C#•6mo ago

collection expression on Span<T>

Just a quick question, wasn't something like

void TestMethod(Span<int> span);

TestMethod([1]);

//Suppose to emit something like:
var _arg = 1;
TestMethod(new(&_arg, 1));
//Or
Span<int> span = stackalloc int[1];
span[0] = 1;
TestMethod(span);

void TestMethod(Span<int> span);

TestMethod([1]);

//Suppose to emit something like:
var _arg = 1;
TestMethod(new(&_arg, 1));
//Or
Span<int> span = stackalloc int[1];
span[0] = 1;
TestMethod(span);

Did they skip anti allocation improvements like this initially? right now it just allocates an array.

5 Replies

Thinker•6mo ago

It should use stackalloc. Not sure why it's using an array, ask in #roslyn perhaps.

jcotton42•6mo ago

@Tacti Tacoz it does not create an array, it creates an inline array https://sharplab.io/#v2:EYLgtghglgdgPgAQEwEYCwAoBAGABAlAOgCEBXAM3IFMAnAZwG5MEBmfJXAYVwG9NcB+NggAsuALIAKAMoAHCDAA8sAC4A+XHXkwAlL36CAvgYEmh+MeKSS9fDIIcTJAbRQBdHU3tHMhoA==

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jcotton42•6mo ago

public class C
{
    public void M(Span<int> span)
    {
    }

    public void M2()
    {
        <>y__InlineArray1<int> buffer = default(<>y__InlineArray1<int>);
        <PrivateImplementationDetails>.InlineArrayElementRef<<>y__InlineArray1<int>, int>(ref buffer, 0) = 1;
        M(<PrivateImplementationDetails>.InlineArrayAsSpan<<>y__InlineArray1<int>, int>(ref buffer, 1));
    }
}
[StructLayout(LayoutKind.Auto)]
[InlineArray(1)]
internal struct <>y__InlineArray1<T>
{
    [CompilerGenerated]
    private T _element0;
}

public class C
{
    public void M(Span<int> span)
    {
    }

    public void M2()
    {
        <>y__InlineArray1<int> buffer = default(<>y__InlineArray1<int>);
        <PrivateImplementationDetails>.InlineArrayElementRef<<>y__InlineArray1<int>, int>(ref buffer, 0) = 1;
        M(<PrivateImplementationDetails>.InlineArrayAsSpan<<>y__InlineArray1<int>, int>(ref buffer, 1));
    }
}
[StructLayout(LayoutKind.Auto)]
[InlineArray(1)]
internal struct <>y__InlineArray1<T>
{
    [CompilerGenerated]
    private T _element0;
}

jcotton42•6mo ago

see https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-12#inline-arrays and https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/builtin-types/struct#inline-arrays for details

Structure types - C# reference - C#

Learn about the struct type in C#

What's new in C# 12 - C# Guide - C#

Get an overview of the new features in C# 12.

Tacti Tacoz•6mo ago

Hmm maybe it doesn't do it when targeting .net framework? or net standard for that matter:

IL_0000: ldarg.0      // this
    IL_0001: ldc.i4.1
    IL_0002: newarr       [netstandard]System.Int32
    IL_0007: dup
    IL_0008: ldc.i4.0
    IL_0009: ldc.i4.1
    IL_000a: stelem.i4
    IL_000b: newobj       instance void valuetype [netstandard]System.Span`1<int32>::.ctor(!0/*int32*/[])
    IL_0010: call         instance void Test::test(valuetype [netstandard]System.Span`1<int32>)

IL_0000: ldarg.0      // this
    IL_0001: ldc.i4.1
    IL_0002: newarr       [netstandard]System.Int32
    IL_0007: dup
    IL_0008: ldc.i4.0
    IL_0009: ldc.i4.1
    IL_000a: stelem.i4
    IL_000b: newobj       instance void valuetype [netstandard]System.Span`1<int32>::.ctor(!0/*int32*/[])
    IL_0010: call         instance void Test::test(valuetype [netstandard]System.Span`1<int32>)

. Not in .net 7.0 either:

IL_0000: ldc.i4.1
    IL_0001: newarr       [System.Runtime]System.Int32
    IL_0006: dup
    IL_0007: ldc.i4.0
    IL_0008: ldc.i4.1
    IL_0009: stelem.i4
    IL_000a: newobj       instance void valuetype [System.Runtime]System.Span`1<int32>::.ctor(!0/*int32*/[])
    IL_000f: call         void Testt::Test(valuetype [System.Runtime]System.Span`1<int32>)

IL_0000: ldc.i4.1
    IL_0001: newarr       [System.Runtime]System.Int32
    IL_0006: dup
    IL_0007: ldc.i4.0
    IL_0008: ldc.i4.1
    IL_0009: stelem.i4
    IL_000a: newobj       instance void valuetype [System.Runtime]System.Span`1<int32>::.ctor(!0/*int32*/[])
    IL_000f: call         void Testt::Test(valuetype [System.Runtime]System.Span`1<int32>)

It's .net 8.0 only:

IL_0000: ldloca.s     V_1
    IL_0002: initobj      valuetype '<>y__InlineArray1`1'<int32>
    IL_0008: ldloca.s     V_1
    IL_000a: ldc.i4.0
    IL_000b: call         !!1/*int32*/& '<PrivateImplementationDetails>'::InlineArrayElementRef<valuetype '<>y__InlineArray1`1'<int32>, int32>(!!0/*valuetype '<>y__InlineArray1`1'<int32>*/&, int32)
    IL_0010: ldc.i4.1
    IL_0011: stind.i4
    IL_0012: ldloca.s     V_1
    IL_0014: ldc.i4.1
    IL_0015: call         valuetype [System.Runtime]System.Span`1<!!1/*int32*/> '<PrivateImplementationDetails>'::InlineArrayAsSpan<valuetype '<>y__InlineArray1`1'<int32>, int32>(!!0/*valuetype '<>y__InlineArray1`1'<int32>*/&, int32)
    IL_001a: call         void Test::Test(valuetype [System.Runtime]System.Span`1<int32>)

IL_0000: ldloca.s     V_1
    IL_0002: initobj      valuetype '<>y__InlineArray1`1'<int32>
    IL_0008: ldloca.s     V_1
    IL_000a: ldc.i4.0
    IL_000b: call         !!1/*int32*/& '<PrivateImplementationDetails>'::InlineArrayElementRef<valuetype '<>y__InlineArray1`1'<int32>, int32>(!!0/*valuetype '<>y__InlineArray1`1'<int32>*/&, int32)
    IL_0010: ldc.i4.1
    IL_0011: stind.i4
    IL_0012: ldloca.s     V_1
    IL_0014: ldc.i4.1
    IL_0015: call         valuetype [System.Runtime]System.Span`1<!!1/*int32*/> '<PrivateImplementationDetails>'::InlineArrayAsSpan<valuetype '<>y__InlineArray1`1'<int32>, int32>(!!0/*valuetype '<>y__InlineArray1`1'<int32>*/&, int32)
    IL_001a: call         void Test::Test(valuetype [System.Runtime]System.Span`1<int32>)

I guess because inline array require runtime support. But was it really too much to ask for them to add a stackalloc fallback though? Seems kinda sloppy to be honest

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collection expression on Span<T>