JIT fails to treat expression as non-negative after range check

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#112,765 1 comment 0 reactions 1 assignee View on GitHub

@EgorBo is already working on this.

Since Feb 20, 2025.

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Description

area-CodeGen-coreclr in-pr

https://godbolt.org/z/Wv5bv99bb

public static short Test1(ref short searchSpace, int searchSpaceLength)
{
    if (searchSpaceLength < Vector128<short>.Count)
        return 42;
    
    return Unsafe.Add(ref searchSpace, searchSpaceLength - Vector128<short>.Count);
}

public static short Test2(ref short searchSpace, int searchSpaceLength)
{
    if (searchSpaceLength < Vector128<short>.Count)
        return 42;
    
    return Unsafe.Add(ref searchSpace, (uint)searchSpaceLength - (nuint)Vector128<short>.Count);
}

-add      esi, -8
-movsxd   rax, esi
-movsx    rax, word  ptr [rdi+2*rax]
+mov      eax, esi
+movsx    rax, word  ptr [rdi+2*rax-0x10]

This occurs in practice in a bunch of places throughput our IndexOf-like logic, e.g.
https://github.com/dotnet/runtime/blob/7e5e573dd32990beedee8b2749a0ffe5af784d3c/src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Packed.cs#L431
https://github.com/dotnet/runtime/blob/7e5e573dd32990beedee8b2749a0ffe5af784d3c/src/libraries/System.Private.CoreLib/src/System/SearchValues/IndexOfAnyAsciiSearcher.cs#L347

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