rts: no timer for wasm32

Due to the lack of threads, on wasm32 there can't be a background
timer that periodically resets the context switch flag. This patch
disables timer for wasm32, and also makes the scheduler default to -C0
on wasm32 to avoid starving threads.

2 files changed, 25 insertions, 0 deletions

diff --git a/rts/Timer.c b/rts/Timer.c
index e6666856a6..332334129d 100644
--- a/rts/Timer.c
+++ b/rts/Timer.c
@@ -26,6 +26,11 @@
 #include "RtsSignals.h"
 #include "rts/EventLogWriter.h"
 
+// See Note [No timer on wasm32]
+#if !defined(wasm32_HOST_ARCH)
+#define HAVE_PREEMPTION
+#endif
+
 // This global counter is used to allow multiple threads to stop the
 // timer temporarily with a stopTimer()/startTimer() pair.  If
 //      timer_enabled  == 0          timer is enabled
@@ -174,37 +179,45 @@ handle_tick(int unused STG_UNUSED)
 void
 initTimer(void)
 {
+#if defined(HAVE_PREEMPTION)
     initProfTimer();
     if (RtsFlags.MiscFlags.tickInterval != 0) {
         initTicker(RtsFlags.MiscFlags.tickInterval, handle_tick);
     }
     SEQ_CST_STORE(&timer_disabled, 1);
+#endif
 }
 
 void
 startTimer(void)
 {
+#if defined(HAVE_PREEMPTION)
     if (atomic_dec(&timer_disabled) == 0) {
         if (RtsFlags.MiscFlags.tickInterval != 0) {
             startTicker();
         }
     }
+#endif
 }
 
 void
 stopTimer(void)
 {
+#if defined(HAVE_PREEMPTION)
     if (atomic_inc(&timer_disabled, 1) == 1) {
         if (RtsFlags.MiscFlags.tickInterval != 0) {
             stopTicker();
         }
     }
+#endif
 }
 
 void
 exitTimer (bool wait)
 {
+#if defined(HAVE_PREEMPTION)
     if (RtsFlags.MiscFlags.tickInterval != 0) {
         exitTicker(wait);
     }
+#endif
 }
diff --git a/rts/include/rts/Flags.h b/rts/include/rts/Flags.h
index eb38e80794..e33d97b17c 100644
--- a/rts/include/rts/Flags.h
+++ b/rts/include/rts/Flags.h
@@ -198,7 +198,19 @@ typedef struct _CONCURRENT_FLAGS {
  *
  * It is changed by the +RTS -V<secs> flag.
  */
+
+// Note [No timer on wasm32]
+// ~~~~~~~~~~~~~~~~~~~~~~~~~
+// Due to the lack of threads and preemption semantics, on wasm32
+// there can't be a background timer that periodically resets the
+// context switch flag. So it makes sense to make the scheduler
+// default to -C0 on wasm32 for better fairness and avoid starving
+// threads.
+#if defined(wasm32_HOST_ARCH)
+#define DEFAULT_TICK_INTERVAL USToTime(0)
+#else
 #define DEFAULT_TICK_INTERVAL USToTime(10000)
+#endif
 
 /*
  * When linkerAlwaysPic is true, the runtime linker assume that all object