Also detect -O flags being set via --swiftcopt when planning parallel compilation.

In the future, we should consider whether we should more strictly control where these flags can be applied, since they don't just control optimizations but also how the driver plans its work.

Cherry-pick: ab55a12

Author: allevato

swift/internal/compiling.bzl

@@ -64,6 +64,7 @@ load(
     "SWIFT_FEATURE_USE_EXPLICIT_SWIFT_MODULE_MAP",
     "SWIFT_FEATURE_VFSOVERLAY",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
+    "SWIFT_FEATURE__OPT_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__WMO_IN_SWIFTCOPTS",
 )
 load(
@@ -823,6 +824,9 @@ def _should_plan_parallel_compilation(
         ) and not is_feature_enabled(
             feature_configuration = feature_configuration,
             feature_name = SWIFT_FEATURE_OPT,
+        ) and not is_feature_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE__OPT_IN_SWIFTCOPTS,
         )
     )
 

swift/internal/feature_names.bzl

@@ -356,6 +356,11 @@ SWIFT_FEATURE_LLD_GC_WORKAROUND = "swift.lld_gc_workaround"
 # objects if you know that isn't required.
 SWIFT_FEATURE_OBJC_LINK_FLAGS = "swift.objc_link_flag"
 
+# A private feature that is set by the toolchain if an optimization flag in the
+# `-O` flag group was passed on the command line using `--swiftcopt`. Users
+# should never manually enable, disable, or query this feature.
+SWIFT_FEATURE__OPT_IN_SWIFTCOPTS = "swift._opt_in_swiftcopts"
+
 # If enabled, requests the `-enforce-exclusivity=checked` swiftc flag which
 # enables runtime checking of exclusive memory access on mutation.
 SWIFT_FEATURE_CHECKED_EXCLUSIVITY = "swift.checked_exclusivity"

swift/internal/optimization.bzl

@@ -17,6 +17,7 @@
 load(
     ":feature_names.bzl",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
+    "SWIFT_FEATURE__OPT_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__WMO_IN_SWIFTCOPTS",
 )
 
@@ -112,6 +113,8 @@ def optimization_features_from_swiftcopts(swiftcopts):
     requested_features = []
     unsupported_features = []
 
+    if is_optimization_manually_requested(user_compile_flags = swiftcopts):
+        requested_features.append(SWIFT_FEATURE__OPT_IN_SWIFTCOPTS)
     if is_wmo_manually_requested(user_compile_flags = swiftcopts):
         requested_features.append(SWIFT_FEATURE__WMO_IN_SWIFTCOPTS)
     if find_num_threads_flag_value(user_compile_flags = swiftcopts) == 1:

test/parallel_compilation_tests.bzl

@@ -29,6 +29,12 @@ opt_actions_create_test = make_actions_created_test_rule(
     },
 )
 
+opt_via_swiftcopt_actions_create_test = make_actions_created_test_rule(
+    config_settings = {
+        "@build_bazel_rules_swift//swift:copt": ["-O"],
+    },
+)
+
 def parallel_compilation_test_suite(name, tags = []):
     """Test suite for parallel compilation.
 
@@ -87,6 +93,15 @@ def parallel_compilation_test_suite(name, tags = []):
         target_under_test = "@build_bazel_rules_swift//test/fixtures/parallel_compilation:no_opt_with_wmo",
     )
 
+    # Force `-O` using the `copt` flag on a non-optimized, with-WMO target and
+    # make sure we don't plan parallel compilation there.
+    opt_via_swiftcopt_actions_create_test(
+        name = "{}_no_opt_with_wmo_but_swiftcopt_dash_O".format(name),
+        mnemonics = ["-SwiftCompileModule", "-SwiftCompileCodegen", "SwiftCompile"],
+        tags = all_tags,
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/parallel_compilation:no_opt_with_wmo",
+    )
+
     # Optimized, with-WMO can be compiled in parallel if library evolution is
     # enabled (which implicitly disables CMO).
     actions_created_test(