7.5 KiB
Efficent Formatting
Summary
This combination of features will allow for effecient and customizable formatting of string values.
Motivation
The allocation of string and params values dominates the performance of many text based applications like MSBuild.
In order to maintain efficiency such applications often need to abandon popular features like params, string
interpolation, etc ... This set of language features will enable applications to avoid the extra allocation overhead
while continuing to use these features.
Detailed Design
There are a set of features that will be used here to achieve these results:
- Expanding
paramsto support more effecient types than array. - Allowing for developers to customize how
stringinterpolation is achieved.
params Span
The language will allow for params in a method signature to have the types Span<T> and ReadOnlySpan<T>. The same
rules will apply to params Span<T> that apply to params T[]:
- Can't overload where the only difference is a
paramskeyword. - Can invoke by passing a series of
Targuments or a singleSpan<T>argument. - Must be the last parameter in a method signature.
- Etc ...
The advantage this variant of params provides is it gives the compiler great flexbility in how it allocations the
backing storage for the Span<T> value. With a params T[] the compiler must allocate a new T[] for every
invocation of a params method because it must assume the callee stored and reused the parameter. Given the
Span<T> and ReadOnlySpan<T> types are ref struct the callee cannot store the argument. Hence the compiler can
safely re-use the value.
One such potential implementation is the following. Consider all params invocation of a given type in a method
body. The compiler could allocate an array which has a size equal to the largest params invocation and use that for
all of the invocations by creating appropriately sized Span<T> instances over the array. For example:
static class OneAllocation {
static void Use(params Span<string> spans) {
...
}
static void Go() {
Use("jaredpar");
Use("hello", "world");
Use("a", "longer", "set");
}
}
The compiler could choose to emit the body of Go as follows:
static void Go() {
var args = new string[3];
args[0] = "jaredpar";
Use(new Span<int>(args, start: 0, length: 1));
args[0] = "hello";
args[1] = "world";
Use(new Span<int>(args, start: 0, length: 2));
args[0] = "a";
args[1] = "longer";
args[2] = "set";
Use(new Span<int>(args, start: 0, length: 3));
}
This can siginficantly reduce the number of arrays allocated in an application. Allocations can be even further reduced if the runtime provides utilities for smarter stack allocation of arrays.
This optimization cannot always be applied though. Even though the callee cannot capture the params argument it can
still be captured in the caller when there is a ref or a out / ref parameter that is itself a ref struct
type.
static class SneakyCapture {
static ref int M(params Span<T> span) => ref span[0];
static void Oops() {
// This now holds onto the memory backing the Span<T>
ref int r = ref M(42);
}
}
These cases are statically dectable though. It potentially occurs whenever there is a ref return or a ref struct
parameter passed by out or ref. In such a case the compiler must allocate a fresh T[] for every invocation.
params IEnumerable
The language will allow for params in a method signature to have the type IEnumerable<T>. The same rules will apply
to params IEnumerable<T> that apply to params T[] (see above for listing of rules).
The compiler will invoke a params IEnumerable<T> method exactly as it invokes a params T[] method. A new array will
be allocated for every call site and passed to the callee.
params VariantCollection
The language will allow for params in a method signature to have the type VariantCollection. This elements of a
VariantCollection are considered to be of type Variant. The same rules will apply to params VariantCollection
that apply to params Variant[] (see above for listing of rules).
The Variant value type is a suggested addition to CoreFX that has a
conversion from any type. For the majority of common framework types (int, double, TimeSpan, etc ..) the
conversion is allocation free.
Methods which take a params of hetrogeneous data today must declare as params object[]. This means that every
value type passed to the method incurs a boxing allocation (in addition to the array the compiler must allocate). The
Variant and VariantCollection type allow such methods to avoid allocations in the most common paths. Invocations
like Console.WriteLine("hello {0] {1}", someInt, DateTime.Now) can be invoked without any boxing or array allocations
here.
When emitting the code for a params VariantCollection invocation the compiler will look for an overload of
VariantCollection.Create(in Variant v1, in Variant v2, ..., in Variant vn) where n matches the number of arguments
being passed to the parameter. Each argument will be passed exactly as if the developer wrote the
VariantCollection.Create call with the arguments. For example:
class VariantCall {
static void Write(params VariantCollection collection) { ... }
static void Use() {
Write(1, DateTime.UtcNow)
// Compiler will evaluate as
Write(VariantCollection.Create(1, DateTime.UtcNow))
// Compiler will eventually emit as
Write(VariantCollection.Create((Variant)1, (Variant)DateTime.UtcNow))
}
}
In the case a VariantCollection.Create method with the correct number of parameters does not exist the compiler will
attempt to invoke Create(Variant[] array) instead. A fresh Variant[] will be allocated for every invocation in this
case. In the case Create(Variant[] array) does not exist the compiler will issue an error.
params overload resolution changes
This proposal means the language now has four variants of params where before it had one. It is also sensible for
methods to define overloads of methods that differ only on params declarations.
Consider that StringBuilder.AppendFormat would certainly add a params VariantCollection overload in addition to the
params object[]. This would allow it to substantially improve performance by reducing boxing and collection
allocations without requiring any changes to the calling code.
To facilitate this the language will introduce the following overload resolution tie breaking rule. When the candidate
methods differ only by the params parameter then the canditates will be preferred in the following order:
VariantCollectionReadOnlySpan<T>Span<T>T[]IEnumerable<T>
This order is the most to the least effecient for the general case.
Customize interopolated strings
existing behavior: interpolated strings have natural type of string but can target type to formattablestring
change to ValueFormattableString
Variant type
Open Issuess
open issue1
Considerations
consideration 1
Future Considerations
CLR helper for stack allocating arrays
Misc
Related issues