diff --git a/Blueprints b/Blueprints
index c8442e7c4..8a2d85f85 100644
--- a/Blueprints
+++ b/Blueprints
@@ -19,6 +19,7 @@ bootstrap_go_binary {
         "soong-cc",
         "soong-common",
         "soong-config",
+        "soong-env",
         "soong-genrule",
     ],
     srcs: [
@@ -27,6 +28,25 @@ bootstrap_go_binary {
     primaryBuilder: true,
 }
 
+bootstrap_go_binary {
+    name: "soong_env",
+    deps: [
+        "soong-env",
+    ],
+    srcs: [
+        "cmd/soong_env/soong_env.go",
+    ],
+}
+
+bootstrap_go_package {
+    name: "soong-env",
+    pkgPath: "android/soong/env",
+    srcs: [
+        "env/env.go",
+    ],
+}
+
+
 bootstrap_go_binary {
     name: "soong_glob",
     deps: [
@@ -54,12 +74,14 @@ bootstrap_go_package {
     deps: [
         "blueprint",
         "blueprint-bootstrap",
+        "soong-env",
         "soong-glob",
     ],
     srcs: [
         "common/arch.go",
         "common/checkbuild.go",
         "common/defs.go",
+        "common/env.go",
         "common/glob.go",
         "common/module.go",
         "common/paths.go",
diff --git a/build.ninja.in b/build.ninja.in
index 2b802d43f..4fb63250e 100644
--- a/build.ninja.in
+++ b/build.ninja.in
@@ -53,7 +53,7 @@ rule g.bootstrap.link
 # Variant:
 # Type:    bootstrap_go_binary
 # Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
-# Defined: build/soong/Blueprints:123:1
+# Defined: build/soong/Blueprints:145:1
 
 build .bootstrap/androidmk/obj/androidmk.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/androidmk/cmd/androidmk/android.go $
@@ -79,7 +79,7 @@ default .bootstrap/bin/androidmk
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:136:1
+# Defined: build/soong/Blueprints:158:1
 
 build .bootstrap/androidmk-parser/pkg/android/soong/androidmk/parser.a: $
         g.bootstrap.gc $
@@ -295,12 +295,13 @@ build .bootstrap/soong-art/pkg/android/soong/art.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a $
         .bootstrap/soong-cc/pkg/android/soong/cc.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg
     pkgPath = android/soong/art
 default .bootstrap/soong-art/pkg/android/soong/art.a
 
@@ -309,7 +310,7 @@ default .bootstrap/soong-art/pkg/android/soong/art.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:82:1
+# Defined: build/soong/Blueprints:104:1
 
 build .bootstrap/soong-cc/pkg/android/soong/cc.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/cc/builder.go $
@@ -327,11 +328,12 @@ build .bootstrap/soong-cc/pkg/android/soong/cc.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg
     pkgPath = android/soong/cc
 default .bootstrap/soong-cc/pkg/android/soong/cc.a
 
@@ -340,12 +342,13 @@ default .bootstrap/soong-cc/pkg/android/soong/cc.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:51:1
+# Defined: build/soong/Blueprints:71:1
 
 build .bootstrap/soong-common/pkg/android/soong/common.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/common/arch.go $
         ${g.bootstrap.srcDir}/build/soong/common/checkbuild.go $
         ${g.bootstrap.srcDir}/build/soong/common/defs.go $
+        ${g.bootstrap.srcDir}/build/soong/common/env.go $
         ${g.bootstrap.srcDir}/build/soong/common/glob.go $
         ${g.bootstrap.srcDir}/build/soong/common/module.go $
         ${g.bootstrap.srcDir}/build/soong/common/paths.go | $
@@ -356,8 +359,9 @@ build .bootstrap/soong-common/pkg/android/soong/common.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg
     pkgPath = android/soong/common
 default .bootstrap/soong-common/pkg/android/soong/common.a
 
@@ -366,7 +370,7 @@ default .bootstrap/soong-common/pkg/android/soong/common.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:69:1
+# Defined: build/soong/Blueprints:91:1
 
 build .bootstrap/soong-config/pkg/android/soong/config.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/config/config.go | $
@@ -377,18 +381,31 @@ build .bootstrap/soong-config/pkg/android/soong/config.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg
     pkgPath = android/soong/config
 default .bootstrap/soong-config/pkg/android/soong/config.a
 
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+# Module:  soong-env
+# Variant:
+# Type:    bootstrap_go_package
+# Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
+# Defined: build/soong/Blueprints:41:1
+
+build .bootstrap/soong-env/pkg/android/soong/env.a: g.bootstrap.gc $
+        ${g.bootstrap.srcDir}/build/soong/env/env.go | ${g.bootstrap.gcCmd}
+    pkgPath = android/soong/env
+default .bootstrap/soong-env/pkg/android/soong/env.a
+
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 # Module:  soong-genrule
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:106:1
+# Defined: build/soong/Blueprints:128:1
 
 build .bootstrap/soong-genrule/pkg/android/soong/genrule.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/genrule/genrule.go | $
@@ -399,10 +416,11 @@ build .bootstrap/soong-genrule/pkg/android/soong/genrule.a: g.bootstrap.gc $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg
     pkgPath = android/soong/genrule
 default .bootstrap/soong-genrule/pkg/android/soong/genrule.a
 
@@ -411,7 +429,7 @@ default .bootstrap/soong-genrule/pkg/android/soong/genrule.a
 # Variant:
 # Type:    bootstrap_go_package
 # Factory: github.com/google/blueprint/bootstrap.newGoPackageModule
-# Defined: build/soong/Blueprints:40:1
+# Defined: build/soong/Blueprints:60:1
 
 build .bootstrap/soong-glob/pkg/android/soong/glob.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/glob/glob.go | ${g.bootstrap.gcCmd} $
@@ -436,31 +454,54 @@ build .bootstrap/soong_build/obj/soong_build.a: g.bootstrap.gc $
         .bootstrap/blueprint-deptools/pkg/github.com/google/blueprint/deptools.a $
         .bootstrap/blueprint-pathtools/pkg/github.com/google/blueprint/pathtools.a $
         .bootstrap/blueprint-bootstrap/pkg/github.com/google/blueprint/bootstrap.a $
+        .bootstrap/soong-env/pkg/android/soong/env.a $
         .bootstrap/soong-glob/pkg/android/soong/glob.a $
         .bootstrap/soong-common/pkg/android/soong/common.a $
         .bootstrap/soong-config/pkg/android/soong/config.a $
         .bootstrap/soong-genrule/pkg/android/soong/genrule.a $
         .bootstrap/soong-cc/pkg/android/soong/cc.a $
         .bootstrap/soong-art/pkg/android/soong/art.a
-    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg -I .bootstrap/soong-art/pkg
+    incFlags = -I .bootstrap/blueprint-parser/pkg -I .bootstrap/blueprint-proptools/pkg -I .bootstrap/blueprint/pkg -I .bootstrap/blueprint-deptools/pkg -I .bootstrap/blueprint-pathtools/pkg -I .bootstrap/blueprint-bootstrap/pkg -I .bootstrap/soong-env/pkg -I .bootstrap/soong-glob/pkg -I .bootstrap/soong-common/pkg -I .bootstrap/soong-config/pkg -I .bootstrap/soong-genrule/pkg -I .bootstrap/soong-cc/pkg -I .bootstrap/soong-art/pkg
     pkgPath = soong_build
 default .bootstrap/soong_build/obj/soong_build.a
 
 build .bootstrap/soong_build/obj/a.out: g.bootstrap.link $
         .bootstrap/soong_build/obj/soong_build.a | ${g.bootstrap.linkCmd}
-    libDirFlags = -L .bootstrap/blueprint-parser/pkg -L .bootstrap/blueprint-proptools/pkg -L .bootstrap/blueprint/pkg -L .bootstrap/blueprint-deptools/pkg -L .bootstrap/blueprint-pathtools/pkg -L .bootstrap/blueprint-bootstrap/pkg -L .bootstrap/soong-glob/pkg -L .bootstrap/soong-common/pkg -L .bootstrap/soong-config/pkg -L .bootstrap/soong-genrule/pkg -L .bootstrap/soong-cc/pkg -L .bootstrap/soong-art/pkg
+    libDirFlags = -L .bootstrap/blueprint-parser/pkg -L .bootstrap/blueprint-proptools/pkg -L .bootstrap/blueprint/pkg -L .bootstrap/blueprint-deptools/pkg -L .bootstrap/blueprint-pathtools/pkg -L .bootstrap/blueprint-bootstrap/pkg -L .bootstrap/soong-env/pkg -L .bootstrap/soong-glob/pkg -L .bootstrap/soong-common/pkg -L .bootstrap/soong-config/pkg -L .bootstrap/soong-genrule/pkg -L .bootstrap/soong-cc/pkg -L .bootstrap/soong-art/pkg
 default .bootstrap/soong_build/obj/a.out
 
 build .bootstrap/bin/soong_build: g.bootstrap.cp $
         .bootstrap/soong_build/obj/a.out
 default .bootstrap/bin/soong_build
 
+# # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
+# Module:  soong_env
+# Variant:
+# Type:    bootstrap_go_binary
+# Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
+# Defined: build/soong/Blueprints:31:1
+
+build .bootstrap/soong_env/obj/soong_env.a: g.bootstrap.gc $
+        ${g.bootstrap.srcDir}/build/soong/cmd/soong_env/soong_env.go | $
+        ${g.bootstrap.gcCmd} .bootstrap/soong-env/pkg/android/soong/env.a
+    incFlags = -I .bootstrap/soong-env/pkg
+    pkgPath = soong_env
+default .bootstrap/soong_env/obj/soong_env.a
+
+build .bootstrap/soong_env/obj/a.out: g.bootstrap.link $
+        .bootstrap/soong_env/obj/soong_env.a | ${g.bootstrap.linkCmd}
+    libDirFlags = -L .bootstrap/soong-env/pkg
+default .bootstrap/soong_env/obj/a.out
+
+build .bootstrap/bin/soong_env: g.bootstrap.cp .bootstrap/soong_env/obj/a.out
+default .bootstrap/bin/soong_env
+
 # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # # #
 # Module:  soong_glob
 # Variant:
 # Type:    bootstrap_go_binary
 # Factory: github.com/google/blueprint/bootstrap.newGoBinaryModule
-# Defined: build/soong/Blueprints:30:1
+# Defined: build/soong/Blueprints:50:1
 
 build .bootstrap/soong_glob/obj/soong_glob.a: g.bootstrap.gc $
         ${g.bootstrap.srcDir}/build/soong/cmd/soong_glob/soong_glob.go | $
@@ -498,7 +539,8 @@ rule s.bootstrap.minibp
 build .bootstrap/main.ninja.in: s.bootstrap.bigbp $
         ${g.bootstrap.srcDir}/Blueprints | .bootstrap/bin/androidmk $
         .bootstrap/bin/bpfmt .bootstrap/bin/bpmodify .bootstrap/bin/minibp $
-        .bootstrap/bin/soong_build .bootstrap/bin/soong_glob
+        .bootstrap/bin/soong_build .bootstrap/bin/soong_env $
+        .bootstrap/bin/soong_glob
 default .bootstrap/main.ninja.in
 build .bootstrap/notAFile: phony
 default .bootstrap/notAFile
diff --git a/cmd/soong_build/main.go b/cmd/soong_build/main.go
index b84b804c1..cb579e9c9 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -64,6 +64,7 @@ func main() {
 
 	// Singletons
 	ctx.RegisterSingletonType("checkbuild", common.CheckbuildSingleton)
+	ctx.RegisterSingletonType("env", common.EnvSingleton)
 
 	configuration, err := config.New(srcDir)
 	if err != nil {
diff --git a/cmd/soong_env/soong_env.go b/cmd/soong_env/soong_env.go
new file mode 100644
index 000000000..933e525ae
--- /dev/null
+++ b/cmd/soong_env/soong_env.go
@@ -0,0 +1,55 @@
+// Copyright 2015 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.
+
+// soong_glob is the command line tool that checks if the list of files matching a glob has
+// changed, and only updates the output file list if it has changed.  It is used to optimize
+// out build.ninja regenerations when non-matching files are added.  See
+// android/soong/common/glob.go for a longer description.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"android/soong/env"
+)
+
+func usage() {
+	fmt.Fprintf(os.Stderr, "usage: soong_env env_file\n")
+	fmt.Fprintf(os.Stderr, "exits with success if the environment varibles in env_file match\n")
+	fmt.Fprintf(os.Stderr, "the current environment\n")
+	flag.PrintDefaults()
+	os.Exit(2)
+}
+
+func main() {
+	flag.Parse()
+
+	if flag.NArg() != 1 {
+		usage()
+	}
+
+	stale, err := env.StaleEnvFile(flag.Arg(0))
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %s\n", err.Error())
+		os.Exit(1)
+	}
+
+	if stale {
+		os.Exit(1)
+	}
+
+	os.Exit(0)
+}
diff --git a/common/env.go b/common/env.go
new file mode 100644
index 000000000..e33a0258b
--- /dev/null
+++ b/common/env.go
@@ -0,0 +1,47 @@
+// Copyright 2015 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 common
+
+import (
+	"android/soong/env"
+
+	"github.com/google/blueprint"
+)
+
+// This file supports dependencies on environment variables.  During build manifest generation,
+// any dependency on an environment variable is added to a list.  During the singleton phase
+// a JSON file is written containing the current value of all used environment variables.
+// The next time the top-level build script is run, it uses the soong_env executable to
+// compare the contents of the environment variables, rewriting the file if necessary to cause
+// a manifest regeneration.
+
+func EnvSingleton() blueprint.Singleton {
+	return &envSingleton{}
+}
+
+type envSingleton struct{}
+
+func (c *envSingleton) GenerateBuildActions(ctx blueprint.SingletonContext) {
+	envDeps := ctx.Config().(Config).EnvDeps()
+
+	envFile := ".soong.environment"
+
+	err := env.WriteEnvFile(envFile, envDeps)
+	if err != nil {
+		ctx.Errorf(err.Error())
+	}
+
+	ctx.AddNinjaFileDeps(envFile)
+}
diff --git a/common/module.go b/common/module.go
index 4b1200d3c..3cb4c09d5 100644
--- a/common/module.go
+++ b/common/module.go
@@ -20,6 +20,13 @@ import (
 	"github.com/google/blueprint"
 )
 
+type Config interface {
+	CpPreserveSymlinksFlags() string
+	SrcDir() string
+	Getenv(string) string
+	EnvDeps() map[string]string
+}
+
 var (
 	DeviceSharedLibrary = "shared_library"
 	DeviceStaticLibrary = "static_library"
diff --git a/common/paths.go b/common/paths.go
index bc75ea59b..abe67bf52 100644
--- a/common/paths.go
+++ b/common/paths.go
@@ -20,11 +20,6 @@ import (
 	"github.com/google/blueprint"
 )
 
-type Config interface {
-	CpPreserveSymlinksFlags() string
-	SrcDir() string
-}
-
 // ModuleOutDir returns the path to the module-specific output directory.
 func ModuleOutDir(ctx AndroidModuleContext) string {
 	return filepath.Join(".intermediates", ctx.ModuleDir(), ctx.ModuleName(), ctx.ModuleSubDir())
diff --git a/config/config.go b/config/config.go
index 6cdc211e7..6cb61e4be 100644
--- a/config/config.go
+++ b/config/config.go
@@ -41,7 +41,8 @@ func NewFileConfigurableOptions() FileConfigurableOptions {
 type Config struct {
 	FileConfigurableOptions
 
-	srcDir string // the path of the root source directory
+	srcDir  string // the path of the root source directory
+	envDeps map[string]string
 }
 
 // loads configuration options from a JSON file in the cwd.
@@ -103,7 +104,10 @@ func saveToConfigFile(config FileConfigurableOptions) error {
 // the root source directory. It also loads the config file, if found.
 func New(srcDir string) (*Config, error) {
 	// Make a config with default options
-	config := &Config{srcDir: srcDir}
+	config := &Config{
+		srcDir:  srcDir,
+		envDeps: make(map[string]string),
+	}
 
 	// Load any configurable options from the configuration file
 	err := loadFromConfigFile(config)
@@ -150,3 +154,17 @@ func (c *Config) CpPreserveSymlinksFlags() string {
 		return ""
 	}
 }
+
+func (c *Config) Getenv(key string) string {
+	var val string
+	var exists bool
+	if val, exists = c.envDeps[key]; !exists {
+		val = os.Getenv(key)
+		c.envDeps[key] = val
+	}
+	return val
+}
+
+func (c *Config) EnvDeps() map[string]string {
+	return c.envDeps
+}
diff --git a/env/env.go b/env/env.go
new file mode 100644
index 000000000..bf58a9914
--- /dev/null
+++ b/env/env.go
@@ -0,0 +1,97 @@
+// Copyright 2015 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.
+
+// env implements the environment JSON file handling for the soong_env command line tool run before
+// the builder and for the env writer in the builder.
+package env
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"sort"
+)
+
+type envFileEntry struct{ Key, Value string }
+type envFileData []envFileEntry
+
+func WriteEnvFile(filename string, envDeps map[string]string) error {
+	contents := make(envFileData, 0, len(envDeps))
+	for key, value := range envDeps {
+		contents = append(contents, envFileEntry{key, value})
+	}
+
+	sort.Sort(contents)
+
+	data, err := json.MarshalIndent(contents, "", "    ")
+	if err != nil {
+		return err
+	}
+
+	data = append(data, '\n')
+
+	err = ioutil.WriteFile(filename, data, 0664)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func StaleEnvFile(filename string) (bool, error) {
+	data, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return true, err
+	}
+
+	var contents envFileData
+
+	err = json.Unmarshal(data, &contents)
+	if err != nil {
+		return true, err
+	}
+
+	var changed []string
+	for _, entry := range contents {
+		key := entry.Key
+		old := entry.Value
+		cur := os.Getenv(key)
+		if old != cur {
+			changed = append(changed, fmt.Sprintf("%s (%q -> %q)", key, old, cur))
+		}
+	}
+
+	if len(changed) > 0 {
+		fmt.Printf("environment variables changed value:\n")
+		for _, s := range changed {
+			fmt.Printf("   %s\n", s)
+		}
+		return true, nil
+	}
+
+	return false, nil
+}
+
+func (e envFileData) Len() int {
+	return len(e)
+}
+
+func (e envFileData) Less(i, j int) bool {
+	return e[i].Key < e[j].Key
+}
+
+func (e envFileData) Swap(i, j int) {
+	e[i], e[j] = e[j], e[i]
+}
diff --git a/soong.bash b/soong.bash
index fc330d0b1..fab15de5f 100755
--- a/soong.bash
+++ b/soong.bash
@@ -27,4 +27,21 @@ fi
 # can regenerate the build manifest.
 export BLUEPRINT_NINJA_HAS_MULTIPASS=1
 
+# Ninja can't depend on environment variables, so do a manual comparison
+# of the relevant environment variables from the last build using the
+# soong_env tool and trigger a build manifest regeneration if necessary
+ENVFILE=${BUILDDIR}/.soong.environment
+ENVTOOL=${BUILDDIR}/.bootstrap/bin/soong_env
+if [ -f ${ENVFILE} ]; then
+    if [ -x ${ENVTOOL} ]; then
+        if ! ${ENVTOOL} ${ENVFILE}; then
+            echo "forcing build manifest regeneration"
+            rm -f ${ENVFILE}
+        fi
+    else
+        echo "Missing soong_env tool, forcing build manifest regeneration"
+        rm -f ${ENVFILE}
+    fi
+fi
+
 ${SRCDIR}/prebuilts/ninja/${PREBUILTOS}/ninja -C ${BUILDDIR} "$@"