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Make SmallVector more general-purpose #37371

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nnethercote opened this issue Oct 24, 2016 · 2 comments
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Make SmallVector more general-purpose #37371

nnethercote opened this issue Oct 24, 2016 · 2 comments

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@nnethercote
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SmallVector is a very useful type. It can avoid lots of heap allocations when used on hot paths, which can give nice speed-ups. I have used it in multiple places, and I have more changes in the pipeline that will use it.

But it's not quite as general-purpose as it could be. Ideas for improvement:

  • Rename it as SmallVec, to mirror Vec.
  • Move it from libsyntax to librustc_data_structures.
  • Increase the number of elements that can be present without requiring heap allocation from 1 to 2 or 4. (Because 1 covers a lot of cases, but 2 or 4 would cover more.)
  • Tweak the API to better match Vec. E.g. change zero() to new().
@nnethercote nnethercote mentioned this issue Oct 24, 2016
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@Mark-Simulacrum
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I've started some part of this work in implementing a SmallVec8, with a proposed rename to ArrayVec and the accompanying possibly-heap variant AccumulateVec, which should land with #37270, but the ETA on that is not well-defined.

@Ms2ger
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Ms2ger commented Oct 24, 2016

I wonder if there's a way to use the existing rust-smallvec crate yet.

@Mark-Simulacrum Mark-Simulacrum mentioned this issue Nov 3, 2016
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eddyb added a commit to eddyb/rust that referenced this issue Nov 11, 2016
@eddyb
Rollup merge of rust-lang#37551 - Mark-Simulacrum:upgrade-accvec, r=e…
f332c3c 
…ddyb

Replace syntax's SmallVector with AccumulateVec

This adds a new type to data_structures, `SmallVec`, which wraps `AccumulateVec` with support for re-allocating onto the heap (`SmallVec::reserve`). `SmallVec` is then used to replace the implementation of `SmallVector` in libsyntax.

r? @eddyb

Fixes rust-lang#37371. Using `SmallVec` instead of libsyntax's `SmallVector` will provide the `N = 2/4` case easily (just needs a few more `Array` impls).

cc @nnethercote, probably interested in this area
eddyb added a commit to eddyb/rust that referenced this issue Nov 12, 2016
@eddyb
Rollup merge of rust-lang#37551 - Mark-Simulacrum:upgrade-accvec, r=e…
f19f939 
…ddyb

Replace syntax's SmallVector with AccumulateVec

This adds a new type to data_structures, `SmallVec`, which wraps `AccumulateVec` with support for re-allocating onto the heap (`SmallVec::reserve`). `SmallVec` is then used to replace the implementation of `SmallVector` in libsyntax.

r? @eddyb

Fixes rust-lang#37371. Using `SmallVec` instead of libsyntax's `SmallVector` will provide the `N = 2/4` case easily (just needs a few more `Array` impls).

cc @nnethercote, probably interested in this area
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@Ms2ger @nnethercote @Mark-Simulacrum