runtime: improve hardfault handler and stack reporting on Cortex-M

- Add reference to the current goroutine stack (PSP) when showing the hardfault info on Cortex-M.

Author: digitalentity

src/internal/task/task_stack_cortexm.c

@@ -11,3 +11,14 @@ uintptr_t SystemStack() {
     );
     return sp;
 }
+
+uintptr_t GoroutineStack() {
+    uintptr_t sp;
+    asm volatile(
+        "mrs %0, PSP"
+        : "=r"(sp)
+        :
+        : "memory"
+    );
+    return sp;
+}

src/internal/task/task_stack_cortexm.go

@@ -70,3 +70,10 @@ func (s *state) pause() {
 //
 //export SystemStack
 func SystemStack() uintptr
+
+// GoroutineStack returns the PSP (goroutine stack pointer). When a fault
+// occurs in goroutine context, the hardware saves the exception frame to PSP;
+// reading PSP+0x18 gives the actual faulting PC.
+//
+//export GoroutineStack
+func GoroutineStack() uintptr

src/runtime/runtime_cortexm_hardfault_debug.go

@@ -4,6 +4,7 @@ package runtime
 
 import (
 	"device/arm"
+	"internal/task"
 	"unsafe"
 )
 
@@ -106,6 +107,18 @@ func HardFault_Handler() {
 			print(" pc=", sp.PC)
 		}
 	}
+
+	// PSP holds the actual exception frame when the fault occurred in a
+	// goroutine (thread mode, PSP active). The MSP-based sp above is the
+	// scheduler's stack and its PC is garbage in that case.
+	pspVal := task.GoroutineStack()
+	if pspVal >= 0x20000000 && pspVal < 0x20040000 {
+		pspFrame := (*interruptStack)(unsafe.Pointer(pspVal))
+		print(" psp=", pspFrame)
+		print(" psp_pc=", pspFrame.PC)
+		print(" psp_lr=", pspFrame.LR)
+	}
+
 	println()
 	abort()
 }