arm: print an error when a HardFault occurs

This is very useful for debugging. It differentiates between a stack
overflow and other errors (because it's easy to see when a stack
overflow occurs) and prints the old stack pointer and program counter if
available.

src/runtime/runtime_cortexm.go

@@ -41,11 +41,61 @@ func preinit() {
 }
 
 func abort() {
+	// disable all interrupts
+	arm.DisableInterrupts()
+
+	// lock up forever
 	for {
 		arm.Asm("wfi")
 	}
 }
 
+// The stack layout at the moment an interrupt occurs.
+// Registers can be accessed if the stack pointer is cast to a pointer to this
+// struct.
+type interruptStack struct {
+	R0  uintptr
+	R1  uintptr
+	R2  uintptr
+	R3  uintptr
+	R12 uintptr
+	LR  uintptr
+	PC  uintptr
+	PSR uintptr
+}
+
+// This function is called at HardFault.
+// Before this function is called, the stack pointer is reset to the initial
+// stack pointer (loaded from addres 0x0) and the previous stack pointer is
+// passed as an argument to this function. This allows for easy inspection of
+// the stack the moment a HardFault occurs, but it means that the stack will be
+// corrupted by this function and thus this handler must not attempt to recover.
+//
+// For details, see:
+// https://community.arm.com/developer/ip-products/system/f/embedded-forum/3257/debugging-a-cortex-m0-hard-fault
+// https://blog.feabhas.com/2013/02/developing-a-generic-hard-fault-handler-for-arm-cortex-m3cortex-m4/
+//go:export handleHardFault
+func handleHardFault(sp *interruptStack) {
+	print("fatal error: ")
+	if uintptr(unsafe.Pointer(sp)) < 0x20000000 {
+		print("stack overflow")
+	} else {
+		// TODO: try to find the cause of the hard fault. Especially on
+		// Cortex-M3 and higher it is possible to find more detailed information
+		// in special status registers.
+		print("HardFault")
+	}
+	print(" with sp=", sp)
+	if uintptr(unsafe.Pointer(&sp.PC)) >= 0x20000000 {
+		// Only print the PC if it points into memory.
+		// It may not point into memory during a stack overflow, so check that
+		// first before accessing the stack.
+		print(" pc=", sp.PC)
+	}
+	println()
+	abort()
+}
+
 // Implement memset for LLVM and compiler-rt.
 //go:export memset
 func libc_memset(ptr unsafe.Pointer, c byte, size uintptr) {