On 12/6/24 20:45, Peter Dimov via Boost wrote:
Andrey Semashev wrote:
Yes, I understand, but the thing is I very rarely have to write `void f2( unsigned char p[4] );` in the first place. Most of the time I get a variable amount of data that I need to process, so I have either an iterator range or a pointer and size. And if there are fixed-sized fragments of that data that I need to process, pretty much always I have checked the entire size (or at least some outer size) of the data beforehand, so no checks needed for those individual fragments.
So, for example, if I have to parse an RTP packet, I would
void on_rtp_packet(const uint8_t* packet, size_t size) { // RTP fixed header is 12 bytes long if (size < 12) throw std::invalid_argument("RTP packet too short");
// Parse 12-byte fixed header uint16_t seqn = read_be16(packet + 2); uint32_t timestamp = read_be32(packet + 4); }
read_be32 takes uint8_t const[4] here, because it has an implicit precondition that the argument has at least 4 valid bytes.
So
uint16_t read_be16( span
p ); uint32_t read_be32( span p ); void on_rtp_packet( span
packet ) { // RTP fixed header is 12 bytes long if (packet.size() < 12) throw std::invalid_argument("RTP packet too short"); // Parse 12-byte fixed header uint16_t seqn = read_be16(packet + 2); uint32_t timestamp = read_be32(packet + 4); }
Since the optimizer sees that packet.size() >= 12, it can elide the checks against 2 and 4.
I don't trust the optimizer. Because it failed me more than once. If you want your code fast, you better write it fast yourself and not hope that the compiler does a good job for you.