A (brief) overview of Span<T>

A (brief) overview of Span<T>
(and ReadOnlySpan<T> and Memory<T> and ReadOnlyMemory<T>)
DAVID WENGIER
@davidwengier

What is Span<T>?
- Stephen Toub, MSDN Magazine, January 2018
https://msdn.microsoft.com/en-us/magazine/mt814808
“System.Span<T> is a new value type at the heart of .NET [that]
enables the representation of contiguous regions of arbitrary
memory”

What is Span<T>?
• struct
• Part of System.Memory, available on Nuget
• Currently at rc1 version (prerelease)
• .NET Standard 1.0 so can be used in .NET 4.5+
• C# 7.2-ish
memory”

What is Span<T>?
• High performance, low (no) overhead
• Framework, CLR, JIT and GC support
• Provides memory and type safety
• Avoids the need for unsafe code
memory”

What is Span<T>?
• Native/unmanaged memory (P/Invoke)
• Managed memory (.NET types)
• Stack memory (stackalloc)
memory”

What is Span<T>?
• Think of it like having an array to access raw memory

var arr = new byte[10];
Span<byte> bytes = arr;
bytes[0] = 42;
Assert.True(arr[0] == 42);
Span<byte> slice = bytes.Slice(4, 3);
slice[0] = 43;
Assert.True(bytes[4] == 43);
Assert.True(arr[4] == 43);
slice[5] = 44; // IndexOutOfRangeException

HeapStack
0x01
0x02
0x03
0x04
0x05
0x06
string plate = “YGG871”;
plate = plate.Substring(3);
int num = int.Parse(plate);
plate 0x01
“YGG871”
plate 0x02
“871”
num 871

HeapStack
0x01
0x02
0x03
0x04
0x05
0x06
ReadOnlySpan<char> s = plate.AsSpan();
s = s.Slice(3);
int num = int.Parse(s);
plate 0x01
“YGG871”
0x01
0
6
3
num 871
span
offset
pointer
length
.NET Framework 4.5+ - slow span

HeapStack
0x01
0x02
0x03
0x04
0x05
0x06
ReadOnlySpan<char> s = plate.AsSpan();
s = s.Slice(3);
int num = int.Parse(s);
plate 0x01
“YGG871”
0x01+3
3
num 871
ref
span
length
pointer
.NET Core 2.0+ – fast span

* From Adam Sitnik - http://adamsitnik.com/Span/
Method Job Mean StdDev
SpanIndexer_G
et
.NET 4.6 0.6054 ns 0.0007 ns
SpanIndexer_G
et
.NET Core 1.1 0.6047 ns 0.0008 ns
SpanIndexer_G
et
.NET Core 2.0 0.5333 ns 0.0006 ns
SpanIndexer_S
et
.NET 4.6 0.6059 ns 0.0007 ns
SpanIndexer_S
et
.NET Core 1.1 0.6042 ns 0.0002 ns
SpanIndexer_S
et
.NET Core 2.0 0.5205 ns 0.0003 n

Method
String
Length
Mean StdDev Scaled Gen 0
Allocated
*
Substring 10 8.277 ns 0.1938 ns 4.54 0.0191 40 B
Slice 10 1.822 ns 0.0383 ns 1.00 - 0 B
Substring 1000 85.518 ns 1.3474 ns 47.22 0.4919 1032 B
Slice 1000 1.811 ns 0.0205 ns 1.00 - 0 B
* Not including original string

Framework Support
https://github.com/dotnet/corefx/blob/master/src/Common/src/CoreLib/System/Int32.cs

Framework Support
// RandomStringLoop
var stringChars = new char[Length];
for (int i = 0; i < stringChars.Length; i++)
{
stringChars[i] = (char)(random.Next(0, 10) + '0');
}
return new string(stringChars);
// RandomStringEnumerable
return new string(Enumerable.Repeat("", Length)
.Select(s => (char)(random.Next(0, 10) + '0')).ToArray());
// RandomStringSpan
return string.Create(Length, random, (Span<char> chars, Random r) =>
{
for (int i = 0; i < chars.Length; i++)
{
chars[i] = (char)(r.Next(0, 10) + '0');
}
});

Method Length Mean Scaled Allocated
RandomStringSpan 10 150.29 ns 1.00 48 B
RandomStringLoop 10 156.16 ns 1.04 96 B
RandomStringEnumerable 10 340.26 ns 2.27 192 B
RandomStringSpan 100 1,314.91 ns 1.00 232 B
RandomStringLoop 100 1,361.57 ns 1.04 456 B
RandomStringEnumerable 100 2,140.91 ns 1.63 552 B
RandomStringSpan 1000 12,461.32 ns 1.00 2032 B
RandomStringLoop 1000 14,073.65 ns 1.13 4056 B
RandomStringEnumerable 1000 20,354.63 ns 1.63 4152 B

When do mere mortals use it?
public static bool ContainsCapitalLetters(string s)
{
for (int i = 0; i < s.Length; i++)
{
if (char.IsUpper(s[i]))
{
return true;
}
}
return false;
}
public static int Sum(int[] a)
{
int sum = 0;
for (int i = 0; i < a.Length; i++)
{
sum += a[i];
}
return sum;
}

When do mere mortals use it?
public static bool ContainsCapitalLetters(ReadOnlySpan<char> s)
{
for (int i = 0; i < s.Length; i++)
{
if (char.IsUpper(s[i]))
{
return true;
}
}
return false;
}
public static int Sum(ReadOnlySpan<int> a)
{
int sum = 0;
for (int i = 0; i < a.Length; i++)
{
sum += a[i];
}
return sum;
}

Limitations
• Can only live on the stack
• Implemented as a ref struct
• Can only be contained by ref structs

ref structs
Structs that can exist only on the stack. New in C# 7.2
• Can’t implement interfaces
• Can’t be used as generic type arguments
• Can’t be boxed to object
• Can’t be passed in to - or used in places inside - of async methods,
iterators, nested functions or query expressions

async Task DoSomethingAsync(Span<byte> buffer)
{
buffer[0] = 0;
await Something();
buffer[0] = 1;
}
{
SomethingClass cl = new SomethingClass()
cl.buffer = buffer;
cl.StartSomething();
}
private class SomethingClass
{
public Span<byte> buffer; // illegal
public void Start_Something()
{
buffer[0] = 0;
DoMagicAsyncStuff().ContinueWith(End_Something);
}
public void End_Something()
{
buffer[0] = 1;
}
}

Memory<T>
• “Normal” struct
• Not as performant as Span
• Can be used in more places than Span (ie, doesn’t have the
limitations)

Memory<T>
{
buffer[0] = 0;
await Something(); // Bang!
buffer[0] = 1;
}
async Task DoSomethingAsync(Memory<byte> buffer)
{
buffer.Span[0] = 0;
await Something(); // Totally fine
buffer.Span[0] = 1;
}

Memory<T>
public struct Memory<T>
{
private int _offset;
private int _length;
private int _pointer;
private byte[] _data; // really OwnedMemory<T>
public Span<T> Span => new Span<T>(_data, _pointer, _offset, _length);
}
* Not real

TechEmpower Plaintext
Round 14
Round 15

DAVID WENGIER
@davidwengier
Thank you!
Questions?

A (brief) overview of Span<T>

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A (brief) overview of Span<T>