cortexm: disable FPU on Cortex-M4

On some boards the FPU is already enabled on startup, probably as part
of the bootloader. On other chips it was enabled as part of the runtime
startup code. In all these cases, enabling the FPU is currently
unsupported: the automatic stack sizing of goroutines assumes that the
processor won't need to reserve space for FPU registers. Enabling the
FPU therefore can lead to a stack overflow.

This commit either removes the code that enables the FPU, or simply
disables it in startup code. A future change should fully enable the FPU
so that operations on float32 can be performed by the FPU instead of in
software, greatly speeding up such code.

src/runtime/runtime_atsamd51.go

@@ -16,6 +16,7 @@ func postinit() {}
 
 //export Reset_Handler
 func main() {
+	arm.SCB.CPACR.Set(0) // disable FPU if it is enabled
 	preinit()
 	run()
 	abort()

src/runtime/runtime_mimxrt1062.go

@@ -102,11 +102,6 @@ func initSystem() {
 
 func initPeripherals() {
 
-	// enable FPU - set CP10, CP11 full access
-	nxp.SystemControl.CPACR.SetBits(
-		((nxp.SCB_CPACR_CP10_CP10_3 << nxp.SCB_CPACR_CP10_Pos) & nxp.SCB_CPACR_CP10_Msk) |
-			((nxp.SCB_CPACR_CP11_CP11_3 << nxp.SCB_CPACR_CP11_Pos) & nxp.SCB_CPACR_CP11_Msk))
-
 	enableTimerClocks() // activate GPT/PIT clock gates
 	initSysTick()       // enable SysTick
 	initRTC()           // enable real-time clock

src/runtime/runtime_nrf.go

@@ -3,6 +3,7 @@
 package runtime
 
 import (
+	"device/arm"
 	"device/nrf"
 	"machine"
 	"runtime/interrupt"
@@ -18,6 +19,9 @@ func postinit() {}
 
 //export Reset_Handler
 func main() {
+	if nrf.FPUPresent {
+		arm.SCB.CPACR.Set(0) // disable FPU if it is enabled
+	}
 	systemInit()
 	preinit()
 	run()

src/runtime/runtime_nxpmk66f18.go

@@ -75,7 +75,6 @@ func initSystem() {
 	nxp.SIM.SCGC3.Set(nxp.SIM_SCGC3_ADC1 | nxp.SIM_SCGC3_FTM2 | nxp.SIM_SCGC3_FTM3)
 	nxp.SIM.SCGC5.Set(0x00043F82) // clocks active to all GPIO
 	nxp.SIM.SCGC6.Set(nxp.SIM_SCGC6_RTC | nxp.SIM_SCGC6_FTM0 | nxp.SIM_SCGC6_FTM1 | nxp.SIM_SCGC6_ADC0 | nxp.SIM_SCGC6_FTF)
-	nxp.SystemControl.CPACR.Set(0x00F00000)
 	nxp.LMEM.PCCCR.Set(0x85000003)
 
 	// release I/O pins hold, if we woke up from VLLS mode