system/bin/tombstoned.gcnodir looks correct (executable)
Test: NATIVE_COVERAGE=true COVERAGE_PATHS=external/libcxxabi m -j libc++
      Confirm that $OUT/coverage/system/lib*/libc++.gcnodir looks correct (whole_static_libs)
Change-Id: I48aaa0ba8d76e50e9c2d1151421c0c6dc8ed79a9

cc/coverage.go

@@ -0,0 +1,124 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cc
+
+import (
+	"android/soong/android"
+	"github.com/google/blueprint"
+)
+
+type CoverageProperties struct {
+	Native_coverage *bool
+
+	CoverageEnabled bool `blueprint:"mutated"`
+}
+
+type coverage struct {
+	Properties CoverageProperties
+
+	// Whether binaries containing this module need --coverage added to their ldflags
+	linkCoverage bool
+}
+
+func (cov *coverage) props() []interface{} {
+	return []interface{}{&cov.Properties}
+}
+
+func (cov *coverage) begin(ctx BaseModuleContext) {}
+
+func (cov *coverage) deps(ctx BaseModuleContext, deps Deps) Deps {
+	return deps
+}
+
+func (cov *coverage) flags(ctx ModuleContext, flags Flags) Flags {
+	if !ctx.DeviceConfig().NativeCoverageEnabled() {
+		return flags
+	}
+
+	if cov.Properties.CoverageEnabled {
+		flags.Coverage = true
+		flags.GlobalFlags = append(flags.GlobalFlags, "--coverage", "-O0")
+		cov.linkCoverage = true
+	}
+
+	// Even if we don't have coverage enabled, if any of our object files were compiled
+	// with coverage, then we need to add --coverage to our ldflags.
+	if !cov.linkCoverage {
+		if ctx.static() && !ctx.staticBinary() {
+			// For static libraries, the only thing that changes our object files
+			// are included whole static libraries, so check to see if any of
+			// those have coverage enabled.
+			ctx.VisitDirectDeps(func(m blueprint.Module) {
+				if ctx.OtherModuleDependencyTag(m) != wholeStaticDepTag {
+					return
+				}
+
+				if cc, ok := m.(*Module); ok && cc.coverage != nil {
+					if cc.coverage.linkCoverage {
+						cov.linkCoverage = true
+					}
+				}
+			})
+		} else {
+			// For executables and shared libraries, we need to check all of
+			// our static dependencies.
+			ctx.VisitDirectDeps(func(m blueprint.Module) {
+				cc, ok := m.(*Module)
+				if !ok || cc.coverage == nil {
+					return
+				}
+
+				if static, ok := cc.linker.(libraryInterface); !ok || !static.static() {
+					return
+				}
+
+				if cc.coverage.linkCoverage {
+					cov.linkCoverage = true
+				}
+			})
+		}
+	}
+
+	if cov.linkCoverage {
+		flags.LdFlags = append(flags.LdFlags, "--coverage")
+	}
+
+	return flags
+}
+
+func coverageLinkingMutator(mctx android.BottomUpMutatorContext) {
+	if c, ok := mctx.Module().(*Module); ok && c.coverage != nil {
+		var enabled bool
+
+		if !mctx.DeviceConfig().NativeCoverageEnabled() {
+			// Coverage is disabled globally
+		} else if mctx.Host() {
+			// TODO(dwillemsen): because of -nodefaultlibs, we must depend on libclang_rt.profile-*.a
+			// Just turn off for now.
+		} else if c.coverage.Properties.Native_coverage != nil {
+			enabled = *c.coverage.Properties.Native_coverage
+		} else {
+			enabled = mctx.DeviceConfig().CoverageEnabledForPath(mctx.ModuleDir())
+		}
+
+		if enabled {
+			// Create a variation so that we don't need to recompile objects
+			// when turning on or off coverage. We'll still relink the necessary
+			// binaries, since we don't know which ones those are until later.
+			m := mctx.CreateLocalVariations("cov")
+			m[0].(*Module).coverage.Properties.CoverageEnabled = true
+		}
+	}
+}