[build] Clean llbuild before building by default

utils/build-script

@@ -796,6 +796,11 @@ class BuildScriptInvocation(object):
                 "--llvm-install-components=%s" % args.llvm_install_components
             ]
 
+        if not args.clean_llbuild:
+            impl_args += [
+                "--skip-clean-llbuild"
+            ]
+
         # Compute the set of host-specific variables, which we pass through to
         # the build script via environment variables.
         host_specific_variables = self.compute_host_specific_variables()

utils/build-script-impl

@@ -162,6 +162,7 @@ KNOWN_SETTINGS=(
 
     ## llbuild Options
     llbuild-enable-assertions                     "1"               "enable assertions in llbuild"
+    skip-clean-llbuild                            "0"               "skip cleaning up llbuild"
 
     ## LLDB Options
     lldb-assertions                               "1"               "build lldb with assertions enabled"
@@ -2189,6 +2190,15 @@ for host in "${ALL_HOSTS[@]}"; do
                         -DSQLite3_INCLUDE_DIR:PATH="$(xcrun -sdk macosx -show-sdk-path)/usr/include"
                     )
                 fi
+
+                if [[ "${SKIP_CLEAN_LLBUILD}" == "0" ]]
+                then
+                  # Ensure llbuild will rebuild from scratch, since
+                  # arbitrary changes to the compiler can prevent
+                  # a successful incremental build
+                  echo "Cleaning the llbuild build directory"
+                  call rm -rf "$(build_directory ${host} llbuild)"
+                fi
                 ;;
             xctest)
                 XCTEST_BUILD_DIR=$(build_directory ${host} xctest)

utils/build_swift/build_swift/driver_arguments.py

@@ -1037,6 +1037,8 @@ def create_argument_parser():
            help='skip testing Android device targets on the host machine (the '
                 'phone itself)')
 
+    option('--skip-clean-llbuild', toggle_false('clean_llbuild'),
+           help='skip cleaning up llbuild')
     option('--skip-clean-swiftpm', toggle_false('clean_swiftpm'),
            help='skip cleaning up swiftpm')
     option('--skip-clean-swift-driver', toggle_false('clean_swift_driver'),

utils/build_swift/tests/expected_options.py

@@ -212,6 +212,7 @@
         defaults.SWIFT_MAX_PARALLEL_LTO_LINK_JOBS,
     'swift_user_visible_version': defaults.SWIFT_USER_VISIBLE_VERSION,
     'symbols_package': None,
+    'clean_llbuild': True,
     'clean_swiftpm': True,
     'clean_swift_driver': True,
     'test': None,
@@ -578,6 +579,7 @@ class BuildScriptImplOption(_BaseOption):
                   dest='build_watchos_device'),
     DisableOption('--skip-build-watchos-simulator',
                   dest='build_watchos_simulator'),
+    DisableOption('--skip-clean-llbuild', dest='clean_llbuild'),
     DisableOption('--skip-clean-swiftpm', dest='clean_swiftpm'),
     DisableOption('--skip-clean-swift-driver', dest='clean_swift_driver'),
     DisableOption('--skip-test-android', dest='test_android'),

validation-test/BuildSystem/skip_clean_llbuild.test

@@ -0,0 +1,15 @@
+# REQUIRES: standalone_build
+
+# RUN: %empty-directory(%t)
+# RUN: mkdir -p %t
+# RUN: SWIFT_BUILD_ROOT=%t %swift_src_root/utils/build-script --dry-run --llbuild --cmake %cmake  2>&1 | %FileCheck --check-prefix=CLEAN-LLBUILD-CHECK %s
+
+# RUN: %empty-directory(%t)
+# RUN: mkdir -p %t
+# RUN: SWIFT_BUILD_ROOT=%t %swift_src_root/utils/build-script --dry-run --llbuild --skip-clean-llbuild --cmake %cmake  2>&1 | %FileCheck --check-prefix=SKIP-CLEAN-LLBUILD-CHECK %s
+
+# CLEAN-LLBUILD-CHECK: Cleaning the llbuild build directory
+# CLEAN-LLBUILD-CHECK-NEXT: rm -rf
+
+# SKIP-CLEAN-LLBUILD-CHECK-NOT: Cleaning the llbuild build directory
+# SKIP-CLEAN-LLBUILD-CHECK-NOT: rm -rf {{.*/llbuild-[^/]*}}