Clarify MoveData::init_loc_map.

[nnethercote]

Change the SmallVec size from 4 to 1, because that's sufficient in the vast majority of cases. (This doesn't affect performance in practice, so it's more of a code clarity change than a performance change.)

r? @cjgillot

[nnethercote]

Let's double-check that this doesn't affect performance:

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 89dfb65 (89dfb65a612a2bb0ccbd33941e17ebb7c04be515, parent: fcd630976c460c819c4bbcaf107d0c94501205d8)

[rust-timer]

Finished benchmarking commit (89dfb65): comparison URL.

Overall result: ✅ improvements - 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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -3.3%)

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
Improvements ✅ (secondary)	-3.3%	[-3.3%, -3.3%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 475.254s -> 475.126s (-0.03%)
Artifact size: 390.85 MiB -> 390.85 MiB (-0.00%)

[nnethercote]

No perf effect, as expected.

@bors rollup