diff --git a/src/test/java/com/google/devtools/build/lib/BUILD b/src/test/java/com/google/devtools/build/lib/BUILD
index cb846a4..ee85750 100644
--- a/src/test/java/com/google/devtools/build/lib/BUILD
+++ b/src/test/java/com/google/devtools/build/lib/BUILD
@@ -839,6 +839,7 @@
         ":testutil",
         "//src/main/java/com/google/devtools/build/lib:analysis-exec-rules-skyframe",
         "//src/main/java/com/google/devtools/build/lib:bazel-core",
+        "//src/main/java/com/google/devtools/build/lib:cmdline",
         "//src/main/java/com/google/devtools/build/lib:util",
         "//src/main/java/com/google/devtools/build/lib:vfs",
         "//src/main/java/com/google/devtools/build/lib/actions",
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationHelper.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationHelper.java
new file mode 100644
index 0000000..f28af6d
--- /dev/null
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationHelper.java
@@ -0,0 +1,111 @@
+// Copyright 2015 The Bazel Authors. 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 com.google.devtools.build.lib.rules.cpp;
+
+import com.google.devtools.build.lib.testutil.TestConstants;
+import com.google.devtools.build.lib.vfs.FileSystemUtils;
+import com.google.devtools.build.lib.vfs.Path;
+import com.google.devtools.build.lib.view.config.crosstool.CrosstoolConfig;
+import com.google.devtools.build.lib.view.config.crosstool.CrosstoolConfig.DefaultCpuToolchain;
+import com.google.devtools.build.lib.view.config.crosstool.CrosstoolConfig.ToolPath;
+import com.google.protobuf.TextFormat;
+
+import java.io.IOException;
+
+/**
+ * Helper class for the creation of crosstool toolchain factories.
+ */
+public class CrosstoolConfigurationHelper {
+  public static Path overwriteCrosstoolFile(Path workspace, String content) throws IOException {
+    Path crosstool = workspace.getRelative(TestConstants.MOCK_CROSSTOOL_PATH + "/CROSSTOOL");
+    long newMTime = crosstool.exists() ? crosstool.getLastModifiedTime() + 1 : -1;
+    crosstool.delete();
+    FileSystemUtils.createDirectoryAndParents(crosstool.getParentDirectory());
+    FileSystemUtils.writeContentAsLatin1(crosstool, content);
+    crosstool.setLastModifiedTime(newMTime);
+    return crosstool;
+  }
+
+  /**
+   * Overwrites the default CROSSTOOL file with one obtained by merging the simple complete
+   * toolchain with the given additional partial toolchain. Only {@code --cpu=piii} is valid.
+   */
+  public static void overwriteCrosstoolWithToolchain(
+      Path workspace, CrosstoolConfig.CToolchain partial) throws IOException {
+    CrosstoolConfig.CrosstoolRelease.Builder release =
+        CrosstoolConfig.CrosstoolRelease.newBuilder();
+    release.mergeFrom(simpleCompleteToolchainProto());
+    CrosstoolConfig.CToolchain.Builder toolchain = CrosstoolConfig.CToolchain.newBuilder();
+    toolchain.mergeFrom(release.getToolchain(0));
+    toolchain.mergeFrom(partial);
+    release.setToolchain(0, toolchain.build());
+    overwriteCrosstoolFile(workspace, TextFormat.printToString(release.build()));
+  }
+
+  /**
+   * Overwrites the default CROSSTOOL file with a reasonable toolchain.
+   */
+  public static void overwriteCrosstoolWithSimpleCompleteToolchain(Path workspace)
+      throws IOException {
+    overwriteCrosstoolFile(workspace, TextFormat.printToString(simpleCompleteToolchainProto()));
+  }
+
+  private static CrosstoolConfig.CrosstoolRelease simpleCompleteToolchainProto() {
+    CrosstoolConfig.CrosstoolRelease.Builder builder =
+        CrosstoolConfig.CrosstoolRelease.newBuilder()
+            .setMajorVersion("12")
+            .setMinorVersion("0")
+            .setDefaultTargetCpu("k8")
+            .addDefaultToolchain(
+                DefaultCpuToolchain.newBuilder()
+                    .setCpu("k8")
+                    .setToolchainIdentifier("k8-toolchain"));
+    CrosstoolConfig.CToolchain.Builder toolchainBuilder = newIncompleteToolchain();
+    toolchainBuilder
+        .setToolchainIdentifier("k8-toolchain")
+        .setHostSystemName("i686-unknown-linux-gnu")
+        .setTargetSystemName("i686-unknown-linux-gnu")
+        .setTargetCpu("k8")
+        .setTargetLibc("glibc-2.3.6-grte")
+        .setCompiler("gcc-4.3.1")
+        .setAbiVersion("gcc-3.4")
+        .setAbiLibcVersion("2.3.2")
+        .addCxxBuiltinIncludeDirectory("/include/directory");
+    builder.addToolchain(toolchainBuilder);
+    return builder.build();
+  }
+
+  private static CrosstoolConfig.CToolchain.Builder newIncompleteToolchain() {
+    CrosstoolConfig.CToolchain.Builder builder = CrosstoolConfig.CToolchain.newBuilder();
+    for (String tool :
+        new String[] {
+          "ar",
+          "cpp",
+          "gcc",
+          "gcov",
+          "ld",
+          "compat-ld",
+          "nm",
+          "objcopy",
+          "objdump",
+          "strip",
+          "dwp",
+          "gcov-tool"
+        }) {
+      builder.addToolPath(ToolPath.newBuilder().setName(tool).setPath(tool));
+    }
+    return builder;
+  }
+}
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationLoaderTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationLoaderTest.java
new file mode 100644
index 0000000..fbb98f0
--- /dev/null
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/CrosstoolConfigurationLoaderTest.java
@@ -0,0 +1,1010 @@
+// Copyright 2015 The Bazel Authors. 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 com.google.devtools.build.lib.rules.cpp;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.common.base.Functions;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.devtools.build.lib.analysis.config.BuildOptions;
+import com.google.devtools.build.lib.analysis.config.CompilationMode;
+import com.google.devtools.build.lib.analysis.config.ConfigurationEnvironment;
+import com.google.devtools.build.lib.analysis.config.InvalidConfigurationException;
+import com.google.devtools.build.lib.analysis.util.AnalysisMock;
+import com.google.devtools.build.lib.analysis.util.AnalysisTestCase;
+import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
+import com.google.devtools.build.lib.cmdline.PackageIdentifier;
+import com.google.devtools.build.lib.rules.cpp.CppConfiguration.Tool;
+import com.google.devtools.build.lib.testutil.TestConstants;
+import com.google.devtools.build.lib.vfs.PathFragment;
+import com.google.devtools.build.lib.view.config.crosstool.CrosstoolConfig.LipoMode;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+
+/**
+ * Tests for {@link CppConfigurationLoader}.
+ */
+public class CrosstoolConfigurationLoaderTest extends AnalysisTestCase {
+  private static final Collection<String> NO_FEATURES = Collections.emptySet();
+
+  @Override
+  protected void setUp() throws Exception {
+    super.setUp();
+  }
+
+  private CppConfiguration create(CppConfigurationLoader loader, String... args) throws Exception {
+    ConfigurationEnvironment env =
+        new ConfigurationEnvironment.TargetProviderEnvironment(
+            skyframeExecutor.getPackageManager(), reporter, directories);
+    return loader.create(env, BuildOptions.of(AnalysisMock.get().getBuildOptions(), args));
+  }
+
+  private CppConfigurationLoader loader(String crosstoolFileContents) throws IOException {
+    CrosstoolConfigurationHelper.overwriteCrosstoolFile(
+        directories.getWorkspace(), crosstoolFileContents);
+    return new CppConfigurationLoader(Functions.<String>identity());
+  }
+
+  private CppConfigurationLoader loaderWithOptionalTool(String optionalTool) throws IOException {
+    return loader(
+        "major_version: \"12\""
+            + "minor_version: \"0\""
+            + "default_target_cpu: \"cpu\""
+            + "default_toolchain {"
+            + "  cpu: \"cpu\""
+            + "  toolchain_identifier: \"toolchain-identifier\""
+            + "}"
+            + "toolchain {"
+            + "  toolchain_identifier: \"toolchain-identifier\""
+            + "  host_system_name: \"host-system-name\""
+            + "  target_system_name: \"target-system-name\""
+            + "  target_cpu: \"piii\""
+            + "  target_libc: \"target-libc\""
+            + "  compiler: \"compiler\""
+            + "  abi_version: \"abi-version\""
+            + "  abi_libc_version: \"abi-libc-version\""
+            + "  tool_path { name: \"ar\" path: \"path-to-ar\" }"
+            + "  tool_path { name: \"cpp\" path: \"path-to-cpp\" }"
+            + "  tool_path { name: \"gcc\" path: \"path-to-gcc\" }"
+            + "  tool_path { name: \"gcov\" path: \"path-to-gcov\" }"
+            + "  tool_path { name: \"ld\" path: \"path-to-ld\" }"
+            + "  tool_path { name: \"nm\" path: \"path-to-nm\" }"
+            + "  tool_path { name: \"objcopy\" path: \"path-to-objcopy\" }"
+            + "  tool_path { name: \"objdump\" path: \"path-to-objdump\" }"
+            + "  tool_path { name: \"strip\" path: \"path-to-strip\" }"
+            + "  tool_path { name: \"dwp\" path: \"path-to-dwp\" }"
+            + optionalTool
+            + "  supports_gold_linker: true"
+            + "  supports_thin_archives: true"
+            + "  supports_normalizing_ar: true"
+            + "  supports_incremental_linker: true"
+            + "  supports_fission: true"
+            + "  compiler_flag: \"c\""
+            + "  cxx_flag: \"cxx\""
+            + "  unfiltered_cxx_flag: \"unfiltered\""
+            + "  linker_flag: \"linker\""
+            + "  dynamic_library_linker_flag: \"solinker\""
+            + "  objcopy_embed_flag: \"objcopy\""
+            + "  compilation_mode_flags {"
+            + "    mode: FASTBUILD"
+            + "    compiler_flag: \"fastbuild\""
+            + "    cxx_flag: \"cxx-fastbuild\""
+            + "    linker_flag: \"linker-fastbuild\""
+            + "  }"
+            + "  compilation_mode_flags {"
+            + "    mode: DBG"
+            + "    compiler_flag: \"dbg\""
+            + "    cxx_flag: \"cxx-dbg\""
+            + "    linker_flag: \"linker-dbg\""
+            + "  }"
+            + "  compilation_mode_flags {"
+            + "    mode: COVERAGE"
+            + "    compiler_flag: \"coverage\""
+            + "    cxx_flag: \"cxx-coverage\""
+            + "    linker_flag: \"linker-coverage\""
+            + "  }"
+            + "  compilation_mode_flags {"
+            + "    mode: OPT"
+            + "    compiler_flag: \"opt\""
+            + "    cxx_flag: \"cxx-opt\""
+            + "    linker_flag: \"linker-opt\""
+            + "  }"
+            + "  linking_mode_flags {"
+            + "    mode: FULLY_STATIC"
+            + "    linker_flag: \"fully static\""
+            + "  }"
+            + "  linking_mode_flags {"
+            + "    mode: MOSTLY_STATIC"
+            + "    linker_flag: \"mostly static\""
+            + "  }"
+            + "  linking_mode_flags {"
+            + "    mode: DYNAMIC"
+            + "    linker_flag: \"dynamic\""
+            + "  }"
+            + "  make_variable {"
+            + "    name: \"SOME_MAKE_VARIABLE\""
+            + "    value: \"make-variable-value\""
+            + "  }"
+            + "  cxx_builtin_include_directory: \"system-include-dir\""
+            + "}");
+  }
+
+  /**
+   * Checks that we do not accidentally change the proto format in incompatible
+   * ways. Do not modify the configuration file in this test, except if you are
+   * absolutely certain that it is backwards-compatible.
+   */
+  public void testSimpleCompleteConfiguration() throws Exception {
+    CppConfigurationLoader loader = loaderWithOptionalTool("");
+
+    CppConfiguration toolchain = create(loader, "--cpu=cpu");
+    assertEquals("toolchain-identifier", toolchain.getToolchainIdentifier());
+
+    assertEquals("host-system-name", toolchain.getHostSystemName());
+    assertEquals("compiler", toolchain.getCompiler());
+    assertEquals("target-libc", toolchain.getTargetLibc());
+    assertEquals("piii", toolchain.getTargetCpu());
+    assertEquals("target-system-name", toolchain.getTargetGnuSystemName());
+
+    assertEquals(getToolPath("/path-to-ar"), toolchain.getToolPathFragment(Tool.AR));
+
+    assertEquals("abi-version", toolchain.getAbi());
+    assertEquals("abi-libc-version", toolchain.getAbiGlibcVersion());
+
+    assertTrue(toolchain.supportsGoldLinker());
+    assertTrue(toolchain.supportsThinArchives());
+    assertFalse(toolchain.supportsStartEndLib());
+    assertFalse(toolchain.supportsInterfaceSharedObjects());
+    assertFalse(toolchain.supportsEmbeddedRuntimes());
+    assertFalse(toolchain.toolchainNeedsPic());
+    assertTrue(toolchain.supportsFission());
+
+    assertEquals(
+        ImmutableList.of(getToolPath("/system-include-dir")),
+        toolchain.getBuiltInIncludeDirectories());
+    assertNull(toolchain.getSysroot());
+
+    assertEquals(Arrays.asList("c", "fastbuild"), toolchain.getCompilerOptions(NO_FEATURES));
+    assertEquals(Arrays.<String>asList(), toolchain.getCOptions());
+    assertEquals(Arrays.asList("cxx", "cxx-fastbuild"), toolchain.getCxxOptions(NO_FEATURES));
+    assertEquals(Arrays.asList("unfiltered"), toolchain.getUnfilteredCompilerOptions(NO_FEATURES));
+
+    assertEquals(Arrays.<String>asList(), toolchain.getLinkOptions());
+    assertEquals(
+        Arrays.asList("linker", "-Wl,-S", "linker-fastbuild", "fully static"),
+        toolchain.getFullyStaticLinkOptions(NO_FEATURES, false));
+    assertEquals(
+        Arrays.asList("linker", "-Wl,-S", "linker-fastbuild", "dynamic"),
+        toolchain.getDynamicLinkOptions(NO_FEATURES, false));
+    assertEquals(
+        Arrays.asList("linker", "-Wl,-S", "linker-fastbuild", "mostly static", "solinker"),
+        toolchain.getFullyStaticLinkOptions(NO_FEATURES, true));
+    assertEquals(
+        Arrays.asList("linker", "-Wl,-S", "linker-fastbuild", "dynamic", "solinker"),
+        toolchain.getDynamicLinkOptions(NO_FEATURES, true));
+
+    assertEquals(Arrays.asList("objcopy"), toolchain.getObjCopyOptionsForEmbedding());
+    assertEquals(Arrays.<String>asList(), toolchain.getLdOptionsForEmbedding());
+    assertEquals(Arrays.asList("rcsD"), toolchain.getArFlags(/*thinArchives=*/ false));
+    assertEquals(Arrays.asList("rcsDT"), toolchain.getArFlags(/*thinArchives=*/ true));
+
+    assertThat(toolchain.getAdditionalMakeVariables().entrySet())
+        .containsExactlyElementsIn(
+            ImmutableMap.of(
+                    "SOME_MAKE_VARIABLE", "make-variable-value",
+                    "STACK_FRAME_UNLIMITED", "",
+                    "CC_FLAGS", "")
+                .entrySet());
+
+    assertEquals(getToolPath("/path-to-ld"), toolchain.getLdExecutable());
+    assertEquals(getToolPath("/path-to-dwp"), toolchain.getToolPathFragment(Tool.DWP));
+  }
+
+  /**
+   * Tests all of the fields and a bunch of the combinations a config can hold,
+   * including non-default toolchains, missing sections and repeated entries
+   * (and their order in the end result.)
+   */
+  public void testComprehensiveCompleteConfiguration() throws Exception {
+    CppConfigurationLoader loader =
+        loader(
+            // Needs to include \n's; as a single line it hits a parser limitation.
+            "major_version: \"12\"\n"
+                + "minor_version: \"0\"\n"
+                + "default_target_cpu: \"piii\"\n"
+                + "default_toolchain {\n"
+                + "  cpu: \"piii\"\n"
+                + "  toolchain_identifier: \"toolchain-identifier-A\"\n"
+                + "}\n"
+                + "default_toolchain {\n"
+                + "  cpu: \"k8\"\n"
+                + "  toolchain_identifier: \"toolchain-identifier-B\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-A\"\n"
+                + "  host_system_name: \"host-system-name-A\"\n"
+                + "  target_system_name: \"target-system-name-A\"\n"
+                + "  target_cpu: \"piii\"\n"
+                + "  target_libc: \"target-libc-A\"\n"
+                + "  compiler: \"compiler-A\"\n"
+                + "  abi_version: \"abi-version-A\"\n"
+                + "  abi_libc_version: \"abi-libc-version-A\"\n"
+                + "  tool_path { name: \"ar\" path: \"path/to/ar-A\" }\n"
+                + "  tool_path { name: \"cpp\" path: \"path/to/cpp-A\" }\n"
+                + "  tool_path { name: \"gcc\" path: \"path/to/gcc-A\" }\n"
+                + "  tool_path { name: \"gcov\" path: \"path/to/gcov-A\" }\n"
+                + "  tool_path { name: \"gcov-tool\" path: \"path-to-gcov-tool-A\" }"
+                + "  tool_path { name: \"ld\" path: \"path/to/ld-A\" }\n"
+                + "  tool_path { name: \"nm\" path: \"path/to/nm-A\" }\n"
+                + "  tool_path { name: \"objcopy\" path: \"path/to/objcopy-A\" }\n"
+                + "  tool_path { name: \"objdump\" path: \"path/to/objdump-A\" }\n"
+                + "  tool_path { name: \"strip\" path: \"path/to/strip-A\" }\n"
+                + "  tool_path { name: \"dwp\" path: \"path/to/dwp\" }\n"
+                + "  supports_gold_linker: true\n"
+                + "  supports_thin_archives: true\n"
+                + "  supports_start_end_lib: true\n"
+                + "  supports_normalizing_ar: true\n"
+                + "  supports_embedded_runtimes: true\n"
+                + "  needsPic: true\n"
+                + "  compiler_flag: \"compiler-flag-A-1\"\n"
+                + "  compiler_flag: \"compiler-flag-A-2\"\n"
+                + "  cxx_flag: \"cxx-flag-A-1\"\n"
+                + "  cxx_flag: \"cxx-flag-A-2\"\n"
+                + "  unfiltered_cxx_flag: \"unfiltered-flag-A-1\"\n"
+                + "  unfiltered_cxx_flag: \"unfiltered-flag-A-2\"\n"
+                + "  linker_flag: \"linker-flag-A-1\"\n"
+                + "  linker_flag: \"linker-flag-A-2\"\n"
+                + "  dynamic_library_linker_flag: \"solinker-flag-A-1\"\n"
+                + "  dynamic_library_linker_flag: \"solinker-flag-A-2\"\n"
+                + "  objcopy_embed_flag: \"objcopy-embed-flag-A-1\"\n"
+                + "  objcopy_embed_flag: \"objcopy-embed-flag-A-2\"\n"
+                + "  ld_embed_flag: \"ld-embed-flag-A-1\"\n"
+                + "  ld_embed_flag: \"ld-embed-flag-A-2\"\n"
+                + "  ar_flag : \"ar-flag-A\"\n"
+                + "  ar_thin_archives_flag : \"ar-thin-archives-flag-A\"\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: FASTBUILD\n"
+                + "    compiler_flag: \"fastbuild-flag-A-1\"\n"
+                + "    compiler_flag: \"fastbuild-flag-A-2\"\n"
+                + "    cxx_flag: \"cxx-fastbuild-flag-A-1\"\n"
+                + "    cxx_flag: \"cxx-fastbuild-flag-A-2\"\n"
+                + "    linker_flag: \"linker-fastbuild-flag-A-1\"\n"
+                + "    linker_flag: \"linker-fastbuild-flag-A-2\"\n"
+                + "  }\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: DBG\n"
+                + "    compiler_flag: \"dbg-flag-A-1\"\n"
+                + "    compiler_flag: \"dbg-flag-A-2\"\n"
+                + "    cxx_flag: \"cxx-dbg-flag-A-1\"\n"
+                + "    cxx_flag: \"cxx-dbg-flag-A-2\"\n"
+                + "    linker_flag: \"linker-dbg-flag-A-1\"\n"
+                + "    linker_flag: \"linker-dbg-flag-A-2\"\n"
+                + "  }\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: COVERAGE\n"
+                + "  }\n"
+                + "  # skip mode OPT to test handling its absence\n"
+                + "  linking_mode_flags {\n"
+                + "    mode: FULLY_STATIC\n"
+                + "    linker_flag: \"fully-static-flag-A-1\"\n"
+                + "    linker_flag: \"fully-static-flag-A-2\"\n"
+                + "  }\n"
+                + "  linking_mode_flags {\n"
+                + "    mode: MOSTLY_STATIC\n"
+                + "  }\n"
+                + "  # skip linking mode DYNAMIC to test handling its absence\n"
+                + "  make_variable {\n"
+                + "    name: \"SOME_MAKE_VARIABLE-A-1\"\n"
+                + "    value: \"make-variable-value-A-1\"\n"
+                + "  }\n"
+                + "  make_variable {\n"
+                + "    name: \"SOME_MAKE_VARIABLE-A-2\"\n"
+                + "    value: \"make-variable-value-A-2 with spaces in\"\n"
+                + "  }\n"
+                + "  cxx_builtin_include_directory: \"system-include-dir-A-1\"\n"
+                + "  cxx_builtin_include_directory: \"system-include-dir-A-2\"\n"
+                + "  builtin_sysroot: \"builtin-sysroot-A\"\n"
+                + "  default_python_top: \"python-top-A\"\n"
+                + "  default_python_version: \"python-version-A\"\n"
+                + "  default_grte_top: \"//some:labelA\""
+                + "  debian_extra_requires: \"a\""
+                + "  debian_extra_requires: \"b\""
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-B\"\n"
+                + "  host_system_name: \"host-system-name-B\"\n"
+                + "  target_system_name: \"target-system-name-B\"\n"
+                + "  target_cpu: \"piii\"\n"
+                + "  target_libc: \"target-libc-B\"\n"
+                + "  compiler: \"compiler-B\"\n"
+                + "  abi_version: \"abi-version-B\"\n"
+                + "  abi_libc_version: \"abi-libc-version-B\"\n"
+                + "  tool_path { name: \"ar\" path: \"path/to/ar-B\" }\n"
+                + "  tool_path { name: \"cpp\" path: \"path/to/cpp-B\" }\n"
+                + "  tool_path { name: \"gcc\" path: \"path/to/gcc-B\" }\n"
+                + "  tool_path { name: \"gcov\" path: \"path/to/gcov-B\" }\n"
+                + "  tool_path { name: \"gcov-tool\" path: \"path/to/gcov-tool-B\" }\n"
+                + "  tool_path { name: \"ld\" path: \"path/to/ld-B\" }\n"
+                + "  tool_path { name: \"nm\" path: \"path/to/nm-B\" }\n"
+                + "  tool_path { name: \"objcopy\" path: \"path/to/objcopy-B\" }\n"
+                + "  tool_path { name: \"objdump\" path: \"path/to/objdump-B\" }\n"
+                + "  tool_path { name: \"strip\" path: \"path/to/strip-B\" }\n"
+                + "  tool_path { name: \"dwp\" path: \"path/to/dwp\" }\n"
+                + "  supports_gold_linker: true\n"
+                + "  supports_thin_archives: true\n"
+                + "  supports_start_end_lib: true\n"
+                + "  supports_normalizing_ar: true\n"
+                + "  supports_embedded_runtimes: true\n"
+                + "  needsPic: true\n"
+                + "  compiler_flag: \"compiler-flag-B-1\"\n"
+                + "  compiler_flag: \"compiler-flag-B-2\"\n"
+                + "  optional_compiler_flag {\n"
+                + "    default_setting_name: \"crosstool_fig\"\n"
+                + "    flag: \"-Wfig\"\n"
+                + "  }\n"
+                + "  cxx_flag: \"cxx-flag-B-1\"\n"
+                + "  cxx_flag: \"cxx-flag-B-2\"\n"
+                + "  unfiltered_cxx_flag: \"unfiltered-flag-B-1\"\n"
+                + "  unfiltered_cxx_flag: \"unfiltered-flag-B-2\"\n"
+                + "  linker_flag: \"linker-flag-B-1\"\n"
+                + "  linker_flag: \"linker-flag-B-2\"\n"
+                + "  dynamic_library_linker_flag: \"solinker-flag-B-1\"\n"
+                + "  dynamic_library_linker_flag: \"solinker-flag-B-2\"\n"
+                + "  objcopy_embed_flag: \"objcopy-embed-flag-B-1\"\n"
+                + "  objcopy_embed_flag: \"objcopy-embed-flag-B-2\"\n"
+                + "  ld_embed_flag: \"ld-embed-flag-B-1\"\n"
+                + "  ld_embed_flag: \"ld-embed-flag-B-2\"\n"
+                + "  ar_flag : \"ar-flag-B\"\n"
+                + "  ar_thin_archives_flag : \"ar-thin-archives-flag-B\"\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: FASTBUILD\n"
+                + "    compiler_flag: \"fastbuild-flag-B-1\"\n"
+                + "    compiler_flag: \"fastbuild-flag-B-2\"\n"
+                + "    cxx_flag: \"cxx-fastbuild-flag-B-1\"\n"
+                + "    cxx_flag: \"cxx-fastbuild-flag-B-2\"\n"
+                + "    linker_flag: \"linker-fastbuild-flag-B-1\"\n"
+                + "    linker_flag: \"linker-fastbuild-flag-B-2\"\n"
+                + "  }\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: DBG\n"
+                + "    compiler_flag: \"dbg-flag-B-1\"\n"
+                + "    compiler_flag: \"dbg-flag-B-2\"\n"
+                + "    cxx_flag: \"cxx-dbg-flag-B-1\"\n"
+                + "    cxx_flag: \"cxx-dbg-flag-B-2\"\n"
+                + "    linker_flag: \"linker-dbg-flag-B-1\"\n"
+                + "    linker_flag: \"linker-dbg-flag-B-2\"\n"
+                + "  }\n"
+                + "  compilation_mode_flags {\n"
+                + "    mode: COVERAGE\n"
+                + "  }\n"
+                + "  # skip mode OPT to test handling its absence\n"
+                + "  lipo_mode_flags {"
+                + "    mode: OFF"
+                + "    compiler_flag: \"lipo_off\""
+                + "    cxx_flag: \"cxx-lipo_off\""
+                + "    linker_flag: \"linker-lipo_off\""
+                + "  }"
+                + "  lipo_mode_flags {"
+                + "    mode: BINARY"
+                + "    compiler_flag: \"lipo_binary\""
+                + "    cxx_flag: \"cxx-lipo_binary\""
+                + "    linker_flag: \"linker-lipo_binary\""
+                + "  }"
+                + "  linking_mode_flags {\n"
+                + "    mode: FULLY_STATIC\n"
+                + "    linker_flag: \"fully-static-flag-B-1\"\n"
+                + "    linker_flag: \"fully-static-flag-B-2\"\n"
+                + "  }\n"
+                + "  linking_mode_flags {\n"
+                + "    mode: MOSTLY_STATIC\n"
+                + "  }\n"
+                + "  # skip linking mode DYNAMIC to test handling its absence\n"
+                + "  make_variable {\n"
+                + "    name: \"SOME_MAKE_VARIABLE-B-1\"\n"
+                + "    value: \"make-variable-value-B-1\"\n"
+                + "  }\n"
+                + "  make_variable {\n"
+                + "    name: \"SOME_MAKE_VARIABLE-B-2\"\n"
+                + "    value: \"make-variable-value-B-2 with spaces in\"\n"
+                + "  }\n"
+                + "  cxx_builtin_include_directory: \"system-include-dir-B-1\"\n"
+                + "  cxx_builtin_include_directory: \"system-include-dir-B-2\"\n"
+                + "  builtin_sysroot: \"builtin-sysroot-B\"\n"
+                + "  default_python_top: \"python-top-B\"\n"
+                + "  default_python_version: \"python-version-B\"\n"
+                + "  default_grte_top: \"//some:labelB\"\n"
+                + "  debian_extra_requires: \"c\""
+                + "  debian_extra_requires: \"d\""
+                + "}\n"
+                + "default_setting {\n"
+                + "  name: \"crosstool_fig\"\n"
+                + "  default_value: false\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-C\"\n"
+                + "  host_system_name: \"host-system-name-C\"\n"
+                + "  target_system_name: \"target-system-name-C\"\n"
+                + "  target_cpu: \"piii\"\n"
+                + "  target_libc: \"target-libc-C\"\n"
+                + "  compiler: \"compiler-C\"\n"
+                + "  abi_version: \"abi-version-C\"\n"
+                + "  abi_libc_version: \"abi-libc-version-C\"\n"
+                + "  tool_path { name: \"ar\" path: \"path/to/ar-C\" }"
+                + "  tool_path { name: \"cpp\" path: \"path/to/cpp-C\" }"
+                + "  tool_path { name: \"gcc\" path: \"path/to/gcc-C\" }"
+                + "  tool_path { name: \"gcov\" path: \"path/to/gcov-C\" }"
+                + "  tool_path { name: \"gcov-tool\" path: \"path/to/gcov-tool-C\" }"
+                + "  tool_path { name: \"ld\" path: \"path/to/ld-C\" }"
+                + "  tool_path { name: \"nm\" path: \"path/to/nm-C\" }"
+                + "  tool_path { name: \"objcopy\" path: \"path/to/objcopy-C\" }"
+                + "  tool_path { name: \"objdump\" path: \"path/to/objdump-C\" }"
+                + "  tool_path { name: \"strip\" path: \"path/to/strip-C\" }"
+                + "  tool_path { name: \"dwp\" path: \"path/to/dwp\" }\n"
+                + "}");
+
+    CppConfiguration toolchainA = create(loader, "--cpu=piii");
+    assertEquals("toolchain-identifier-A", toolchainA.getToolchainIdentifier());
+    assertEquals("host-system-name-A", toolchainA.getHostSystemName());
+    assertEquals("target-system-name-A", toolchainA.getTargetGnuSystemName());
+    assertEquals("piii", toolchainA.getTargetCpu());
+    assertEquals("target-libc-A", toolchainA.getTargetLibc());
+    assertEquals("compiler-A", toolchainA.getCompiler());
+    assertEquals("abi-version-A", toolchainA.getAbi());
+    assertEquals("abi-libc-version-A", toolchainA.getAbiGlibcVersion());
+    assertEquals(getToolPath("path/to/ar-A"), toolchainA.getToolPathFragment(Tool.AR));
+    assertEquals(getToolPath("path/to/cpp-A"), toolchainA.getToolPathFragment(Tool.CPP));
+    assertEquals(getToolPath("path/to/gcc-A"), toolchainA.getToolPathFragment(Tool.GCC));
+    assertEquals(getToolPath("path/to/gcov-A"), toolchainA.getToolPathFragment(Tool.GCOV));
+    assertEquals(getToolPath("path/to/ld-A"), toolchainA.getToolPathFragment(Tool.LD));
+    assertEquals(getToolPath("path/to/nm-A"), toolchainA.getToolPathFragment(Tool.NM));
+    assertEquals(getToolPath("path/to/objcopy-A"), toolchainA.getToolPathFragment(Tool.OBJCOPY));
+    assertEquals(getToolPath("path/to/objdump-A"), toolchainA.getToolPathFragment(Tool.OBJDUMP));
+    assertEquals(getToolPath("path/to/strip-A"), toolchainA.getToolPathFragment(Tool.STRIP));
+    assertTrue(toolchainA.supportsGoldLinker());
+    assertTrue(toolchainA.supportsThinArchives());
+    assertTrue(toolchainA.supportsStartEndLib());
+    assertTrue(toolchainA.supportsEmbeddedRuntimes());
+    assertTrue(toolchainA.toolchainNeedsPic());
+
+    assertEquals(
+        Arrays.asList(
+            "compiler-flag-A-1", "compiler-flag-A-2", "fastbuild-flag-A-1", "fastbuild-flag-A-2"),
+        toolchainA.getCompilerOptions(NO_FEATURES));
+    assertEquals(
+        Arrays.asList(
+            "cxx-flag-A-1", "cxx-flag-A-2", "cxx-fastbuild-flag-A-1", "cxx-fastbuild-flag-A-2"),
+        toolchainA.getCxxOptions(NO_FEATURES));
+    assertEquals(
+        Arrays.asList("--sysroot=some", "unfiltered-flag-A-1", "unfiltered-flag-A-2"),
+        toolchainA.getUnfilteredCompilerOptions(NO_FEATURES));
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-A-1",
+            "linker-flag-A-2",
+            "-Wl,-S",
+            "linker-fastbuild-flag-A-1",
+            "linker-fastbuild-flag-A-2",
+            "solinker-flag-A-1",
+            "solinker-flag-A-2"),
+        toolchainA.getDynamicLinkOptions(NO_FEATURES, true));
+
+    // Only test a couple of compilation/lipo/linking mode combinations
+    // (but test each mode at least once.)
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-A-1",
+            "linker-flag-A-2",
+            "-Wl,-S",
+            "linker-fastbuild-flag-A-1",
+            "linker-fastbuild-flag-A-2",
+            "fully-static-flag-A-1",
+            "fully-static-flag-A-2"),
+        toolchainA.configureLinkerOptions(
+            CompilationMode.FASTBUILD,
+            LipoMode.OFF,
+            LinkingMode.FULLY_STATIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-A-1",
+            "linker-flag-A-2",
+            "-Wl,-S",
+            "linker-dbg-flag-A-1",
+            "linker-dbg-flag-A-2"),
+        toolchainA.configureLinkerOptions(
+            CompilationMode.DBG,
+            LipoMode.OFF,
+            LinkingMode.DYNAMIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-A-1",
+            "linker-flag-A-2",
+            "-Wl,-S",
+            "fully-static-flag-A-1",
+            "fully-static-flag-A-2"),
+        toolchainA.configureLinkerOptions(
+            CompilationMode.OPT,
+            LipoMode.OFF,
+            LinkingMode.FULLY_STATIC,
+            new PathFragment("hello-world/ld"),
+            true));
+
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-A-1",
+            "linker-flag-A-2",
+            "-Wl,-S",
+            "fully-static-flag-A-1",
+            "fully-static-flag-A-2"),
+        toolchainA.configureLinkerOptions(
+            CompilationMode.OPT,
+            LipoMode.BINARY,
+            LinkingMode.FULLY_STATIC,
+            new PathFragment("hello-world/ld"),
+            true));
+
+    assertEquals(
+        Arrays.asList("objcopy-embed-flag-A-1", "objcopy-embed-flag-A-2"),
+        toolchainA.getObjCopyOptionsForEmbedding());
+    assertEquals(
+        Arrays.asList("ld-embed-flag-A-1", "ld-embed-flag-A-2"),
+        toolchainA.getLdOptionsForEmbedding());
+    assertEquals(Arrays.asList("ar-flag-A"), toolchainA.getArFlags(/*thinArchives=*/ false));
+    assertEquals(
+        Arrays.asList("ar-thin-archives-flag-A"), toolchainA.getArFlags(/*thinArchives=*/ true));
+
+    assertThat(toolchainA.getAdditionalMakeVariables().entrySet())
+        .containsExactlyElementsIn(
+            ImmutableMap.<String, String>builder()
+                .put("SOME_MAKE_VARIABLE-A-1", "make-variable-value-A-1")
+                .put("SOME_MAKE_VARIABLE-A-2", "make-variable-value-A-2 with spaces in")
+                .put("CC_FLAGS", "--sysroot=some")
+                .put("STACK_FRAME_UNLIMITED", "")
+                .build()
+                .entrySet());
+    assertEquals(
+        Arrays.asList(
+            getToolPath("/system-include-dir-A-1"), getToolPath("/system-include-dir-A-2")),
+        toolchainA.getBuiltInIncludeDirectories());
+    assertEquals(new PathFragment("some"), toolchainA.getSysroot());
+
+    // Cursory testing of the "B" toolchain only; assume that if none of
+    // toolchain B bled through into toolchain A, the reverse also didn't occur. And
+    // we test more of it with the "C" toolchain below.
+    checkToolchainB(loader, LipoMode.OFF, "--cpu=k8", "--lipo=off");
+    checkToolchainB(loader, LipoMode.BINARY, "--cpu=k8", "--lipo=binary");
+
+    // Make sure nothing bled through to the nearly-empty "C" toolchain. This is also testing for
+    // all the defaults.
+    CppConfiguration toolchainC =
+        create(loader, "--compiler=compiler-C", "--glibc=target-libc-C", "--cpu=piii");
+    assertEquals("toolchain-identifier-C", toolchainC.getToolchainIdentifier());
+    assertEquals("host-system-name-C", toolchainC.getHostSystemName());
+    assertEquals("target-system-name-C", toolchainC.getTargetGnuSystemName());
+    assertEquals("piii", toolchainC.getTargetCpu());
+    assertEquals("target-libc-C", toolchainC.getTargetLibc());
+    assertEquals("compiler-C", toolchainC.getCompiler());
+    assertEquals("abi-version-C", toolchainC.getAbi());
+    assertEquals("abi-libc-version-C", toolchainC.getAbiGlibcVersion());
+    // Don't bother with testing the list of tools again.
+    assertFalse(toolchainC.supportsGoldLinker());
+    assertFalse(toolchainC.supportsThinArchives());
+    assertFalse(toolchainC.supportsStartEndLib());
+    assertFalse(toolchainC.supportsInterfaceSharedObjects());
+    assertFalse(toolchainC.supportsEmbeddedRuntimes());
+    assertFalse(toolchainC.toolchainNeedsPic());
+    assertFalse(toolchainC.supportsFission());
+
+    assertThat(toolchainC.getCompilerOptions(NO_FEATURES)).isEmpty();
+    assertThat(toolchainC.getCOptions()).isEmpty();
+    assertThat(toolchainC.getCxxOptions(NO_FEATURES)).isEmpty();
+    assertThat(toolchainC.getUnfilteredCompilerOptions(NO_FEATURES)).isEmpty();
+    assertEquals(Arrays.asList("-Wl,-S"), toolchainC.getDynamicLinkOptions(NO_FEATURES, true));
+    assertEquals(
+        Arrays.asList("-Wl,-S"),
+        toolchainC.configureLinkerOptions(
+            CompilationMode.FASTBUILD,
+            LipoMode.OFF,
+            LinkingMode.FULLY_STATIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertEquals(
+        Arrays.asList("-Wl,-S"),
+        toolchainC.configureLinkerOptions(
+            CompilationMode.DBG,
+            LipoMode.OFF,
+            LinkingMode.DYNAMIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertEquals(
+        Arrays.asList("-Wl,-S"),
+        toolchainC.configureLinkerOptions(
+            CompilationMode.OPT,
+            LipoMode.OFF,
+            LinkingMode.FULLY_STATIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertThat(toolchainC.getObjCopyOptionsForEmbedding()).isEmpty();
+    assertThat(toolchainC.getLdOptionsForEmbedding()).isEmpty();
+
+    assertThat(toolchainC.getAdditionalMakeVariables().entrySet())
+        .containsExactlyElementsIn(
+            ImmutableMap.<String, String>builder()
+                .put("CC_FLAGS", "")
+                .put("STACK_FRAME_UNLIMITED", "")
+                .build()
+                .entrySet());
+    assertThat(toolchainC.getBuiltInIncludeDirectories()).isEmpty();
+    assertNull(toolchainC.getSysroot());
+  }
+
+  protected PathFragment getToolPath(String path) throws LabelSyntaxException {
+    PackageIdentifier packageIdentifier =
+        PackageIdentifier.create(
+            TestConstants.TOOLS_REPOSITORY,
+            new PathFragment(
+                new PathFragment(TestConstants.MOCK_CROSSTOOL_PATH), new PathFragment(path)));
+    return packageIdentifier.getPathFragment();
+  }
+
+  private void checkToolchainB(CppConfigurationLoader loader, LipoMode lipoMode, String... args)
+      throws Exception {
+    String lipoSuffix = lipoMode.toString().toLowerCase();
+    CppConfiguration toolchainB = create(loader, args);
+    assertEquals("toolchain-identifier-B", toolchainB.getToolchainIdentifier());
+    assertEquals(
+        Arrays.asList(
+            "linker-flag-B-1",
+            "linker-flag-B-2",
+            "-Wl,-S",
+            "linker-dbg-flag-B-1",
+            "linker-dbg-flag-B-2",
+            "linker-lipo_" + lipoSuffix),
+        toolchainB.configureLinkerOptions(
+            CompilationMode.DBG,
+            lipoMode,
+            LinkingMode.DYNAMIC,
+            new PathFragment("hello-world/ld"),
+            true));
+    assertEquals(
+        ImmutableList.<String>of(
+            "compiler-flag-B-1",
+            "compiler-flag-B-2",
+            "fastbuild-flag-B-1",
+            "fastbuild-flag-B-2",
+            "lipo_" + lipoSuffix,
+            "-Wfig"),
+        toolchainB.getCompilerOptions(ImmutableList.of("crosstool_fig")));
+  }
+
+  /**
+   * Tests that we can select a toolchain using a subset of the --compiler and
+   * --glibc flags, as long as they select a unique result. Also tests the error
+   * messages we get when they don't.
+   */
+  public void testCompilerLibcSearch() throws Exception {
+    CppConfigurationLoader loader =
+        loader(
+            // Needs to include \n's; as a single line it hits a parser limitation.
+            "major_version: \"12\"\n"
+                + "minor_version: \"0\"\n"
+                + "default_target_cpu: \"k8\"\n"
+                + "default_toolchain {\n"
+                + "  cpu: \"piii\"\n"
+                + "  toolchain_identifier: \"toolchain-identifier-AA-piii\"\n"
+                + "}\n"
+                + "default_toolchain {\n"
+                + "  cpu: \"k8\"\n"
+                + "  toolchain_identifier: \"toolchain-identifier-BB\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-AA\"\n"
+                + "  host_system_name: \"host-system-name-AA\"\n"
+                + "  target_system_name: \"target-system-name-AA\"\n"
+                + "  target_cpu: \"k8\"\n"
+                + "  target_libc: \"target-libc-A\"\n"
+                + "  compiler: \"compiler-A\"\n"
+                + "  abi_version: \"abi-version-A\"\n"
+                + "  abi_libc_version: \"abi-libc-version-A\"\n"
+                + "}\n"
+                // AA-piii is uniquely determined by libc and compiler.
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-AA-piii\"\n"
+                + "  host_system_name: \"host-system-name-AA\"\n"
+                + "  target_system_name: \"target-system-name-AA\"\n"
+                + "  target_cpu: \"piii\"\n"
+                + "  target_libc: \"target-libc-A\"\n"
+                + "  compiler: \"compiler-A\"\n"
+                + "  abi_version: \"abi-version-A\"\n"
+                + "  abi_libc_version: \"abi-libc-version-A\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-AB\"\n"
+                + "  host_system_name: \"host-system-name-AB\"\n"
+                + "  target_system_name: \"target-system-name-AB\"\n"
+                + "  target_cpu: \"k8\"\n"
+                + "  target_libc: \"target-libc-A\"\n"
+                + "  compiler: \"compiler-B\"\n"
+                + "  abi_version: \"abi-version-B\"\n"
+                + "  abi_libc_version: \"abi-libc-version-A\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-BA\"\n"
+                + "  host_system_name: \"host-system-name-BA\"\n"
+                + "  target_system_name: \"target-system-name-BA\"\n"
+                + "  target_cpu: \"k8\"\n"
+                + "  target_libc: \"target-libc-B\"\n"
+                + "  compiler: \"compiler-A\"\n"
+                + "  abi_version: \"abi-version-A\"\n"
+                + "  abi_libc_version: \"abi-libc-version-B\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-BB\"\n"
+                + "  host_system_name: \"host-system-name-BB\"\n"
+                + "  target_system_name: \"target-system-name-BB\"\n"
+                + "  target_cpu: \"k8\"\n"
+                + "  target_libc: \"target-libc-B\"\n"
+                + "  compiler: \"compiler-B\"\n"
+                + "  abi_version: \"abi-version-B\"\n"
+                + "  abi_libc_version: \"abi-libc-version-B\"\n"
+                + "}\n"
+                + "toolchain {\n"
+                + "  toolchain_identifier: \"toolchain-identifier-BC\"\n"
+                + "  host_system_name: \"host-system-name-BC\"\n"
+                + "  target_system_name: \"target-system-name-BC\"\n"
+                + "  target_cpu: \"k8\"\n"
+                + "  target_libc: \"target-libc-B\"\n"
+                + "  compiler: \"compiler-C\"\n"
+                + "  abi_version: \"abi-version-C\"\n"
+                + "  abi_libc_version: \"abi-libc-version-B\"\n"
+                + "}");
+
+    // Uses the default toolchain for k8.
+    assertEquals("toolchain-identifier-BB", create(loader, "--cpu=k8").getToolchainIdentifier());
+    // Defaults to --cpu=k8.
+    assertEquals(
+        "toolchain-identifier-BA",
+        create(loader, "--compiler=compiler-A", "--glibc=target-libc-B").getToolchainIdentifier());
+    // Uses the default toolchain for piii.
+    assertEquals(
+        "toolchain-identifier-AA-piii", create(loader, "--cpu=piii").getToolchainIdentifier());
+
+    // We can select the unique piii toolchain with either its compiler or glibc.
+    assertEquals(
+        "toolchain-identifier-AA-piii",
+        create(loader, "--cpu=piii", "--compiler=compiler-A").getToolchainIdentifier());
+    assertEquals(
+        "toolchain-identifier-AA-piii",
+        create(loader, "--cpu=piii", "--glibc=target-libc-A").getToolchainIdentifier());
+
+    // compiler-C uniquely identifies a toolchain, so we can use it.
+    assertEquals(
+        "toolchain-identifier-BC",
+        create(loader, "--compiler=compiler-C").getToolchainIdentifier());
+
+    try {
+      create(loader, "--compiler=nonexistent-compiler");
+      fail("Expected an error that no toolchain matched.");
+    } catch (InvalidConfigurationException e) {
+      assertThat(e)
+          .hasMessage(
+              "No toolchain found for --cpu='k8' --compiler='nonexistent-compiler'. "
+                  + "Valid toolchains are: [\n"
+                  + "  --cpu='k8' --compiler='compiler-A' --glibc='target-libc-A',\n"
+                  + "  --cpu='piii' --compiler='compiler-A' --glibc='target-libc-A',\n"
+                  + "  --cpu='k8' --compiler='compiler-B' --glibc='target-libc-A',\n"
+                  + "  --cpu='k8' --compiler='compiler-A' --glibc='target-libc-B',\n"
+                  + "  --cpu='k8' --compiler='compiler-B' --glibc='target-libc-B',\n"
+                  + "  --cpu='k8' --compiler='compiler-C' --glibc='target-libc-B',\n"
+                  + "]");
+    }
+
+    try {
+      create(loader, "--glibc=target-libc-A");
+      fail("Expected an error that multiple toolchains matched.");
+    } catch (InvalidConfigurationException e) {
+      assertThat(e)
+          .hasMessage(
+              "Multiple toolchains found for --cpu='k8' --glibc='target-libc-A': [\n"
+                  + "  --cpu='k8' --compiler='compiler-A' --glibc='target-libc-A',\n"
+                  + "  --cpu='k8' --compiler='compiler-B' --glibc='target-libc-A',\n"
+                  + "]");
+    }
+  }
+
+  private void assertStringStartsWith(String expected, String text) {
+    if (!text.startsWith(expected)) {
+      fail("expected <" + expected + ">, but got <" + text + ">");
+    }
+  }
+
+  public void testIncompleteFile() throws Exception {
+    try {
+      CrosstoolConfigurationLoader.toReleaseConfiguration("/CROSSTOOL", "major_version: \"12\"");
+      fail();
+    } catch (IOException e) {
+      assertStringStartsWith(
+          "Could not read the crosstool configuration file "
+              + "'/CROSSTOOL', because of an incomplete protocol buffer",
+          e.getMessage());
+    }
+  }
+
+  /**
+   * Returns a test crosstool config with the specified tool missing from the tool_path
+   * set. Also allows injection of custom fields.
+   */
+  private static String getConfigWithMissingToolDef(Tool missingTool, String... customFields) {
+    StringBuilder s =
+        new StringBuilder(
+            "major_version: \"12\""
+                + "minor_version: \"0\""
+                + "default_target_cpu: \"cpu\""
+                + "default_toolchain {"
+                + "  cpu: \"cpu\""
+                + "  toolchain_identifier: \"toolchain-identifier\""
+                + "}"
+                + "toolchain {"
+                + "  toolchain_identifier: \"toolchain-identifier\""
+                + "  host_system_name: \"host-system-name\""
+                + "  target_system_name: \"target-system-name\""
+                + "  target_cpu: \"piii\""
+                + "  target_libc: \"target-libc\""
+                + "  compiler: \"compiler\""
+                + "  abi_version: \"abi-version\""
+                + "  abi_libc_version: \"abi-libc-version\"");
+
+    for (String customField : customFields) {
+      s.append(customField);
+    }
+    for (Tool tool : Tool.values()) {
+      if (tool != missingTool) {
+        String toolName = tool.getNamePart();
+        s.append("  tool_path { name: \"" + toolName + "\" path: \"path-to-" + toolName + "\" }");
+      }
+    }
+    s.append("}");
+    return s.toString();
+  }
+
+  public void testConfigWithMissingToolDefs() throws Exception {
+    CppConfigurationLoader loader = loader(getConfigWithMissingToolDef(Tool.STRIP));
+    try {
+      create(loader, "--cpu=cpu");
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e.getMessage()).contains("Tool path for 'strip' is missing");
+    }
+  }
+
+  /**
+   * For a fission-supporting crosstool: check the dwp tool path.
+   */
+  public void testFissionConfigWithMissingDwp() throws Exception {
+    CppConfigurationLoader loader =
+        loader(getConfigWithMissingToolDef(Tool.DWP, "supports_fission: true"));
+    try {
+      create(loader, "--cpu=cpu");
+      fail("Expected failed check on 'dwp' tool path");
+    } catch (IllegalArgumentException e) {
+      assertThat(e.getMessage()).contains("Tool path for 'dwp' is missing");
+    }
+  }
+
+  /**
+   * For a non-fission-supporting crosstool, there's no need to check the dwp tool path.
+   */
+  public void testNonFissionConfigWithMissingDwp() throws Exception {
+    CppConfigurationLoader loader =
+        loader(getConfigWithMissingToolDef(Tool.DWP, "supports_fission: false"));
+    // The following line throws an IllegalArgumentException if an expected tool path is missing.
+    create(loader, "--cpu=cpu");
+  }
+
+  public void testInvalidFile() throws Exception {
+    try {
+      CrosstoolConfigurationLoader.toReleaseConfiguration("/CROSSTOOL", "some xxx : yak \"");
+      fail();
+    } catch (IOException e) {
+      assertStringStartsWith(
+          "Could not read the crosstool configuration file "
+              + "'/CROSSTOOL', because of a parser error",
+          e.getMessage());
+    }
+  }
+
+  /**
+   * Tests interpretation of static_runtimes_filegroup / dynamic_runtimes_filegroup.
+   */
+  public void testCustomRuntimeLibraryPaths() throws Exception {
+    CppConfigurationLoader loader =
+        loader(
+            "major_version: \"v17\""
+                + "minor_version: \"0\""
+                + "default_target_cpu: \"cpu\""
+                + "default_toolchain {"
+                + "  cpu: \"piii\""
+                + "  toolchain_identifier: \"default-libs\""
+                + "}"
+                + "default_toolchain {"
+                + "  cpu: \"k8\""
+                + "  toolchain_identifier: \"custom-libs\""
+                + "}"
+                + "toolchain {" // "default-libs": runtime libraries in default locations.
+                + "  toolchain_identifier: \"default-libs\""
+                + "  host_system_name: \"host-system-name\""
+                + "  target_system_name: \"target-system-name\""
+                + "  target_cpu: \"piii\""
+                + "  target_libc: \"target-libc\""
+                + "  compiler: \"compiler\""
+                + "  abi_version: \"abi-version\""
+                + "  abi_libc_version: \"abi-libc-version\""
+                + "  supports_embedded_runtimes: true"
+                + "}\n"
+                + "toolchain {" // "custom-libs" runtime libraries in toolchain-specified locations.
+                + "  toolchain_identifier: \"custom-libs\""
+                + "  host_system_name: \"host-system-name\""
+                + "  target_system_name: \"target-system-name\""
+                + "  target_cpu: \"k8\""
+                + "  target_libc: \"target-libc\""
+                + "  compiler: \"compiler\""
+                + "  abi_version: \"abi-version\""
+                + "  abi_libc_version: \"abi-libc-version\""
+                + "  supports_embedded_runtimes: true"
+                + "  static_runtimes_filegroup: \"static-group\""
+                + "  dynamic_runtimes_filegroup: \"dynamic-group\""
+                + "}\n");
+
+    final String ctTop = TestConstants.TOOLS_REPOSITORY + "//" + TestConstants.MOCK_CROSSTOOL_PATH;
+
+    CppConfiguration defaultLibs = create(loader, "--cpu=piii");
+    assertEquals(
+        ctTop + ":static-runtime-libs-piii", defaultLibs.getStaticRuntimeLibsLabel().toString());
+    assertEquals(
+        ctTop + ":dynamic-runtime-libs-piii", defaultLibs.getDynamicRuntimeLibsLabel().toString());
+
+    CppConfiguration customLibs = create(loader, "--cpu=k8");
+    assertEquals(ctTop + ":static-group", customLibs.getStaticRuntimeLibsLabel().toString());
+    assertEquals(ctTop + ":dynamic-group", customLibs.getDynamicRuntimeLibsLabel().toString());
+  }
+
+  /*
+   * Crosstools should load fine with or without 'gcov-tool'. Those that define 'gcov-tool'
+   * should also add a make variable.
+   */
+  public void testOptionalGcovTool() throws Exception {
+    // Crosstool with gcov-tool
+    CppConfigurationLoader loader =
+        loaderWithOptionalTool("  tool_path { name: \"gcov-tool\" path: \"path-to-gcov-tool\" }");
+    CppConfiguration cppConfig = create(loader, "--cpu=cpu");
+    ImmutableMap.Builder<String, String> builder = ImmutableMap.builder();
+    cppConfig.addGlobalMakeVariables(builder);
+    assertNotNull(builder.build().get("GCOVTOOL"));
+
+    // Crosstool without gcov-tool
+    loader = loaderWithOptionalTool("");
+    cppConfig = create(loader, "--cpu=cpu");
+    builder = ImmutableMap.builder();
+    cppConfig.addGlobalMakeVariables(builder);
+    assertThat(builder.build()).doesNotContainKey("GCOVTOOL");
+  }
+}
diff --git a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
index 1c51bd5..b6d3e54 100644
--- a/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
+++ b/src/test/java/com/google/devtools/build/lib/testutil/TestConstants.java
@@ -68,4 +68,6 @@
   public static final String GCC_INCLUDE_PATH = "external/bazel_tools/tools/cpp/gcc3";
 
   public static final String TOOLS_REPOSITORY = "@bazel_tools";
+
+  public static final String MOCK_CROSSTOOL_PATH = "tools/cpp";
 }