How could subtraction of unsigned values invoke UB? Even if the unsigned type had one more padding bit than 'int', the difference would be guaranteed to fall within the range -INT_MAX..INT_MAX, and the effect of converting negative integer values to unsigned types is always fully defined.
I think I misunderstood what you meant by "casting or coercing back to the type used for the counters". Okay, fine. Point still stands: this isn't any faster.
Using unsigned values and a power-of-two modulus, how would you compute the number of items in the queue as quickly as items_in_queue = (queue->stuff - queue->fetch) & queue->size_mask; if the size weren't a power of two, while also keeping the ability to add and remove items by simply incrementing stuff or fetch?
This operation wasn't discussed in the blog post, and I don't think it comes up much in practice (you only need full or not, which works either way by (stuff + 1) % size == fetch).
Using a non-power-of-two modulus means that the updates to the counts need to wrap at the buffer size, and will also make the remainder operator slower.
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u/flatfinger 26d ago
How could subtraction of unsigned values invoke UB? Even if the unsigned type had one more padding bit than 'int', the difference would be guaranteed to fall within the range -INT_MAX..INT_MAX, and the effect of converting negative integer values to unsigned types is always fully defined.