Optimize unchecked indexing into chunks and chunks_mut

[the8472]

Fixes #53340

# BEFORE

$ rustc +nightly -Copt-level=3 -Ccodegen-units=1 -Clto=fat chunks.rs
$ perf stat ./chunks

 Performance counter stats for './chunks':

          3,177.03 msec task-clock                #    1.000 CPUs utilized
                 4      context-switches          #    0.001 K/sec
                 0      cpu-migrations            #    0.000 K/sec
           984,006      page-faults               #    0.310 M/sec
    13,092,199,322      cycles                    #    4.121 GHz                      (83.29%)
       384,543,475      stalled-cycles-frontend   #    2.94% frontend cycles idle     (83.35%)
     7,414,280,722      stalled-cycles-backend    #   56.63% backend cycles idle      (83.38%)
    50,493,980,662      instructions              #    3.86  insn per cycle
                                                  #    0.15  stalled cycles per insn  (83.29%)
     6,625,375,297      branches                  # 2085.396 M/sec                    (83.38%)
         3,087,652      branch-misses             #    0.05% of all branches          (83.31%)

       3.178079469 seconds time elapsed

       2.327156000 seconds user
       0.762041000 seconds sys

# AFTER

$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc -Copt-level=3 -Ccodegen-units=1 -Clto=fat chunks.rs
$ perf stat ./chunks

 Performance counter stats for './chunks':

          2,705.76 msec task-clock                #    1.000 CPUs utilized
                 4      context-switches          #    0.001 K/sec
                 0      cpu-migrations            #    0.000 K/sec
           984,005      page-faults               #    0.364 M/sec
    11,156,763,039      cycles                    #    4.123 GHz                      (83.26%)
       342,198,882      stalled-cycles-frontend   #    3.07% frontend cycles idle     (83.37%)
     6,486,263,637      stalled-cycles-backend    #   58.14% backend cycles idle      (83.37%)
    40,553,476,617      instructions              #    3.63  insn per cycle
                                                  #    0.16  stalled cycles per insn  (83.37%)
     6,668,429,113      branches                  # 2464.532 M/sec                    (83.37%)
         3,099,636      branch-misses             #    0.05% of all branches          (83.26%)

       2.706725288 seconds time elapsed

       1.782083000 seconds user
       0.848424000 seconds sys

[rust-highfive]

r? @kennytm

(rust-highfive has picked a reviewer for you, use r? to override)

[the8472]

Grepping through the compiler source doesn't show any uses of chunks or chunks_mut, even less so in combination with Zip, so I don't really expect a change in perf rlo results, but maybe some dependencies do.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 24094a0 with merge 5c392fe307a7b9c6ca1d328ad7dbed69fb03897d...

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[klensy]

Comparing CI build nightly with manually build stage1 can show unrelated results.

[the8472]

I'd expect the compiler internals to be built differently, but shouldn't the output of the compiler remain comparable?

[bors]

☀️ Try build successful - checks-actions
Build commit: 5c392fe307a7b9c6ca1d328ad7dbed69fb03897d (5c392fe307a7b9c6ca1d328ad7dbed69fb03897d)

[rust-timer]

Queued 5c392fe307a7b9c6ca1d328ad7dbed69fb03897d with parent 2545459, future comparison URL.

[rust-timer]

Finished benchmarking try commit (5c392fe307a7b9c6ca1d328ad7dbed69fb03897d): comparison url.

Summary: This benchmark run did not return any significant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression

[kennytm]

@bors r+

[bors]

📌 Commit 24094a0 has been approved by kennytm

[bors]

⌛ Testing commit 24094a0 with merge 0cd0709...

[bors]

☀️ Test successful - checks-actions
Approved by: kennytm
Pushing 0cd0709 to master...

[pedrocr]

Awesome, thanks for working on this!