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Closed world optimization of wasm modules that have function types in import/export boundary

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
18/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
wasm
Área
compilers

Línea de trabajo

Start from the repro in the issue: the module exports callForeignUninhabited and callForeignDevirtualized, which take function references from outside, and the bug appears when wasm-opt --closed-world --traps-never-happen --gufa runs on it. Trace how closed-world and GUFA decide which functions can reach a given type, since the wrong unreachable and devirtualized target both come from that. Done means the optimizer no longer assumes it knows every function of an externally passed signature, and the repro's call_ref sites keep their real targets. The maintainers have not yet settled the fix, so check the comments first.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

Let's say we have a module that can be optimized under closed world assumptions but we want to export some functions that the outside can invoke and pass callbacks of a specific type in. Concretely:

(module
  (type $sig1 (func (param i32 i64 f64) (result i32)))
  (type $sig2 (func (param i32 i32 i64) (result i32)))

  (func $unrelated (type $sig2) (param i32 i32 i64) (result i32)
    (i32.const 12345)
  )

  (export "getUnrelated" (func $getUnrelated))
  (func $getUnrelated (result (ref $sig2))
    (ref.func $unrelated)
  )

  ;; Takes a (ref $sig1) directly from outside and calls it.
  (export "callForeignUninhabited" (func $callForeignUninhabited))
  (func $callForeignUninhabited (param $fn (ref $sig1)) (param $a i32) (param $b i64) (param $c f64) (result i32)
    (call_ref $sig1 (local.get $a) (local.get $b) (local.get $c) (local.get $fn))
  )

  ;; Takes a (ref $sig2) directly from outside and calls it.
  (export "callForeignDevirtualized" (func $callForeignDevirtualized))
  (func $callForeignDevirtualized (param $fn (ref $sig2)) (param $a i32) (param $b i32) (param $c i64) (result i32)
    (call_ref $sig2 (local.get $a) (local.get $b) (local.get $c) (local.get $fn))
  )
)

If we compile this with

% wasm-opt -all --closed-world --traps-never-happen --gufa -Os -S repro.wat -o repro.opt.wat

we get

(module
 (type $sig2 (func (param i32 i32 i64) (result i32)))
 (type $sig1 (func (param i32 i64 f64) (result i32)))
 (type $2 (func (result (ref $sig2))))
 (type $3 (func (param (ref $sig1) i32 i64 f64) (result i32)))
 (type $4 (func (param (ref $sig2) i32 i32 i64) (result i32)))
 (elem declare func $unrelated)
 (export "getUnrelated" (func $getUnrelated))
 (export "callForeignUninhabited" (func $callForeignUninhabited))
 (export "callForeignDevirtualized" (func $callForeignDevirtualized))
 (func $unrelated (type $sig2) (param $0 i32) (param $1 i32) (param $2 i64) (result i32)
  (unreachable)
 )
 (func $getUnrelated (type $2) (result (ref $sig2))
  (ref.func $unrelated)
 )
 (func $callForeignUninhabited (type $3) (param $0 (ref $sig1)) (param $1 i32) (param $2 i64) (param $3 f64) (result i32)
  (unreachable)
 )
 (func $callForeignDevirtualized (type $4) (param $0 (ref $sig2)) (param $1 i32) (param $2 i32) (param $3 i64) (result i32)
  (i32.const 12345)
 )
)

So the call_ref may either be turned into an unreachable trap or devirtualized to the wrong target.

I would assume that even under --closed-world assumptions, if binaryen sees that outside world passes a function reference in that it may pass a function from a different module in (i.e. that binaryen cannot assume it knows all functions of said signature type).

Note: For the other way around, where we take the reference of a wasm function defined in our module and pass it to the outside world, we can annotate it with the (binaryen.js-called) annotation and that will prevent binaryen from changing the function type. Isn't there a mechanism for the reverse: An outside function enters our module and we call it?

/cc @tlively @kripken Is this expected? Is there a way around this?

Lenguaje dominante
WebAssembly
Estrellas
8.7k
Forks
895
Merge medio
1 d 23 h
PR fusionados (30 d)
105

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