chacha20poly1305: make gcc 8.1 happy

GCC 8.1 does not know about the invariant `0 <= ctx->num < POLY1305_BLOCK_SIZE`.
This results in a warning that `memcpy(ctx->data + num, inp, len);` may
overflow the `data` field, which is correct for arbitrary values of `num`.

To make the invariant explicit we ensure that `num` is in the required range.
An alternative would be to change `ctx->num` to a 4-bit bitfield at the point
of declaration.

This changes the code from `test ebp, ebp; jz end` to `and ebp, 15; jz
end`, which have identical performance characteristics.

Signed-off-by: Samuel Neves <sneves@dei.uc.pt>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

0 files changed, 0 insertions, 0 deletions