Re: NaN trick

[David Given <dg@...>]

Peter Cawley wrote:
[...]
> Though you then need to get slightly more creative about encoding the
> type. A double has 52 bits of mantissa; with 48 taken for a
> pointer-sized value, and 1 taken to ensure a quiet NaN, you only have
> 3 bits left to encode the type. If you borrow the sign bit as well,
> then you have 4 bits. For reference, the current implementation in 5.2
> beta uses 6 bits for encoding type.

If the thing you're pointing at is 4-aligned, then you get two free bits
at the bottom of the pointer. If the thing you're pointing at has been
allocated with malloc(), then you get more --- the result of malloc() is
guaranteed to be correctly aligned for any kind of variable, which in
practice usually gives you four bits.

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