Description

NoMemCopy will alter the way the output file is generated.

It will employ a hack that allows us to read the file data directly from the compiled program's `.rodata` section. This ensures that when we call call our generated function, we omit unnecessary mem copies.

The downside of this, is that it requires dependencies on the `reflect` and `unsafe` packages. These may be restricted on platforms like AppEngine and thus prevent you from using this mode.

Another disadvantage is that the byte slice we create, is strictly read-only. For most use-cases this is not a problem, but if you ever try to alter the returned byte slice, a runtime panic is thrown. Use this mode only on target platforms where memory constraints are an issue.

The default behaviour is to use the old code generation method. This prevents the two previously mentioned issues, but will employ at least one extra memcopy and thus increase memory requirements.

For instance, consider the following two examples:

This would be the default mode, using an extra memcopy but gives a safe implementation without dependencies on `reflect` and `unsafe`:

func myfile() []byte {
	return []byte{0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a}
}

Here is the same functionality, but uses the `.rodata` hack. The byte slice returned from this example can not be written to without generating a runtime error.

var _myfile = "\x89\x50\x4e\x47\x0d\x0a\x1a"

func myfile() []byte {
	var empty [0]byte
	sx := (*reflect.StringHeader)(unsafe.Pointer(&_myfile))
	b := empty[:]
	bx := (*reflect.SliceHeader)(unsafe.Pointer(&b))
	bx.Data = sx.Data
	bx.Len = len(_myfile)
	bx.Cap = bx.Len
	return b
}