Add work_pool_size, IntRange, item_range_for_worker, and ThreadIndex

We'll use these later in this PR.

kernel/threading.h

@@ -8,6 +8,7 @@
 
 #include "kernel/yosys_common.h"
 #include "kernel/log.h"
+#include "kernel/utils.h"
 
 #ifndef YOSYS_THREADING_H
 #define YOSYS_THREADING_H
@@ -131,6 +132,11 @@ public:
 	// The result may be 0.
 	static int pool_size(int reserved_cores, int max_worker_threads);
 
+	// Computes the number of worker threads to use, by dividing work_units among threads.
+	// For testing purposes you can set YOSYS_WORK_UNITS_PER_THREAD to override `work_units_per_thread`.
+	// The result may be 0.
+	static int work_pool_size(int reserved_cores, int work_units, int work_units_per_thread);
+
 	// Create a pool of threads running the given closure (parameterized by thread number).
 	// `pool_size` must be the result of a `pool_size()` call.
 	ThreadPool(int pool_size, std::function<void(int)> b);
@@ -154,6 +160,16 @@ private:
 #endif
 };
 
+// Divides some number of items into `num_threads` subranges and returns the
+// `thread_num`'th subrange. If `num_threads` is zero, returns the whole range.
+IntRange item_range_for_worker(int num_items, int thread_num, int num_threads);
+
+// A type that encapsulates the index of a thread in some list of threads. Useful for
+// stronger typechecking and code readability.
+struct ThreadIndex {
+	int thread_num;
+};
+
 template <class T>
 class ConcurrentStack
 {