[stdlib] [bugfix] Make memcmp function robust against overflow and provide correct SIMD implementation

[mzaks]

The bugs are reproduced in the unit tests.

[soraros]

Would this work?

Suggested change

	var smaller = s1i < s2i
	var bigger = s1i > s2i
	if smaller or bigger:
	return -int(smaller) + int(bigger)
	if s1i != s2i:
	return 1 if s1i > s2i else -1

[mzaks]

Yeah, that is even simpler, thanks!

[soraros]

Can we do similar things to the SIMD part?

[mzaks]

I actually realised why I did it the way I did before I wanted to make a branchless implementation, but stoped half way :)

  var result = 0
  var i = 0
  while i < count:
    var s1i = s1[i]
    var s2i = s2[i]
    var smaller = s1i < s2i
    var bigger = s1i > s2i
    i += 1 + count * int(smaller or bigger)
    result = -1 * int(smaller) + 1 * int(bigger)
  return result

This would be branchless implementation, is a bit more complex but my guess it should be faster. Anyways let's go with the simple one for now. I will create a gist for myself. When we have benchmarks infrastructure setup we can do such micro optimisations.

[soraros]

Somewhat relevant reference. I still think we could these to a cmp function in SIMD so you can use as much trick as possible.

[mzaks]

@soraros you were right regarding simplifying SIMD part, I just pushed an update.

[soraros]

• Should we restrict the type to unsigned as well?
  • If we do, it's hardly "mem"-compare anymore, IMO
• Maybe we could add a lexcmp method on SIMD.

[JoeLoser]

Nice. Thank you for the fixes! Always good to make the low-level functions robust to avoid lingering bugs here.

[modularbot]

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