facebook
Swap positions of `ptr` and `extra` in PackedSyncPtr
Summary: Change layout: [63:48][47:0] -> [63:16][ 15:0] [extra][ ptr] -> [ ptr][extra]
Previous PackedSyncPtr was organized s.t: [63:48][47:0] [extra][ ptr]
So access pointer required a 48-bit bitmask:
data_.getData() & (-1ull >> 16)
This isn't great on x86 because imm operands max out at imm32 so this requires two instructions: 1. movabs + and 2. movl + andn
Either way its almost certainly cheaper to just just a shift which can be done by swap the placements of extra and ptr s.t:
[63:16][ 15:0]
[ ptr][extra]
As a slightly plus, extra can also be accessed essentially for free with movzwl (which can move-elim).
On aarch64 get() might be slightly more expensive now (and extra() correspondingly cheaper) because lsl is sometimes slightly slower than and.
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@Orvid has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.
@Orvid merged this pull request in facebook/folly@5c4c2530dd85cf2bea6d9b46a3e7613d70160660.