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IL: hold custom attributes in fields rather than a union case #20287

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IL: hold custom attributes in fields rather than a union case #20287

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IL: hold custom attributes in fields rather than a union case
auduchinok Aug 14, 2026
e0a9274
Fantomas
auduchinok Aug 18, 2026
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
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Expand Up @@ -166,6 +166,7 @@
* Add symbol and type highlighting to F# diagnostics ([PR #20097](https://github.com/dotnet/fsharp/pull/20097))
* IL: add `ILPreNamespace`, make `ILPreTypeDef` creation lazy ([PR #20092](https://github.com/dotnet/fsharp/pull/20092))
* IL: use empty tables for members when possible ([PR #20249](https://github.com/dotnet/fsharp/pull/20249))
* IL: hold custom attributes in fields rather than a union case

### Improved

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33 changes: 19 additions & 14 deletions src/Compiler/AbstractIL/il.fs
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Expand Up @@ -1262,27 +1262,31 @@ type WellKnownILAttributes =
| OverloadResolutionPriorityAttribute = (1u <<< 26)
| NotComputed = (1u <<< 31)

type internal ILAttributesStoredRepr =
| Reader of (int32 -> ILAttribute[])
| Given of ILAttributes

[<Sealed; NoEquality; NoComparison>]
type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStoredRepr) =
type ILAttributesStored private (metadataIndex: int32, reader: int32 -> ILAttribute[], given: ILAttribute[] | null) =

/// Stands in for the reader when the attributes are already in hand, so the field can stay non-null.
static let noReader: int32 -> ILAttribute[] = fun _ -> [||]

// Holds the array rather than an ILAttributesStoredRepr. The reader function is shared per metadata
// reader per attribute table, so it is held directly; wrapping it cost one object per owner, and most
// owners are never forced, so most of those existed only to say "not read yet". ILAttributes is a
// struct over the array, so rewrapping on each read allocates nothing.
[<VolatileField>]
let mutable repr = initial
let mutable attrArray: ILAttribute[] | null = given

[<VolatileField>]
let mutable wellKnownFlags = WellKnownILAttributes.NotComputed

member _.MetadataIndex = metadataIndex

member x.CustomAttrs: ILAttributes =
match repr with
| Given a -> a
| Reader f ->
let r = ILAttributes(f metadataIndex)
repr <- Given r
r
match attrArray with
| null ->
let a = reader metadataIndex
attrArray <- a
ILAttributes a
| a -> ILAttributes a

member x.HasWellKnownAttribute(flag: WellKnownILAttributes, compute: ILAttributes -> WellKnownILAttributes) : bool =
x.GetOrComputeWellKnownFlags(compute) &&& flag <> WellKnownILAttributes.None
Expand All @@ -1298,9 +1302,10 @@ type ILAttributesStored private (metadataIndex: int32, initial: ILAttributesStor
wellKnownFlags <- computed
computed

static member CreateReader(idx: int32, f: int32 -> ILAttribute[]) = ILAttributesStored(idx, Reader f)
static member CreateReader(idx: int32, f: int32 -> ILAttribute[]) = ILAttributesStored(idx, f, null)

static member CreateGiven(attrs: ILAttributes) = ILAttributesStored(-1, Given attrs)
static member CreateGiven(attrs: ILAttributes) =
ILAttributesStored(-1, noReader, attrs.AsArray())

let emptyILCustomAttrs = ILAttributes [||]

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