machine: make sure DMA buffers do not escape unnecessarily

Writing the pointer of a buffer to memory-mapped I/O will normally cause
it to escape, which forces the compiler to heap-allocate the buffer. But
we do know how long the value stays alive, so we can tell the compiler
to keep it alive exactly until it is not needed anymore - and tell it to
not treat the pointer-to-uintptr cast as escaping.

src/machine/machine_nrf52xxx.go

@@ -145,7 +145,9 @@ func (a *ADC) Get() uint16 {
 	nrf.SAADC.CH[0].PSELP.Set(pwmPin)
 
 	// Destination for sample result.
-	nrf.SAADC.RESULT.PTR.Set(uint32(uintptr(unsafe.Pointer(&rawValue))))
+	// Note: rawValue doesn't need to be kept alive for the GC, since the
+	// volatile read later will force it to stay alive.
+	nrf.SAADC.RESULT.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(&rawValue))))
 	nrf.SAADC.RESULT.MAXCNT.Set(1) // One sample
 
 	// Start tasks.
@@ -312,7 +314,7 @@ func (spi *SPI) Tx(w, r []byte) error {
 			if nr > spiMaxBufferSize {
 				nr = spiMaxBufferSize
 			}
-			spi.Bus.RXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&r[0]))))
+			spi.Bus.RXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(r)))))
 			r = r[nr:]
 		}
 		spi.Bus.RXD.MAXCNT.Set(nr)
@@ -323,7 +325,7 @@ func (spi *SPI) Tx(w, r []byte) error {
 			if nw > spiMaxBufferSize {
 				nw = spiMaxBufferSize
 			}
-			spi.Bus.TXD.PTR.Set(uint32(uintptr(unsafe.Pointer(&w[0]))))
+			spi.Bus.TXD.PTR.Set(uint32(unsafeNoEscape(unsafe.Pointer(unsafe.SliceData(w)))))
 			w = w[nw:]
 		}
 		spi.Bus.TXD.MAXCNT.Set(nw)
@@ -337,6 +339,11 @@ func (spi *SPI) Tx(w, r []byte) error {
 		spi.Bus.EVENTS_END.Set(0)
 	}
 
+	// Make sure the w and r buffers stay alive for the GC until this point,
+	// since they are used by the hardware but not otherwise visible.
+	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(r)))
+	keepAliveNoEscape(unsafe.Pointer(unsafe.SliceData(w)))
+
 	return nil
 }