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Store vtables sorted in Rust builder (#6765)

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* benchmark many vtables

* Rust: Store written_table rev-positions sorted.

The previous implementation was slow if there were too many tables.

Asymototically when inserting the n^th vtable: The old implementation
took O(n) lookup steps and O(1) insertion. The new implementation is
O(log n) lookup and O(n) insertion. This might be improved further by
using a balanced btree.

Benchmarking, create_many_tables is 7.5x faster (on my laptop):

// Simple vector cache
test create_many_tables ... bench: 728,875 ns/iter (+/- 12,279) = 44 MB/s

// Sorted vector cache
test create_many_tables ... bench: 97,843 ns/iter (+/- 4,430) = 334 MB/s

* Fix lints

Co-authored-by: Casper Neo <cneo@google.com>
This commit is contained in:
Casper
2021-08-03 15:31:45 -04:00
committed by GitHub
parent c39fc9dd9c
commit 35e2cac6eb
4 changed files with 51 additions and 60 deletions
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12
rust/flatbuffers/src/vtable_writer.rs
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@@ -16,7 +16,7 @@
use std::ptr::write_bytes;
use crate::endian_scalar::{emplace_scalar, read_scalar_at};
use crate::endian_scalar::emplace_scalar;
use crate::primitives::*;
/// VTableWriter compartmentalizes actions needed to create a vtable.
@@ -54,16 +54,6 @@ impl<'a> VTableWriter<'a> {
}
}
/// Gets an object field offset from the vtable. Only used for debugging.
///
/// Note that this expects field offsets (which are like pointers), not
/// field ids (which are like array indices).
#[inline(always)]
pub fn get_field_offset(&self, vtable_offset: VOffsetT) -> VOffsetT {
let idx = vtable_offset as usize;
unsafe { read_scalar_at::<VOffsetT>(&self.buf, idx) }
}
/// Writes an object field offset into the vtable.
///
/// Note that this expects field offsets (which are like pointers), not