riscv: support sleeping in QEMU

QEMU doesn't support the RTC peripheral yet so work around it for now.

This makes the following command work:

    tinygo run -target=hifive1-qemu ./testdata/coroutines.go

src/runtime/runtime_fe310.go

@@ -81,27 +81,4 @@ func putchar(c byte) {
 	machine.UART0.WriteByte(c)
 }
 
-func ticks() timeUnit {
-	// Combining the low bits and the high bits yields a time span of over 270
-	// years without counter rollover.
-	highBits := sifive.RTC.RTCHI.Get()
-	for {
-		lowBits := sifive.RTC.RTCLO.Get()
-		newHighBits := sifive.RTC.RTCHI.Get()
-		if newHighBits == highBits {
-			// High bits stayed the same.
-			return timeUnit(lowBits) | (timeUnit(highBits) << 32)
-		}
-		// Retry, because there was a rollover in the low bits (happening every
-		// 1.5 days).
-		highBits = newHighBits
-	}
-}
-
 const asyncScheduler = false
-
-func sleepTicks(d timeUnit) {
-	target := ticks() + d
-	for ticks() < target {
-	}
-}

src/runtime/runtime_fe310_baremetal.go

@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"device/riscv"
+	"device/sifive"
 )
 
 func abort() {
@@ -12,3 +13,27 @@ func abort() {
 		riscv.Asm("wfi")
 	}
 }
+
+func ticks() timeUnit {
+	// Combining the low bits and the high bits yields a time span of over 270
+	// years without counter rollover.
+	highBits := sifive.RTC.RTCHI.Get()
+	for {
+		lowBits := sifive.RTC.RTCLO.Get()
+		newHighBits := sifive.RTC.RTCHI.Get()
+		if newHighBits == highBits {
+			// High bits stayed the same.
+			println("bits:", highBits, lowBits)
+			return timeUnit(lowBits) | (timeUnit(highBits) << 32)
+		}
+		// Retry, because there was a rollover in the low bits (happening every
+		// 1.5 days).
+		highBits = newHighBits
+	}
+}
+
+func sleepTicks(d timeUnit) {
+	target := ticks() + d
+	for ticks() < target {
+	}
+}

src/runtime/runtime_fe310_qemu.go

@@ -10,7 +10,19 @@ import (
 // Special memory-mapped device to exit tests, created by SiFive.
 var testExit = (*volatile.Register32)(unsafe.Pointer(uintptr(0x100000)))
 
+var timestamp timeUnit
+
 func abort() {
 	// Signal a successful exit.
 	testExit.Set(0x5555)
 }
+
+func ticks() timeUnit {
+	return timestamp
+}
+
+func sleepTicks(d timeUnit) {
+	// Note: QEMU doesn't seem to support the RTC peripheral at the time of
+	// writing so just simulate sleeping here.
+	timestamp += d
+}