Potential optimization for is ""

[Youssef1313]

Based on the current master in sharplab:

https://sharplab.io/#v2:EYLgtghgzgLgpgJwD4AEDMACFAmDBhAWACgBvYjCrTYAexoBsMBZAChuACs4BjGDADwCU5SmSKUJWAOwCMASygYAREoDcIigF9imoA==

The following:

public bool M(object x)
{
    return x is "";
}

is compiled as:

public bool M(object x)
{
    string text = x as string;
    if (text != null)
    {
        return text == "";
    }
    return false;
}

It's a bit more performant if return text == "" is text.Length == 0.
That would be a special-casing for this, which I'm unsure if it's acceptable to the compiler team or not.

category:cq
theme:basic-cq
skill-level:intermediate
cost:small

[CyrusNajmabadi]

It's a bit more performant if return text == "" is text.Length == 0.

That feels like a runtime optimziation to make. text == "" is totally idiomatic and should be jittable to whatever the fastest string check is we can muster. @jkotas thoughts? Should this move to jit?

[jkotas]

This is a special case of #33338 .

[Dotnet-GitSync-Bot]

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[jkotas]

cc @EgorBo

[EgorBo]

Yes, if we could tune the inliner to be able to inline string.Equals (called via == operator) for constant arguments we could achieve the desired behavior and fold redundant null-checks.

// Determines whether two Strings match.
public static bool Equals(string? a, string? b)
{
    if (object.ReferenceEquals(a, b))
    {
        return true;
    }

    if (a is null || b is null || a.Length != b.Length)
    {
        return false;
    }

    return EqualsHelper(a, b);
}

(the idea was to tell the inliner "if you see a get_Length call on a constant string especially in a condition - try harder")

However, I guess redundant object.ReferenceEquals(a, b) and EqualsHelper(a, b); would still be presented in the final codegen. So probably makes sense to intrinsify string.Equals and if its argument is a constant string - emit GT_ARRLEN check.

[EgorBo]

Prototype with an intrinsic: EgorBo@7c4ef9f

it optimizes x == "" or x is "" or string.Equals(x, "") to x != null && x.Length == 0

[EgorBo]

Moving to future
First, we need to be able to import string.Empty as GT_CNS_STR to maximize impact of this.