runtime: add Windows vectored exception handler for recoverable panics

Register a vectored exception handler at startup so Windows hardware
exceptions can be translated into Go panics. Access violations become
nil pointer panics, and integer divide-by-zero exceptions become divide
by zero panics, allowing defer/recover to handle them like ordinary
runtime panics.

compileopts/target.go

@@ -485,6 +485,8 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 			"--no-insert-timestamp",
 			"--no-dynamicbase",
 		)
+		spec.ExtraFiles = append(spec.ExtraFiles,
+			"src/runtime/runtime_windows.c")
 	case "wasm", "wasip1", "wasip2":
 		return nil, fmt.Errorf("GOOS=%s but GOARCH is unset. Please set GOARCH to wasm", options.GOOS)
 	default:

src/runtime/runtime_windows.c

@@ -0,0 +1,29 @@
+//go:build none
+
+// This file is included on Windows (despite the //go:build line above).
+
+#include <windows.h>
+#include <stdint.h>
+
+void tinygo_sigpanic_windows(int32_t exception_code);
+
+static LONG WINAPI tinygo_exception_handler(EXCEPTION_POINTERS *info) {
+	DWORD code = info->ExceptionRecord->ExceptionCode;
+	switch (code) {
+	case EXCEPTION_ACCESS_VIOLATION:
+	case EXCEPTION_IN_PAGE_ERROR:
+	case EXCEPTION_INT_DIVIDE_BY_ZERO:
+	case EXCEPTION_INT_OVERFLOW:
+		tinygo_sigpanic_windows((int32_t)code);
+		// If runtimePanic triggers longjmp, we never reach here.
+		// If it doesn't (no defer frame), it will abort and we also
+		// never reach here.
+		return EXCEPTION_CONTINUE_SEARCH;
+	default:
+		return EXCEPTION_CONTINUE_SEARCH;
+	}
+}
+
+void tinygo_init_exception_handler(void) {
+	AddVectoredExceptionHandler(1, tinygo_exception_handler);
+}

src/runtime/runtime_windows.go

@@ -60,6 +60,10 @@ func mainCRTStartup() int {
 		_QueryPerformanceFrequency(&performanceFrequency)
 	}
 
+	// Register vectored exception handler so that access violations and
+	// divide-by-zero exceptions can be recovered with defer/recover.
+	tinygo_init_exception_handler()
+
 	// Obtain the initial stack pointer right before calling the run() function.
 	// The run function has been moved to a separate (non-inlined) function so
 	// that the correct stack pointer is read.
@@ -294,3 +298,27 @@ func hardwareRand() (n uint64, ok bool) {
 //
 //export SystemFunction036
 func _RtlGenRandom(buf unsafe.Pointer, len int) bool
+
+const (
+	_EXCEPTION_ACCESS_VIOLATION   = 0xC0000005
+	_EXCEPTION_IN_PAGE_ERROR      = 0xC0000006
+	_EXCEPTION_INT_DIVIDE_BY_ZERO = 0xC0000094
+	_EXCEPTION_INT_OVERFLOW       = 0xC0000095
+)
+
+//export tinygo_init_exception_handler
+func tinygo_init_exception_handler()
+
+//export tinygo_sigpanic_windows
+func tinygo_sigpanic_windows(exceptionCode int32) {
+	switch uint32(exceptionCode) {
+	case _EXCEPTION_ACCESS_VIOLATION, _EXCEPTION_IN_PAGE_ERROR:
+		runtimePanic("nil pointer dereference")
+	case _EXCEPTION_INT_DIVIDE_BY_ZERO:
+		runtimePanic("divide by zero")
+	case _EXCEPTION_INT_OVERFLOW:
+		runtimePanic("integer overflow")
+	default:
+		runtimePanic("unknown exception")
+	}
+}