Feature flag to set num-threads to host threads

Currently, num-threads is hardcoded to 12. This PR sets the value to the
number of cores return by sysctl, or "host threads". This may improve
vertical scaling in some situations for larger core machines.

swift/internal/compiling.bzl

@@ -60,6 +60,7 @@ load(
     "SWIFT_FEATURE_USE_C_MODULES",
     "SWIFT_FEATURE_USE_GLOBAL_INDEX_STORE",
     "SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE",
+    "SWIFT_FEATURE_USE_HOST_NUM_THREADS",
     "SWIFT_FEATURE_VFSOVERLAY",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__WMO_IN_SWIFTCOPTS",
@@ -767,6 +768,16 @@ def compile_action_configs(
             ],
             not_features = [SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS],
         ),
+        swift_toolchain_config.action_config(
+            actions = [
+                swift_action_names.COMPILE,
+                swift_action_names.DERIVE_FILES,
+            ],
+            configurators = [_wmo_host_thread_count_configurator],
+            features = [
+                [SWIFT_FEATURE_USE_HOST_NUM_THREADS, SWIFT_FEATURE__WMO_IN_SWIFTCOPTS],
+            ],
+        ),
         swift_toolchain_config.action_config(
             actions = [
                 swift_action_names.COMPILE,
@@ -1385,6 +1396,11 @@ def _wmo_thread_count_configurator(should_check_flags, prerequisites, args):
         # Force threaded mode for WMO builds.
         args.add("-num-threads", str(_DEFAULT_WMO_THREAD_COUNT))
 
+def _wmo_host_thread_count_configurator(prerequisites, args):
+    """Sets num-threads to number of threads on the host.
+    """
+    args.add("-Xwrapped-swift=-use-host-num-threads")
+
 def _is_wmo_manually_requested(user_compile_flags):
     """Returns `True` if a WMO flag is in the given list of compiler flags.
 

swift/internal/feature_names.bzl

@@ -266,3 +266,6 @@ SWIFT_FEATURE__WMO_IN_SWIFTCOPTS = "swift._wmo_in_swiftcopts"
 # were passed on the command line using `--swiftcopt`. Users should never
 # manually enable, disable, or query this feature.
 SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS = "swift._num_threads_0_in_swiftcopts"
+
+# If enabled, it passes the number of threads that the host has
+SWIFT_FEATURE_USE_HOST_NUM_THREADS = "swift.use_host_num_threads"

tools/worker/swift_runner.cc

@@ -14,6 +14,9 @@
 
 #include "tools/worker/swift_runner.h"
 
+#include <sys/sysctl.h>
+#include <sys/types.h>
+
 #include <fstream>
 
 #include "tools/common/bazel_substitutions.h"
@@ -102,7 +105,7 @@ static bool StripPrefix(const std::string &prefix, std::string &str) {
 
 SwiftRunner::SwiftRunner(const std::vector<std::string> &args,
                          bool force_response_file)
-    : force_response_file_(force_response_file) {
+    : force_response_file_(force_response_file), use_host_num_threads_(false) {
   args_ = ProcessArguments(args);
 }
 
@@ -257,6 +260,26 @@ bool SwiftRunner::ProcessPossibleResponseFile(
   return changed;
 }
 
+static int GetNumCPU() {
+  int sysctlarg[4];
+  int numCPU;
+  std::size_t len = sizeof(numCPU);
+
+  // set the sysctlarg for hw.ncpu
+  sysctlarg[0] = CTL_HW;
+  sysctlarg[1] = HW_AVAILCPU;
+
+  // Gets NCPU from sysctl
+  sysctl(sysctlarg, 2, &numCPU, &len, NULL, 0);
+
+  if (numCPU < 1) {
+    sysctlarg[1] = HW_NCPU;
+    sysctl(sysctlarg, 2, &numCPU, &len, NULL, 0);
+    if (numCPU < 1) numCPU = 1;
+  }
+  return numCPU;
+}
+
 template <typename Iterator>
 bool SwiftRunner::ProcessArgument(
     Iterator &itr, const std::string &arg,
@@ -318,6 +341,14 @@ bool SwiftRunner::ProcessArgument(
         ++itr;
         new_arg = output_file_map_path_;
         changed = true;
+      } else if (use_host_num_threads_ && arg == "-num-threads") {
+        consumer("-num-threads");
+
+        // If we're using the number of threads on the host, set this value to
+        // the NumCPU.
+        ++itr;
+        new_arg = std::to_string(GetNumCPU());
+        changed = true;
       }
 
       // Apply any other text substitutions needed in the argument (i.e., for
@@ -347,6 +378,8 @@ std::vector<std::string> SwiftRunner::ParseArguments(Iterator itr) {
         global_index_store_import_path_ = arg;
       } else if (StripPrefix("-generated-header-rewriter=", arg)) {
         generated_header_rewriter_path_ = arg;
+      } else if (StripPrefix("-use-host-num-threads", arg)) {
+        use_host_num_threads_ = true;
       }
     } else {
       if (arg == "-output-file-map") {

tools/worker/swift_runner.h

@@ -146,6 +146,9 @@ class SwiftRunner {
   // `-index-store-path`. After running `swiftc` `index-import` copies relevant
   // index outputs into the `index_store_path` to integrate outputs with Bazel.
   std::string global_index_store_import_path_;
+
+  // If the worker should set num-threads to number of threads on the host.
+  bool use_host_num_threads_;
 };
 
 #endif  // BUILD_BAZEL_RULES_SWIFT_TOOLS_WORKER_SWIFT_RUNNER_H_