On Zen 5, a hashtable benchmark's result depends on where its timed loop
lands in memory. With a fixed hash seed, `BM_MapContainsMiss<Map<int,
int>>/64` takes 5.5-6.3 cycles per lookup at 8 of the 16 possible loop
positions within 256 bytes, and 6.7-7.1 at the other 8, through effects
in the op cache, the BTB, and the branch predictor. Which position a
build gets is an accident of the surrounding code, so comparing two
builds conflates a change's effect with where its code landed.
Each timed loop now runs from one of 16 out-of-line copies, aligned to
256 bytes and padded in 16-byte steps, which place the loop at each of
those 16 positions. A process picks a copy at random, or the one named
in `CARBON_BENCH_LAYOUT`, and reports it in the `Layout` counter.
Running `bench_runner.py` with enough runs to sample every position, 48
for example, makes its medians and confidence intervals account for
layout.
In a sweep over 1024 bytes, performance depended only on the loop's
address modulo 256, and loop heads are 16-byte aligned, so finer or
wider padding found nothing more on this CPU.
`CARBON_BENCH_LAYOUT_PAD_STEP` and `CARBON_BENCH_LAYOUT_PAD_RANGE`
adjust it. The copies grow `map_benchmark` from 4.8 MB to 7.6 MB.
On AArch64, LLVM doesn't align loop heads for generic or Apple CPUs, so
they land on any 4-byte boundary and 16-byte steps only reach a quarter
of the positions. AArch64 builds pad in 4-byte steps instead, giving 64
layouts and growing `map_benchmark` there to 16.6 MB. Every copy,
including the unpadded one, also emits the padding `asm` statement,
because its presence changes how the surrounding code is compiled.
Assisted-by: Claude Code
---------
Co-authored-by: Nicholas Bishop <nbishop@nbishop.net>
OwningArrayRef is being removed upstream, per
https://github.com/llvm/llvm-project/pull/169126. This replaces uses
with `SmallVector`.
I've also made a separate commit which does init changes; these aren't
strictly necessary, but I added to make it a little more idiomatic in
spots.
---------
Co-authored-by: Dana Jansens <danakj@orodu.net>
These have unique challenges for our hashing scheme, and so its useful
to make sure the hash functions we use can handle them.
Some other work on Abseil's hash tables uncovered that this might be
risky and may have surfaced some improvements to reduce the impact here,
but the first step seems to try and start covering this path in the
benchmarks.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
This is essentially the result of looking at `.begin()` uses. We also
frequently do `std::shuffle`, but unfortunately STLExtras doesn't
provide a wrapper for that.
This switches `DCHECK` and `FATAL` as well.
The goal is to reduce the code size impact of these assertions so that
we can keep more of them enabled. Currently, the largest cost I see from
`CHECK` is not the actual check or the cold code itself, but actually
the failure to inline trivial functions due to the presence of the cold
code. This means that our goal isn't to reduce apparent code size in the
final binary but the LLVM IR cost assessed for these routines in the
inliner, which closely correlates with code size but is a bit different.
As discussed in #4283, experimentation shows that a single function call
with a minimal number of arguments is the lowest cost model for these.
This is easily achieved with a format-string API that internally uses
`llvm::formatv`. This PR is essentially the `CHECK` version of #4283.
However, the check macros are substantially harder to make work with
both format strings and streaming because they also take a condition.
Also, unexpectedly, I was very successful at devising a regular
expression based automated rewrite from the streaming to the format
string form with only low 10s of manual fixes. This includes compacting
strings broken up across lines, etc. Given how well that went, I've
prepared this PR which just directly switches to the format string API
and migrate everything to use it.
One nice side-effect is that the format string approach ends up greatly
simplifying the implementation here as well.
This is ... *shockingly* effective. Parsing speeds up by more than 3%
with just this change. And checking speeds up by **8%** with this change
alone:
```
BM_CompileAPIFileDenseDecls<Phase::Parse>/256 86.3µs ± 1% 82.9µs ± 1% -3.94% (p=0.000 n=17+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/1024 431µs ± 1% 415µs ± 1% -3.76% (p=0.000 n=18+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/4096 1.77ms ± 1% 1.71ms ± 1% -3.18% (p=0.000 n=18+19)
BM_CompileAPIFileDenseDecls<Phase::Parse>/16384 7.44ms ± 1% 7.17ms ± 2% -3.56% (p=0.000 n=18+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/65536 30.7ms ± 1% 29.7ms ± 1% -3.15% (p=0.000 n=18+20)
BM_CompileAPIFileDenseDecls<Phase::Parse>/262144 131ms ± 1% 127ms ± 1% -2.81% (p=0.000 n=18+18)
BM_CompileAPIFileDenseDecls<Phase::Check>/256 878µs ± 2% 800µs ± 1% -8.91% (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/1024 1.88ms ± 2% 1.72ms ± 1% -8.56% (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/4096 5.78ms ± 2% 5.28ms ± 1% -8.70% (p=0.000 n=20+18)
BM_CompileAPIFileDenseDecls<Phase::Check>/16384 21.9ms ± 1% 20.1ms ± 1% -8.02% (p=0.000 n=18+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/65536 90.4ms ± 2% 83.1ms ± 1% -8.04% (p=0.000 n=19+20)
BM_CompileAPIFileDenseDecls<Phase::Check>/262144 381ms ± 2% 352ms ± 1% -7.79% (p=0.000 n=19+19)
```
---------
Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
The hash table design is heavily based on Abseil's ["Swiss
Tables"][swiss-tables] design. It uses an array of bytes storing
metadata about each entry and an array of entries where each is a pair
of key and value. The metadata byte consists of 7-bits of hash of the
key (distinct from the bits used to index the table), and one bit
indicating the presence of a special entry -- either empty or deleted.
[swiss-tables]: https://abseil.io/about/design/swisstables
There are a large range of optimizations and other nuanced aspects of
this hash table design and implementation, a good point to understand
that context is `raw_hashtable.h` which has an overview of the design
and references to various other files for relevant details.
---------
Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>
Chandler CarruthandNicholas Bishop
Jon Ross-PerkinsandDana Jansens
Thomas Köppe
