Add tail-call based interleaving approach

CMakeLists.txt

@@ -358,6 +358,11 @@ if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND BUILD_TESTING)
   set_target_properties(server_bench PROPERTIES SKIP_BUILD_RPATH ON)
 
   add_executable(interleaving_test InterleavingTest.cpp)
+  # work around lack of musttail for gcc
+  if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_BUILD_TYPE STREQUAL "Debug")
+    target_compile_options(interleaving_test PRIVATE -Og
+                                                     -foptimize-sibling-calls)
+  endif()
   target_link_libraries(interleaving_test PRIVATE nanobench)
 endif()
 

InterleavingTest.cpp

@@ -22,9 +22,6 @@ void *stepJob(Job *j) {
   return done ? nullptr : (void *)stepJob;
 }
 
-// So we can look at the disassembly more easily
-
-extern "C" {
 void sequential(Job **jobs, int count) {
   for (int i = 0; i < count; ++i) {
     do {
@@ -94,6 +91,87 @@ void interleaveBoundedCyclicList(Job **jobs, int count) {
   }
 }
 
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+#if __has_attribute(musttail)
+#define MUSTTAIL __attribute__((musttail))
+#else
+#define MUSTTAIL
+#endif
+
+struct Context {
+  constexpr static int kConcurrent = 32;
+  Job **jobs;
+  Job *inProgress[kConcurrent];
+  void (*continuation[kConcurrent])(Context *, int64_t prevJob, int64_t job,
+                                    int64_t started, int64_t count);
+  int nextJob[kConcurrent];
+};
+
+void keepGoing(Context *context, int64_t prevJob, int64_t job, int64_t started,
+               int64_t count) {
+  prevJob = job;
+  job = context->nextJob[job];
+  MUSTTAIL return context->continuation[job](context, prevJob, job, started,
+                                             count);
+}
+
+void stepJobTailCall(Context *context, int64_t prevJob, int64_t job,
+                     int64_t started, int64_t count);
+
+void complete(Context *context, int64_t prevJob, int64_t job, int64_t started,
+              int64_t count) {
+  if (started == count) {
+    if (prevJob == job) {
+      return;
+    }
+    context->nextJob[prevJob] = context->nextJob[job];
+    job = prevJob;
+  } else {
+    context->inProgress[job] = context->jobs[started++];
+    context->continuation[job] = stepJobTailCall;
+  }
+  prevJob = job;
+  job = context->nextJob[job];
+  MUSTTAIL return context->continuation[job](context, prevJob, job, started,
+                                             count);
+}
+
+void stepJobTailCall(Context *context, int64_t prevJob, int64_t job,
+                     int64_t started, int64_t count) {
+  auto *j = context->inProgress[job];
+  auto done = --(*j->input) == 0;
+#ifdef __x86_64__
+  _mm_clflush(j->input);
+#endif
+  if (done) {
+    MUSTTAIL return complete(context, prevJob, job, started, count);
+  } else {
+    context->continuation[job] = stepJobTailCall;
+    MUSTTAIL return keepGoing(context, prevJob, job, started, count);
+  }
+}
+
+void useTailCalls(Job **jobs, int count) {
+  if (count == 0) {
+    return;
+  }
+  Context context;
+  context.jobs = jobs;
+  int64_t started = std::min(Context::kConcurrent, count);
+  for (int i = 0; i < started; i++) {
+    context.inProgress[i] = jobs[i];
+    context.nextJob[i] = i + 1;
+    context.continuation[i] = stepJobTailCall;
+  }
+  context.nextJob[started - 1] = 0;
+  int prevJob = started - 1;
+  int job = 0;
+  return context.continuation[job](&context, prevJob, job, started, count);
+}
+
 void interleaveCyclicList(Job **jobs, int count) {
   auto *nextJob = (int *)alloca(sizeof(int) * count);
 
@@ -117,12 +195,11 @@ void interleaveCyclicList(Job **jobs, int count) {
     job = nextJob[job];
   }
 }
-}
 
 int main() {
   ankerl::nanobench::Bench bench;
 
-  constexpr int kNumJobs = 100;
+  constexpr int kNumJobs = 10000;
   bench.relative(true);
 
   Job jobs[kNumJobs];
@@ -140,6 +217,7 @@ int main() {
   for (auto [scheduler, name] :
        {std::make_pair(sequentialNoFuncPtr, "sequentialNoFuncPtr"),
         std::make_pair(sequential, "sequential"),
+        std::make_pair(useTailCalls, "useTailCalls"),
         std::make_pair(interleaveSwapping, "interleavingSwapping"),
         std::make_pair(interleaveBoundedCyclicList,
                        "interleaveBoundedCyclicList"),