dotnet/roslyn

Slice can be simplified does not produce the same code

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#47,629 opened on Sep 11, 2020

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 (10 comments) (5 reactions) (0 assignees)C# (4,257 forks)batch import
Area-CompilersCode Gen QualityFeature - Rangehelp wanted

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Description

Currently the IDE suggest with Span not to use slice but instead use the Range indexer (IDE0057: Slice can be simplified to this). So the original code:

new Vector<T>(B.Slice(Idx))

is then translated to

new Vector<T>(B[Idx..])

the resulting generated code is different (which is OK)

    public void WithSlice() {
        ReadOnlySpan<float> A = new float[100];
        
        for (int i=0; i<A.Length; i+= Vector<float>.Count)
            new Vector<float>(A.Slice(i));   
    }
    
       public void WithRange() {
        ReadOnlySpan<float> A = new float[100];
        
        for (int i=0; i<A.Length; i+= Vector<float>.Count)
            new Vector<float>(A[i..]);
    }

and the result taken from sharplab:

    public void WithSlice()
    {
        ReadOnlySpan<float> readOnlySpan = new float[100];
        for (int i = 0; i < readOnlySpan.Length; i += Vector<float>.Count)
        {
            new Vector<float>(readOnlySpan.Slice(i));
        }
    }

    public void WithRange()
    {
        ReadOnlySpan<float> readOnlySpan = new float[100];
        for (int i = 0; i < readOnlySpan.Length; i += Vector<float>.Count)
        {
            ReadOnlySpan<float> readOnlySpan2 = readOnlySpan;
            int length = readOnlySpan2.Length;
            int num = i;
            int length2 = length - num;
            new Vector<float>(readOnlySpan2.Slice(num, length2));
        }
    }

but the JIT Asm seems to be different. I didn't make any performance comparisons, but for my environment it is crucial that the suggested rewrite does not alter the performance of the code.

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