cortexm: optimize code size for the HardFault_Handler

builder/sizes_test.go

@@ -43,8 +43,8 @@ func TestBinarySize(t *testing.T) {
 	tests := []sizeTest{
 		// microcontrollers
 		{"hifive1b", "examples/echo", 3884, 280, 0, 2268},
-		{"microbit", "examples/serial", 2924, 388, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7365, 1491, 116, 6912},
+		{"microbit", "examples/serial", 2852, 360, 8, 2272},
+		{"wioterminal", "examples/pininterrupt", 7337, 1491, 116, 6912},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the

src/device/arm/cortexm.S

@@ -1,29 +1,6 @@
 .syntax unified
 .cfi_sections .debug_frame
 
-.section .text.HardFault_Handler
-.global  HardFault_Handler
-.type    HardFault_Handler, %function
-HardFault_Handler:
-    .cfi_startproc
-    // Put the old stack pointer in the first argument, for easy debugging. This
-    // is especially useful on Cortex-M0, which supports far fewer debug
-    // facilities.
-    mov r0, sp
-
-    // Load the default stack pointer from address 0 so that we can call normal
-    // functions again that expect a working stack. However, it will corrupt the
-    // old stack so the function below must not attempt to recover from this
-    // fault.
-    movs r3, #0
-    ldr r3, [r3]
-    mov sp, r3
-
-    // Continue handling this error in Go.
-    bl handleHardFault
-    .cfi_endproc
-.size HardFault_Handler, .-HardFault_Handler
-
 // This is a convenience function for semihosting support.
 // At some point, this should be replaced by inline assembly.
 .section .text.SemihostingCall

src/runtime/panic.go

@@ -93,6 +93,9 @@ func runtimePanicAt(addr unsafe.Pointer, msg string) {
 		trap()
 	}
 	if hasReturnAddr {
+		// Note: the string "panic: runtime error at " is also used in
+		// runtime_cortexm_hardfault.go. It is kept the same so that the string
+		// can be deduplicated by the compiler.
 		printstring("panic: runtime error at ")
 		printptr(uintptr(addr) - callInstSize)
 		printstring(": ")

src/runtime/runtime.go

@@ -57,6 +57,12 @@ func memzero(ptr unsafe.Pointer, size uintptr)
 // the current stack pointer in a platform-independent way.
 func stacksave() unsafe.Pointer
 
+// Special LLVM intrinsic that returns the SP register on entry to the calling
+// function.
+//
+//export llvm.sponentry.p0
+func llvm_sponentry() unsafe.Pointer
+
 //export strlen
 func strlen(ptr unsafe.Pointer) uintptr
 

src/runtime/runtime_cortexm_hardfault.go

@@ -2,39 +2,25 @@
 
 package runtime
 
-import (
-	"unsafe"
-)
-
 // This function is called at HardFault.
-// Before this function is called, the stack pointer is reset to the initial
-// stack pointer (loaded from address 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/
 //
-//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)
-	}
+//export HardFault_Handler
+func HardFault_Handler() {
+	// Obtain the stack pointer as it was on entry to the HardFault. It contains
+	// the registers that were pushed by the NVIC and that we can now read back
+	// to print the PC value at the time of the hard fault, for example.
+	sp := (*interruptStack)(llvm_sponentry())
+
+	// Note: by reusing the string "panic: runtime error at " we save a little
+	// bit in terms of code size as the string can be deduplicated.
+	print("panic: runtime error at ", sp.PC, ": HardFault with sp=", sp)
+	// 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.
 	println()
 	abort()
 }

src/runtime/runtime_cortexm_hardfault_debug.go

@@ -18,8 +18,13 @@ const (
 
 // See runtime_cortexm_hardfault.go
 //
-//go:export handleHardFault
-func handleHardFault(sp *interruptStack) {
+//export HardFault_Handler
+func HardFault_Handler() {
+	// Obtain the stack pointer as it was on entry to the HardFault. It contains
+	// the registers that were pushed by the NVIC and that we can now read back
+	// to print the PC value at the time of the hard fault, for example.
+	sp := (*interruptStack)(llvm_sponentry())
+
 	fault := GetFaultStatus()
 	spValid := !fault.Bus().ImpreciseDataBusError()