Implement minimal bump pointer allocator

Useful for MCUs, until a real garbage collector has been implemented.

src/runtime/gc.go

@@ -0,0 +1,31 @@
+
+// +build !linux
+
+package runtime
+
+import (
+	"unsafe"
+)
+
+var (
+	_extern__heap_start unsafe.Pointer // defined by the linker
+	heapptr             = uintptr(unsafe.Pointer(&_extern__heap_start))
+)
+
+func alloc(size uintptr) unsafe.Pointer {
+	// TODO: this can be optimized by not casting between pointers and ints so
+	// much. And by using platform-native data types (e.g. *uint8 for 8-bit
+	// systems).
+	size = align(size)
+	addr := heapptr
+	heapptr += size
+	for i := uintptr(0); i < uintptr(size); i += 4 {
+		ptr := (*uint32)(unsafe.Pointer(addr + i))
+		*ptr = 0
+	}
+	return unsafe.Pointer(addr)
+}
+
+func free(ptr unsafe.Pointer) {
+	// TODO: use a GC
+}

src/runtime/runtime_unix.go

@@ -3,6 +3,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 // #include <stdio.h>
 // #include <stdlib.h>
 // #include <unistd.h>
@@ -21,3 +25,15 @@ func Sleep(d Duration) {
 func abort() {
 	C.abort()
 }
+
+func alloc(size uintptr) unsafe.Pointer {
+	buf := C.calloc(1, C.size_t(size))
+	if buf == nil {
+		panic("cannot allocate memory")
+	}
+	return buf
+}
+
+func free(ptr unsafe.Pointer) {
+	C.free(ptr)
+}