Optimize Vec::shrink_to_fit() #21401
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This uses `Vec::shrink_to_fit()` internally so it's really benchmarking that.
`Vec::shrink_to_fit()` may be called on vectors that are already the
correct length. Calling out to `reallocate()` in this case is a bad idea
because there is no guarantee that `reallocate()` won't allocate a new
buffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.
Before:
test string::tests::bench_exact_size_shrink_to_fit ... bench: 45 ns/iter (+/- 2)
After:
test string::tests::bench_exact_size_shrink_to_fit ... bench: 26 ns/iter (+/- 1)
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(rust_highfive has picked a reviewer for you, use r? to override) |
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How does this impact code that actually needs to shrink? |
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(I imagine the overhead is negligable if not free, but am still curious) |
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@gankro I would imagine it's negligible too. It's a single comparison, of values that are already being used anyway so no cache faults or anything like that. I would welcome some actual profiling but I doubt I can get any meaningful measurements. |
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Jan 26, 2015
Don't reallocate when capacity is already equal to length
`Vec::shrink_to_fit()` may be called on vectors that are already the
correct length. Calling out to `reallocate()` in this case is a bad idea
because there is no guarantee that `reallocate()` won't allocate a new
buffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.
Before:
test string::tests::bench_exact_size_shrink_to_fit ... bench: 45 ns/iter (+/- 2)
After:
test string::tests::bench_exact_size_shrink_to_fit ... bench: 26 ns/iter (+/- 1)
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Don't reallocate when capacity is already equal to length
Vec::shrink_to_fit()may be called on vectors that are already thecorrect length. Calling out to
reallocate()in this case is a bad ideabecause there is no guarantee that
reallocate()won't allocate a newbuffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.
Before:
After: