Add support for parallel compilation

Author: luispadron

swift/internal/action_names.bzl

@@ -20,9 +20,17 @@
 SWIFT_ACTION_AUTOLINK_EXTRACT = "SwiftAutolinkExtract"
 
 # Compiles one or more `.swift` source files into a `.swiftmodule` and
-# object files.
+# object files. This is the legacy mode that emits all outputs from a single
+# driver invocation.
 SWIFT_ACTION_COMPILE = "SwiftCompile"
 
+# Emits the object files and other per-source-file outputs for a a batch of
+# `.swift` source files in a module.
+SWIFT_ACTION_COMPILE_CODEGEN = "SwiftCompileCodegen"
+
+# Compiles one or more `.swift` source files into a `.swiftmodule`.
+SWIFT_ACTION_COMPILE_MODULE = "SwiftCompileModule"
+
 # Compiles a `.swiftinterface` file into a `.swiftmodule` file.
 SWIFT_ACTION_COMPILE_MODULE_INTERFACE = "SwiftCompileModuleInterface"
 
@@ -54,6 +62,8 @@ def all_action_names():
     return (
         SWIFT_ACTION_AUTOLINK_EXTRACT,
         SWIFT_ACTION_COMPILE,
+        SWIFT_ACTION_COMPILE_CODEGEN,
+        SWIFT_ACTION_COMPILE_MODULE,
         SWIFT_ACTION_COMPILE_MODULE_INTERFACE,
         SWIFT_ACTION_DERIVE_FILES,
         SWIFT_ACTION_DUMP_AST,
@@ -67,5 +77,6 @@ def all_compile_action_names():
     """Returns all actions that compile source files."""
     return [
         SWIFT_ACTION_COMPILE,
-        SWIFT_ACTION_COMPILE_MODULE_INTERFACE,
+        SWIFT_ACTION_COMPILE_CODEGEN,
+        SWIFT_ACTION_COMPILE_MODULE,
     ]

swift/internal/feature_names.bzl

@@ -369,6 +369,10 @@ SWIFT_FEATURE_INTERNALIZE_AT_LINK = "swift.internalize_at_link"
 # This disables checking for potentially unavailable APIs.
 SWIFT_FEATURE_DISABLE_AVAILABILITY_CHECKING = "swift.disable_availability_checking"
 
+# If enabled, parallelize the compilation of Swift modules and their object
+# files by registering separate actions for each.
+SWIFT_FEATURE_COMPILE_IN_PARALLEL = "swift.compile_in_parallel"
+
 # A private feature that is set by the toolchain if it supports the
 # `-enable-{experimental,upcoming}-feature` flag (Swift 5.8 and above). Users
 # should never manually, enable, disable, or query this feature.

swift/providers.bzl

@@ -336,6 +336,10 @@ C/Objective-C modules:
 
 For ease of use, this field is never `None`; it will always be a valid `struct`
 containing the fields described above, even if those lists are empty.
+""",
+        "codegen_batch_size": """\
+The number of files to pass to the compiler in a single code generation action
+(one that compiles object files from Swift source files).
 """,
         "const_protocols_to_gather": """\
 `File`. A JSON file specifying a list of protocols for extraction of