func init() {
	if runtime.GOARCH == "amd64" {
		// On amd64 we have assembly and can use stronger compression.
		compressOpts = append(compressOpts, s2.WriterBetterCompression())
	}
}

// newS2CompressReader will read data from r, compress it and return the compressed data as a Reader.
// Use Close to ensure resources are released on incomplete streams.
//
// input 'on' is always recommended such that this function works

	// Compress only if the compression is enabled during initial multipart.
	var idxCb func() []byte
	if isCompressed {
		wantEncryption := crypto.Requested(r.Header) || isEncrypted
		s2c, cb := newS2CompressReader(reader, actualPartSize, wantEncryption)
		idxCb = cb
		defer s2c.Close()
		reader = etag.Wrap(s2c, reader)
		length = -1
	}