Perf: Optimize target_for_value lookup

[malik672]

Replaced the iter().zip().find_map() chain with a slice based iter().position() scan.

The zip iterator forced a runtime cmp/cmovb check between values.len() and targets.len() to establish loop bounds. The new implementation depends solely on values, decoupling the dependency chain.

https://godbolt.org/z/hGda7PYzz

[rustbot]

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[lqd]

Is that code path particularly hot in your experience?

It doesn't hurt to check, even if it may be unlikely (and the reason why "no one noticed").

@bors try @rust-timer queue

[malik672]

Is that code path particularly hot in your experience?

It doesn't hurt to check, even if it may be unlikely (and the reason why "no one noticed").

@bors try @rust-timer queue

Idk, was just reading some part of the codebase and picked it out
decided to check on godbolt

[rust-bors]

☀️ Try build successful (CI)
Build commit: 6014c74 (6014c74d5b6b9c8bdc7133a8bb8291f57ac4df2e, parent: e22dab387f6b4f6a87dfc54ac2f6013dddb41e68)

[rust-timer]

Finished benchmarking commit (6014c74): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.6%	[0.5%, 0.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results (primary -2.0%, secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

Results (primary -0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	4

Bootstrap: 473.456s -> 472.515s (-0.20%)
Artifact size: 388.90 MiB -> 388.91 MiB (0.00%)