Skip to content

Resumable state machines: Don't capture definitions by "builder@" prefix. EXPERIMENTAL #19482

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
majocha wants to merge 4 commits into
base: main
Choose a base branch
from
+626 −541
Draft

Resumable state machines: Don't capture definitions by "builder@" prefix. EXPERIMENTAL #19482

Show file tree
Hide file tree
Changes from 2 commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
fe2e2d2
Simplify state machine binding detection logic
majocha Mar 23, 2026
858ccfc
Merge branch 'main' into builder-resumable
majocha Mar 25, 2026
e8521de
eliminate the elimination
majocha Apr 1, 2026
9b92de4
Merge branch 'main' into builder-resumable
majocha Apr 1, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
11 changes: 1 addition & 10 deletions src/Compiler/Optimize/LowerStateMachines.fs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -102,15 +102,8 @@ let RepresentBindingAsStateVar g (bind: Binding) (resBody: StateMachineConversio
let isExpandVar g (v: Val) =
isReturnsResumableCodeTy g v.TauType

// We allow a prefix of bindings prior to the state machine, e.g.
// task { .. }
// becomes
// let builder@ = task
// ....
let isStateMachineBindingVar g (v: Val) =
isExpandVar g v ||
(let nm = v.LogicalName
(nm.StartsWithOrdinal("builder@") || v.IsMemberThisVal))
Copy link
Copy Markdown
Member

@T-Gro T-Gro Apr 1, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Do we know why it was needed before?
Are the recent state machine bug fixes the reason why we no longer need this special casing?

Copy link
Copy Markdown
Contributor Author

@majocha majocha Apr 1, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Are the recent state machine bug fixes the reason why we no longer need this special casing?

No, this is completely independent. I did a checkout of old commit and applied this fix to check it.

Do we know why it was needed before?

Ha, this is not entirely clear. That's why I'm a bit wary about this.

isExpandVar g v || v.IsMemberThisVal

type env =
{
Expand Down Expand Up @@ -361,7 +354,6 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
TryReduceExpr env f (args2 @ args) (fun f2 -> remake (Expr.App (f2, _fty, _tyargs, args2, _m)))

| _ ->
//let (env, expr) = BindResumableCodeDefinitions env expr
match TryReduceApp env expr args with
| Some expandedExpr ->
if sm_verbose then printfn "reduction = %A, args = %A --> %A..." expr args expandedExpr
Expand Down Expand Up @@ -408,7 +400,6 @@ type LowerStateMachine(g: TcGlobals, outerResumableCodeDefns: ValMap<Expr>) =
// Detect a state machine with a single method override
[<return: Struct>]
let (|ExpandedStateMachineInContext|_|) inputExpr =
// All expanded resumable code state machines e.g. 'task { .. }' begin with a bind of @builder or 'defn'
// Seed the env with any expand-var definitions from outer scopes (e.g. across lambda boundaries)
let initialEnv = { env.Empty with ResumableCodeDefns = outerResumableCodeDefns }
let env, expr = BindResumableCodeDefinitions initialEnv inputExpr
Expand Down
51 changes: 27 additions & 24 deletions tests/fsharp/Compiler/CodeGen/EmittedIL/TaskGeneratedCode.fs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -1100,7 +1100,7 @@ type Generic1InGeneric1<'T>() =
""",
(fun verifier -> verifier.VerifyIL [
"""
.class public abstract auto ansi sealed Test
.class public abstract auto ansi sealed Test
extends [runtime]System.Object
{
.custom instance void [FSharp.Core]Microsoft.FSharp.Core.CompilationMappingAttribute::.ctor(valuetype [FSharp.Core]Microsoft.FSharp.Core.SourceConstructFlags) = ( 01 00 07 00 00 00 00 00 )
Expand Down Expand Up @@ -1356,29 +1356,32 @@ type Generic1InGeneric1<'T>() =
.custom instance void [runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 )

.maxstack 4
.locals init (valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A> V_0,
valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>& V_1)
IL_0000: ldloca.s V_0
IL_0002: initobj valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>
IL_0008: ldloca.s V_0
IL_000a: stloc.1
IL_000b: ldloc.1
IL_000c: ldarg.1
IL_000d: stfld class [runtime]System.Threading.Tasks.Task`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::computation
IL_0012: ldloc.1
IL_0013: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_0018: call valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!A>::Create()
IL_001d: stfld valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_0022: ldloc.1
IL_0023: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_0028: ldflda valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_002d: ldloc.1
IL_002e: call instance void valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!0>::Start<valuetype Test/Generic1InGeneric1`1/clo@7<!0,!!0>>(!!0&)
IL_0033: ldloc.1
IL_0034: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_0039: ldflda valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_003e: call instance class [netstandard]System.Threading.Tasks.Task`1<!0> valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!A>::get_Task()
IL_0043: ret
.locals init (class [FSharp.Core]Microsoft.FSharp.Control.TaskBuilder V_0,
valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A> V_1,
valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>& V_2)
IL_0000: call class [FSharp.Core]Microsoft.FSharp.Control.TaskBuilder [FSharp.Core]Microsoft.FSharp.Control.TaskBuilderModule::get_task()
Copy link
Copy Markdown
Contributor Author

@majocha majocha Mar 24, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

this getter call is the only substantial change in the IL. It gets eliminated with "--optimize+"

IL_0005: stloc.0
IL_0006: ldloca.s V_1
IL_0008: initobj valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>
IL_000e: ldloca.s V_1
IL_0010: stloc.2
IL_0011: ldloc.2
IL_0012: ldarg.1
IL_0013: stfld class [runtime]System.Threading.Tasks.Task`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::computation
IL_0018: ldloc.2
IL_0019: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_001e: call valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!A>::Create()
IL_0023: stfld valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_0028: ldloc.2
IL_0029: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_002e: ldflda valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_0033: ldloc.2
IL_0034: call instance void valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!0>::Start<valuetype Test/Generic1InGeneric1`1/clo@7<!0,!!0>>(!!0&)
IL_0039: ldloc.2
IL_003a: ldflda valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!1> valuetype Test/Generic1InGeneric1`1/clo@7<!T,!!A>::Data
IL_003f: ldflda valuetype [runtime]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!0> valuetype [FSharp.Core]Microsoft.FSharp.Control.TaskStateMachineData`1<!!A>::MethodBuilder
IL_0044: call instance class [netstandard]System.Threading.Tasks.Task`1<!0> valuetype [netstandard]System.Runtime.CompilerServices.AsyncTaskMethodBuilder`1<!!A>::get_Task()
IL_0049: ret
}

}
Expand Down
Loading