[SR-13945] [AutoDiff] runtime segfault involving generic methods, arrays, and control flow

[marcrasi]

Previous ID	SR-13945
Radar	rdar://problem/72160850
Original Reporter	@marcrasi
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug, AutoDiff
Assignee	@marcrasi
Priority	Medium

md5: 8e8401d9c9641d0f9a1d5a8d944d8be4

Issue Description:

I believe this is a regression that happened some time between November 20 and December 4, because we have some code that was working around Nov 20 but started triggering this crash when we pulled in Dec 4 changes.

import _Differentiation

struct Reverser {
  func apply(_ input: [Float]) -> [Float] {
    var result = [Float]()
    for i in 0..<withoutDerivative(at: input.count) {
      result.append(input[withoutDerivative(at: input.count - 1 - i)])
    }
    return result
  }
}

let g = gradient(at: [1.0, 2.0, 3.0]) { (x: [Float]) -> Float in
  let reverser = Reverser()
  return reverser.apply(x)[0]
}

Compile and run to get a segfault.

The backtrace is:

* thread #​1, name = 'main', stop reason = signal SIGSEGV: invalid address (fault address: 0x841f0e)
  * frame #&#8203;0: 0x00007ffff7ddbc4d libswiftCore.so`swift_getAssociatedTypeWitnessSlowImpl(swift::MetadataRequest, swift::TargetWitnessTable<swift::InProcess>*, swift::TargetMetadata<swift::InProcess> const*, swift::TargetProtocolRequirement<swift::InProcess> const*, swift::TargetProtocolRequirement<swift::InProcess> const*) + 93
    frame #&#8203;1: 0x00007ffff7dd9dde libswiftCore.so`swift_getAssociatedTypeWitness + 142
    frame #&#8203;2: 0x0000555555559a2d main`AD__$s4main8ReverserV5applyySayxGAEF__pullback_src_0_wrt_0_16_Differentiation14DifferentiableRzl + 93
    frame #&#8203;3: 0x0000555555557cc9 main`AD__$s4mainSfSaySfGcfU___pullback_src_0_wrt_0 + 121
    frame #&#8203;4: 0x0000555555557954 main`reabstraction thunk helper from @escaping @callee_guaranteed (@unowned Swift.Float) -> (@owned Swift.Array<Swift.Float>.DifferentiableView) to @escaping @callee_guaranteed (@in_guaranteed Swift.Float) -> (@out Swift.Array<Swift.Float>.DifferentiableView) + 20
    frame #&#8203;5: 0x000055555555a741 main`partial apply forwarder for reabstraction thunk helper from @escaping @callee_guaranteed (@unowned Swift.Float) -> (@owned Swift.Array<Swift.Float>.DifferentiableView) to @escaping @callee_guaranteed (@in_guaranteed Swift.Float) -> (@out Swift.Array<Swift.Float>.DifferentiableView) + 17
    frame #&#8203;6: 0x00007ffff7f82503 libswift_Differentiation.so`_Differentiation.gradient<τ_0_0, τ_0_1 where τ_0_0: _Differentiation.Differentiable, τ_0_1: Swift.FloatingPoint, τ_0_1: _Differentiation.Differentiable, τ_0_1 == τ_0_1._Differentiation.Differentiable.TangentVector>(at: τ_0_0, in: @differentiable (τ_0_0) -> τ_0_1) -> τ_0_0.TangentVector + 131
    frame #&#8203;7: 0x0000555555556fbd main`main + 525
    frame #&#8203;8: 0x00007ffff75f4cca libc.so.6`__libc_start_main(main=(main`main), argc=1, argv=0x00007fffffffa118, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, stack_end=0x00007fffffffa108) at libc-start.c:308:16
    frame #&#8203;9: 0x0000555555556b5a main`_start + 42

[rxwei]

@swift-ci create

[marcrasi]

I bisected this to #34886

[marcrasi]

I managed to eliminate generics from the reproducer. I have updated the description with the simpler reproducer.

[rxwei]

Thanks for bisecting this. I'm happy to fix this if you haven't started.

[marcrasi]

I think that I found a fix! Generated pullback sil functions should always be "convention(thin)" but the code currently makes them use the convention from the original function. e.g. the pullback of Reverser.apply gets "convention(method)". This never caused any actual problems until PR 34886 uncovered it.

The change is really simple and I'm just finishing up some tests now.

[marcrasi]

Fix in #35236