Martin Storsjö 19a2d26177 random_seed: Improve behaviour with small timer increments with high precision timers ...

On a Zen 5, on Ubuntu 24.04 (with CLOCKS_PER_SEC 1000000), the
value of clock() in this loop increments by 0 most of the time,
and when it does increment, it usually increments by 1 compared
to the previous round.

Due to the "last_t + 2*last_td + (CLOCKS_PER_SEC > 1000) >= t"
expression, we only manage to take one step forward in this loop
(incrementing i) if clock() increments by 2, while it incremented
by 0 in the previous iteration (last_td).

This is similar to the change done in
c4152fc42e, to speed it up on
systems with very small CLOCKS_PER_SEC. However in this case,
CLOCKS_PER_SEC is still very large, but the machine is fast enough
to hit every clock increment repeatedly.

For this case, use the number of repetitions of each timer value
as entropy source; require a change in the number of repetitions
in order to proceed to the next buffer index.

This helps the fate-random-seed test to actually terminate within
a reasonable time on such a system (where it previously could hang,
running for many minutes).

Signed-off-by: Martin Storsjö <martin@martin.st>

2025-02-13 11:41:51 +02:00

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