JIT can't elide span.Slice range checks after a manual check

For a case like this:

public static ReadOnlySpan<char> Test(ReadOnlySpan<char> span, int offset)
{
    if ((uint)offset < (uint)span.Length)
    {
        return span.Slice(offset + 1);
    }
    
    return span;
}

runtime/src/libraries/System.Private.CoreLib/src/System/ReadOnlySpan.cs

Lines 357 to 363 in 08298b4

    
           public ReadOnlySpan<T> Slice(int start) 
        
           { 
        
               if ((uint)start > (uint)_length) 
        
                   ThrowHelper.ThrowArgumentOutOfRangeException(); 
        
               return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference, (nint)(uint)start /* force zero-extension */), _length - start); 
        
           }

The JIT isn't removing the if ((uint)start > (uint)_length) check.

You can hit that in IndexOf + slice loops, e.g.

int count = 0;
int pos;
while ((uint)(pos = span.IndexOf(c)) < (uint)span.Length)
{
    count++;
    span = span.Slice(pos + 1);
}

@JulieLeeMSFT

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Hopefully, should be handled by the PRs I'm currently working on (e.g. #112907 + #112824 + more changes)

@EgorBo

@EgorBo can this be closed now?

@EgorBo can this be closed now?

Nope, still needs work

JIT: Remove more redundant Span.Slice range checks (dotnet#136615)

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JIT can't elide span.Slice range checks after a manual check #112953

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	public ReadOnlySpan<T> Slice(int start)
	{
	if ((uint)start > (uint)_length)
	ThrowHelper.ThrowArgumentOutOfRangeException();

	return new ReadOnlySpan<T>(ref Unsafe.Add(ref _reference, (nint)(uint)start /* force zero-extension */), _length - start);
	}

JIT can't elide span.Slice range checks after a manual check #112953

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dotnet-policy-service commented on Feb 26, 2025

EgorBo commented on Feb 26, 2025

tannergooding commented on Apr 28, 2025

EgorBo commented on May 2, 2025

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