Refactor binary_search_by to use conditional moves

[okaneco]

Refactor the if/else checking on cmp::Ordering variants to a "branchless" reassignment of left and right.

This change results in fewer branches and instructions.
https://rust.godbolt.org/z/698eYffTx

---

I saw consistent benchmark improvements locally. Performance of worst case seems about the same, maybe slightly faster for the L3 test.

Current

slice::binary_search_l1             43.00ns/iter +/- 3.00ns
slice::binary_search_l1_with_dups   25.00ns/iter +/- 0.00ns
slice::binary_search_l1_worst_case  10.00ns/iter +/- 0.00ns
slice::binary_search_l2             64.00ns/iter +/- 1.00ns
slice::binary_search_l2_with_dups   42.00ns/iter +/- 0.00ns
slice::binary_search_l2_worst_case  16.00ns/iter +/- 0.00ns
slice::binary_search_l3            132.00ns/iter +/- 2.00ns
slice::binary_search_l3_with_dups  108.00ns/iter +/- 2.00ns
slice::binary_search_l3_worst_case  33.00ns/iter +/- 3.00ns

This PR

slice::binary_search_l1            21.00ns/iter +/- 0.00ns
slice::binary_search_l1_with_dups  14.00ns/iter +/- 0.00ns
slice::binary_search_l1_worst_case  9.00ns/iter +/- 0.00ns
slice::binary_search_l2            34.00ns/iter +/- 0.00ns
slice::binary_search_l2_with_dups  23.00ns/iter +/- 0.00ns
slice::binary_search_l2_worst_case 16.00ns/iter +/- 0.00ns
slice::binary_search_l3            92.00ns/iter +/- 3.00ns
slice::binary_search_l3_with_dups  63.00ns/iter +/- 1.00ns
slice::binary_search_l3_worst_case 29.00ns/iter +/- 0.00ns

[rustbot]

r? @thomcc

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[thomcc]

@bors r+ rollup=never

[bors]

📌 Commit d585eec has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit d585eec with merge 8abf920...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 8abf920 to master...

[rust-timer]

Finished benchmarking commit (8abf920): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	1.3%	[1.3%, 1.4%]	2
Improvements ✅ (primary)	-1.4%	[-1.9%, -0.2%]	5
Improvements ✅ (secondary)	-1.8%	[-2.6%, -1.3%]	8
All ❌✅ (primary)	-1.1%	[-1.9%, 0.4%]	6

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.3%	[1.8%, 4.9%]	2
Regressions ❌ (secondary)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-4.9%	[-7.2%, -2.7%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.8%	[-7.2%, 4.9%]	4

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.6%	[-1.9%, -1.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.6%	[-1.9%, -1.0%]	7

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	3
Improvements ✅ (primary)	-0.9%	[-1.9%, -0.1%]	9
Improvements ✅ (secondary)	-0.7%	[-1.3%, -0.2%]	2
All ❌✅ (primary)	-0.8%	[-1.9%, 0.1%]	10

Bootstrap: 676.001s -> 675.075s (-0.14%)
Artifact size: 313.55 MiB -> 313.48 MiB (-0.02%)

[pnkfelix]

• The single primary regression here seems to be a measurement blip, based on the 30-day history.
• Even if it weren't, the improvements would outweigh the regression.
• Marked as triaged.

@rustbot label: +perf-regression-triaged