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https://github.com/carbon-language/carbon-lang.git
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Chandler Carruth
d037848a96
Replaces the callback-based `ForEach` methods on `RawHashtable`, `Map`,
and
`Set` with a range object supporting range-for loops, structured
bindings, and
the standard range concepts.
- Adds `.entries()` on `Map`, `Set`, and `RawHashtable`, returning a
range that
models `std::ranges::forward_range` and `std::ranges::common_range`.
Obtaining one is an explicit call rather than `begin()`/`end()` on the
container, as scanning a whole table is costly and shouldn't be hidden.
- Iterating a `Map` yields a `std::pair` of key and value references,
which
fits in two registers and is returned without being materialized in
memory.
- `Map::Range` and `Set::Range` are aliases of the raw hashtable's range
rather
than wrappers around it. The raw iterator produces the user-facing
reference
itself -- a `KeyT&` for a set, a pair of references for a map -- picked
by
`StorageEntry`, which is already specialized on whether there is a value
type. That leaves one iterator to reason about instead of three.
- Deletes the rvalue `.entries()` overloads on the owning containers, as
a
range built from a temporary table would dangle. Views don't own their
storage, so the operation remains available on them.
- In release builds, the walk over the groups is a single induction
variable: a
negative byte offset counting up to zero, anchored at the ends of the
metadata and entry arrays. Both arrays are then reached by indexed
addressing
off a base that stays put, and the entry pointer is formed only once a
group
with a present entry has been found.
- In debug builds, the range hashes the table's metadata when it is
built and
re-checks that hash when it is destroyed, catching mutation of the table
while a range is live. It also picks a random starting group and a
random odd
group stride, which varies the traversal order between ranges while
still
visiting every group exactly once. That entropy is drawn when the range
is
built rather than in `begin()`, so `begin()` stays a pure function of
the
range and the multi-pass guarantee holds.
- Removes `ForEachEntry` and all of its callers.
Measured against the iteration benchmark added in its own commit, a
traversal is at or ahead of what the callback compiled to across nearly
the
whole size range. The largest tables spend 3-5% fewer cycles, small
`Set`s as
much as 24% fewer, and instruction counts stay within about 1%. What
remains
behind is a handful of mid-sized `Map`s by up to 1%, and `Set` at 65536,
which
sits at exactly half its load factor, by 2%.
Both revisions were built with `-c opt --copt=-march=x86-64-v3` and
compared
with:
```
./scripts/bench_runner.py --exp_benchmark=... --base_benchmark=... \
--benchmark_args=--benchmark_perf_counters=INSTRUCTIONS,CYCLES \
--benchmark_args='--benchmark_filter=(Set|Map)Iterate<(Set|Map)<' \
--extra_metrics_filter='(INSTRUCTIONS|CYCLES)'
```
Trimmed below to the primary integer configurations and to the two
counters;
the pointer- and string-keyed configurations follow the same pattern.
```
Benchmark ┃ CYCLES ┃ INSTRUCTIONS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
BM_MapIterate<Map<int, int>>/1....... │ 👍 -6.032% p=1.14e-05 │ ?? p=0.752
baseline: │ 12.06 ± 1.520% │ 64 ± 3.125%
experiment: │ 11.33 ± 2.765% │ 65.5 ± 3.817%
│ │
BM_MapIterate<Map<int, int>>/2....... │ ?? p=0.155 │ ?? p=0.343
baseline: │ 7.587 ± 1.285% │ 41 ± 0.000%
experiment: │ 7.652 ± 0.865% │ 41 ± 2.439%
│ │
BM_MapIterate<Map<int, int>>/3....... │ ?? p=0.343 │ 👍 -1.020% p=0.0039
baseline: │ 6.663 ± 4.260% │ 32.67 ± 2.041%
experiment: │ 6.368 ± 12.224% │ 32.33 ± 2.062%
│ │
BM_MapIterate<Map<int, int>>/4....... │ ?? p=0.343 │ 👍 -1.786% p=0.0297
baseline: │ 6.091 ± 15.470% │ 28 ± 3.571%
experiment: │ 5.957 ± 8.932% │ 27.5 ± 3.636%
│ │
BM_MapIterate<Map<int, int>>/8....... │ ?? p=0.323 │ 👍 -1.220% p=0.000148
baseline: │ 4.845 ± 0.800% │ 20.5 ± 0.000%
experiment: │ 4.814 ± 3.585% │ 20.25 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/16...... │ 👍 -2.195% p=0.00908 │ 👍 0.769% p=6.58e-06
baseline: │ 4.312 ± 0.187% │ 16.25 ± 0.000%
experiment: │ 4.218 ± 2.368% │ 16.13 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/32...... │ ?? p=0.236 │ 👍 0.442% p=9.53e-06
baseline: │ 4.051 ± 1.084% │ 14.13 ± 0.000%
experiment: │ 4.063 ± 0.737% │ 14.06 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/64...... │ ?? p=0.693 │ 👎 0.227% p=4.52e-06
baseline: │ 4.021 ± 0.239% │ 13.75 ± 0.000%
experiment: │ 4.019 ± 0.417% │ 13.78 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/256..... │ 👍 0.360% p=0.00119 │ 👎 0.754% p=1.37e-05
baseline: │ 3.996 ± 0.173% │ 13.47 ± 0.000%
experiment: │ 3.982 ± 0.272% │ 13.57 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/4096.... │ 👍 0.581% p=1.96e-05 │ 👎 0.923% p=1.96e-05
baseline: │ 4.005 ± 0.816% │ 13.38 ± 0.000%
experiment: │ 3.981 ± 0.192% │ 13.5 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/65536... │ 👍 -4.957% p=1.14e-05 │ 👎 0.934% p=1.14e-05
baseline: │ 5.307 ± 0.501% │ 13.38 ± 0.000%
experiment: │ 5.044 ± 1.746% │ 13.5 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/1048576. │ 👍 -3.947% p=9.09e-05 │ 👎 0.935% p=3.3e-05
baseline: │ 6.074 ± 0.807% │ 13.38 ± 0.000%
experiment: │ 5.834 ± 2.159% │ 13.5 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/16777216 │ ?? p=0.155 │ 👎 0.935% p=2.11e-05
baseline: │ 5.082 ± 3.650% │ 13.38 ± 0.000%
experiment: │ 5.012 ± 1.316% │ 13.5 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/56...... │ 👎 0.825% p=0.0268 │ 👍 0.270% p=1.14e-05
baseline: │ 3.918 ± 0.501% │ 13.21 ± 0.000%
experiment: │ 3.951 ± 0.342% │ 13.18 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/224..... │ 👎 0.788% p=0.000504 │ 👎 0.346% p=1.64e-05
baseline: │ 3.895 ± 0.111% │ 12.89 ± 0.000%
experiment: │ 3.926 ± 0.285% │ 12.94 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/3584.... │ 👎 1.028% p=0.000148 │ 👎 0.545% p=1.14e-05
baseline: │ 3.913 ± 0.427% │ 12.79 ± 0.000%
experiment: │ 3.954 ± 0.325% │ 12.86 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/57344... │ ?? p=0.236 │ 👎 0.558% p=2.55e-06
baseline: │ 4.574 ± 0.721% │ 12.79 ± 0.000%
experiment: │ 4.51 ± 3.709% │ 12.86 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/917504.. │ 👍 -3.826% p=6.58e-06 │ 👎 0.559% p=2.33e-05
baseline: │ 5.221 ± 0.507% │ 12.79 ± 0.000%
experiment: │ 5.021 ± 0.556% │ 12.86 ± 0.000%
│ │
BM_MapIterate<Map<int, int>>/14680064 │ 👍 -3.839% p=1.37e-05 │ 👎 0.559% p=3.31e-05
baseline: │ 5.129 ± 1.194% │ 12.79 ± 0.000%
experiment: │ 4.932 ± 1.475% │ 12.86 ± 0.000%
│ │
Benchmark ┃ CYCLES ┃ INSTRUCTIONS
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
BM_SetIterate<Set<int>>/1....... │ 👍 -3.104% p=0.000583 │ ?? p=0.206
baseline: │ 11.2 ± 6.323% │ 60 ± 3.333%
experiment: │ 10.85 ± 5.820% │ 61 ± 3.279%
│ │
BM_SetIterate<Set<int>>/2....... │ 👍 -7.037% p=0.0362 │ ?? p=0.155
baseline: │ 7.086 ± 16.857% │ 37 ± 0.000%
experiment: │ 6.587 ± 0.479% │ 36 ± 4.167%
│ │
BM_SetIterate<Set<int>>/3....... │ 👎 1.400% p=2.34e-05 │ 👍 -1.163% p=0.00136
baseline: │ 5.363 ± 0.463% │ 28.67 ± 2.326%
experiment: │ 5.438 ± 32.763% │ 28.33 ± 1.176%
│ │
BM_SetIterate<Set<int>>/4....... │ ?? p=0.968 │ ?? p=0.286
baseline: │ 4.642 ± 32.751% │ 23.5 ± 2.128%
experiment: │ 4.658 ± 38.416% │ 23.63 ± 3.704%
│ │
BM_SetIterate<Set<int>>/8....... │ 👍 -23.823% p=3.74e-06 │ 👍 -1.515% p=5.52e-05
baseline: │ 4.701 ± 6.589% │ 16.5 ± 0.000%
experiment: │ 3.581 ± 7.790% │ 16.25 ± 0.000%
│ │
BM_SetIterate<Set<int>>/16...... │ 👍 -4.502% p=1.37e-05 │ 👍 -1.020% p=3.31e-05
baseline: │ 3.124 ± 0.585% │ 12.25 ± 0.000%
experiment: │ 2.983 ± 0.625% │ 12.13 ± 0.000%
│ │
BM_SetIterate<Set<int>>/32...... │ 👍 -4.032% p=5.46e-06 │ 👍 0.617% p=1.96e-05
baseline: │ 2.957 ± 0.260% │ 10.13 ± 0.000%
experiment: │ 2.838 ± 0.434% │ 10.06 ± 0.000%
│ │
BM_SetIterate<Set<int>>/64...... │ 👍 -5.054% p=4.52e-06 │ 👎 0.321% p=1.37e-05
baseline: │ 2.937 ± 0.301% │ 9.75 ± 0.000%
experiment: │ 2.788 ± 1.143% │ 9.781 ± 0.000%
│ │
BM_SetIterate<Set<int>>/256..... │ 👍 -5.325% p=1.14e-05 │ 👎 1.073% p=6.58e-06
baseline: │ 2.916 ± 0.220% │ 9.469 ± 0.000%
experiment: │ 2.761 ± 0.142% │ 9.57 ± 0.000%
│ │
BM_SetIterate<Set<int>>/4096.... │ 👍 -4.865% p=4.52e-06 │ 👎 1.317% p=2.34e-05
baseline: │ 2.921 ± 0.194% │ 9.381 ± 0.000%
experiment: │ 2.779 ± 0.224% │ 9.504 ± 0.000%
│ │
BM_SetIterate<Set<int>>/65536... │ 👎 1.961% p=3.93e-05 │ 👎 1.332% p=1.49e-05
baseline: │ 4.015 ± 0.482% │ 9.375 ± 0.000%
experiment: │ 4.094 ± 0.613% │ 9.5 ± 0.000%
│ │
BM_SetIterate<Set<int>>/1048576. │ 👍 -4.843% p=1.14e-05 │ 👎 1.333% p=5.38e-06
baseline: │ 5.239 ± 0.144% │ 9.375 ± 0.000%
experiment: │ 4.986 ± 0.139% │ 9.5 ± 0.000%
│ │
BM_SetIterate<Set<int>>/16777216 │ 👍 0.840% p=0.0362 │ 👎 1.333% p=2.52e-06
baseline: │ 3.719 ± 1.420% │ 9.375 ± 0.000%
experiment: │ 3.688 ± 1.308% │ 9.5 ± 0.000%
│ │
BM_SetIterate<Set<int>>/56...... │ 👍 -2.857% p=9.53e-06 │ 👍 0.388% p=3.31e-05
baseline: │ 2.942 ± 0.439% │ 9.214 ± 0.000%
experiment: │ 2.858 ± 0.619% │ 9.179 ± 0.000%
│ │
BM_SetIterate<Set<int>>/224..... │ 👍 -2.161% p=2.34e-05 │ 👎 0.502% p=4.52e-06
baseline: │ 2.888 ± 0.347% │ 8.893 ± 0.000%
experiment: │ 2.826 ± 0.450% │ 8.938 ± 0.000%
│ │
BM_SetIterate<Set<int>>/3584.... │ 👍 -1.750% p=6.58e-06 │ 👎 0.793% p=2.34e-05
baseline: │ 2.89 ± 0.261% │ 8.792 ± 0.000%
experiment: │ 2.84 ± 0.411% │ 8.862 ± 0.000%
│ │
BM_SetIterate<Set<int>>/57344... │ ?? p=0.502 │ 👎 0.812% p=2.78e-05
baseline: │ 3.684 ± 4.246% │ 8.786 ± 0.000%
experiment: │ 3.644 ± 4.431% │ 8.857 ± 0.000%
│ │
BM_SetIterate<Set<int>>/917504.. │ 👍 -2.629% p=0.000148 │ 👎 0.813% p=3.08e-06
baseline: │ 4.372 ± 0.693% │ 8.786 ± 0.000%
experiment: │ 4.257 ± 0.210% │ 8.857 ± 0.000%
│ │
BM_SetIterate<Set<int>>/14680064 │ 👍 -2.927% p=0.0219 │ 👎 0.813% p=3.03e-06
baseline: │ 4.154 ± 3.286% │ 8.786 ± 0.000%
experiment: │ 4.032 ± 3.198% │ 8.857 ± 0.000%
│ │
```
Assisted-by: Antigravity with Opus