On 13-07-24 09:33 AM, Brendan Zabarauskas wrote:
On 25/07/2013, at 2:15 AM, Evan Martin <[email protected]
<mailto:[email protected]>> wrote:
Is an Option<u8> implemented as a pair of (type, value) or is it
packed into a single word?
A quick test shows:
rusti> std::sys::size_of::<Option<u8>>()
16
We represent enums as (tag-word, union-of-data-fields) pairs, except in
the optimized representation case of option<~T> where we use compress
the nullary None tag into a sentinel pointer (null) and drop to 1 word.
I expect we'll eventually do one other obvious optimization here, to
define tags as the smallest datum width that can accommodate all the tag
values and maintan the alignment "requirement" (or absence of speed
penalty) of the data fields, not just "1 word". That sort of definition
would mean option<u8> drops to 2 bytes (not 16) and on targets with
cheap unaligned access (ivy bridge etc) most tags (with <256 variants)
drop from 8 bytes to 1 (if you pass the appropriate --target-feature flag).
To get much more clever than that involves hunting in the
union-of-data-fields for overlapping fields into which you can either
scrounge bits or use sentinel values. This is not totally implausible:
the low 3 bits are available in most pointers from a malloc, and the
entire zero page on almost all platforms is unmapped for picking
sentinels. On x64 there are tons of spare bits in every pointer, and
virtually unlimited sentinel values due to the noncanonical address
range. Plus there are NaN bits and the unused >21st bits in char and
whatnot. But ... I think it'll be a while before we bother with that
sort of thing, if ever.
-Graydon
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