Re: [hash2][review] Early review (due to holidays)

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-06 17:45:19

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<uint8_t const, 2> p );
uint32_t read_be32( span<uint8_t const, 4> p );

void on_rtp_packet( span<uint8_t const> 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.