runtime: implement NumCPU for -scheduler=threads

For the threads scheduler, it makes sense to have NumCPU available.
For all other schedulers, the number of available CPUs is practically
limited to one by the scheduler (even though the system might have more
CPUs).

src/internal/task/task_threads.c

@@ -6,6 +6,7 @@
 #include <signal.h>
 #include <stdint.h>
 #include <stdio.h>
+#include <unistd.h>
 
 // BDWGC also uses SIGRTMIN+6 on Linux, which seems like a reasonable choice.
 #ifdef __linux__
@@ -29,7 +30,7 @@ struct state_pass {
 void tinygo_task_gc_pause(int sig);
 
 // Initialize the main thread.
-void tinygo_task_init(void *mainTask, pthread_t *thread, void *context) {
+void tinygo_task_init(void *mainTask, pthread_t *thread, int *numCPU, void *context) {
     // Make sure the current task pointer is set correctly for the main
     // goroutine as well.
     current_task = mainTask;
@@ -43,6 +44,14 @@ void tinygo_task_init(void *mainTask, pthread_t *thread, void *context) {
     act.sa_flags = SA_SIGINFO;
     act.sa_handler = &tinygo_task_gc_pause;
     sigaction(taskPauseSignal, &act, NULL);
+
+    // Obtain the number of CPUs available on program start (for NumCPU).
+    int num = sysconf(_SC_NPROCESSORS_ONLN);
+    if (num <= 0) {
+        // Fallback in case there is an error.
+        num = 1;
+    }
+    *numCPU = num;
 }
 
 void tinygo_task_exited(void*);

src/internal/task/task_threads.go

@@ -38,6 +38,8 @@ type state struct {
 // Goroutine counter, starting at 0 for the main goroutine.
 var goroutineID uintptr
 
+var numCPU int32
+
 var mainTask Task
 
 // Queue of tasks (see QueueNext) that currently exist in the program.
@@ -53,7 +55,7 @@ func OnSystemStack() bool {
 // startup, before starting any other goroutines.
 func Init(sp uintptr) {
 	mainTask.state.stackTop = sp
-	tinygo_task_init(&mainTask, &mainTask.state.thread)
+	tinygo_task_init(&mainTask, &mainTask.state.thread, &numCPU)
 }
 
 // Return the task struct for the current thread.
@@ -259,7 +261,7 @@ func runtimePanic(msg string)
 // that the 't' parameter won't escape (because it will).
 //
 //go:linkname tinygo_task_init tinygo_task_init
-func tinygo_task_init(t *Task, thread *threadID)
+func tinygo_task_init(t *Task, thread *threadID, numCPU *int32)
 
 // Here same as for tinygo_task_init.
 //
@@ -273,3 +275,7 @@ func tinygo_task_send_gc_signal(threadID)
 
 //export tinygo_task_current
 func tinygo_task_current() unsafe.Pointer
+
+func NumCPU() int {
+	return int(numCPU)
+}