Use an explicit unsigned comparison for span index checks

[stephentyrone]

LLVM ought to be able to do this transformation for us, but it currently fails to do so. We can code around it easily enough.

[stephentyrone]

@swift-ci test

[meg-gupta]

This seems to be the same underlying issue as rdar://154950810 for which @eeckstein has a PR - #82804

This change looks good as well especially if you plan to cherry-pick

[glessard]

We should have the same changes in RawSpan and MutableRawSpan.
I'd already written the bounds checks that way in RawSpan and MutableRawSpan, so no change needed there.

[stephentyrone]

@meg-gupta right, hopefully we can land #82804 and fix the problem more generally; this is a stop-gap

[stephentyrone]

@swift-ci smoke test

[stephentyrone]

@swift-ci smoke test

[eeckstein]

LLVM ought to be able to do this transformation for us

LLVM can't do it because it doesn't know that count is >= 0

This seems to be the same underlying issue as rdar://154950810 for which @eeckstein has a PR - #82804

No, it's a different issue.

[...] and fix the problem more generally; this is a stop-gap

Yes, we can. We can add _assumeNonNegative to count:

  public var count: Int { _assumeNonNegative(_count) }

and add a small peephole optimization which takes this constraint into account for converting the two comparisons into a single one.

[eeckstein]

and add a small peephole optimization which takes this constraint into account for converting the two comparisons into a single one.

If we pass the information of _assumeNonNegative to LLVM, LLVM can do the work for us: (last two commits of) #83172

[stephentyrone]

Looking at the macOS failure here, we're getting a null pointer check that we also absolutely should not have (due to the invariant that the pointer may only be null when count is zero). However, that does not reproduce at desk for me. I assume this is due to the stdlib being built with --swift-stdlib-enable-assertions=true in CI (for macOS only), which is really counter-productive for this sort of testing where we want to ensure that no extra checks are present, even though it's beneficial for other sorts of testing.

[stephentyrone]

@swift-ci smoke test

[stephentyrone]

@swift-ci smoke test macOS