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swift-mirror/test/SILOptimizer/closure_specialization_recurse1.sil
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Daniil Kovalev 619b66e639 [SILOptimizer] Fix thunk handling in closure specialization pass (#91093) 
Closure specialization pass does the specialization against so-called
root closures, meaning that closures which are results of converions or
reabstractions are not treated as root closure, and the closure
converted or reabstracted is (applies transitively through
conversion/reabstraction chains).

Previously, `partial_apply` instructions satisfying the following
criteria were treated as such transparent conversions/reabstractions:

```
self.numArguments == 1,
let fun = self.referencedFunction,
fun.thunkKind == .reabstractionThunk || fun.thunkKind == .thunk,
self.arguments[0].type.isLoweredFunction,
self.arguments[0].type.isReferenceCounted(in: self.parentFunction) || self.callee.type.isThickFunction
```

This check is incorrect because we cannot guarantee that a thunk with
kind `thunk` is indeed just a conversion/reabstraction or something more
complex with different behavior - even if all other criteria are
satisfied. The previous set of criteria was just a heuristic working in
many cases.

As a result, we might end up with miscompile in a following situation.
If we have two call sites with identical callee, specializing against
identical root closure, but with different intermediate `partial_apply`
of a non-reabstraction thunk treated as behavior-preserving conversion
or reabstraction (per criteria above), these would yield identical
mangled specialized callee function name. So, the the closure
specialization pass would emit the specialized callee when processing
the 1st call site, and would reuse that specialized callee when
processing the 2nd call site. That would be wrong and would result in
incorrect runtime behavior since the intermediate `partial_apply`'s were
different and might have different behavior, while after specialization
only the behavior of the 1st call site would be preserved.

See the following tests resulting in such miscompile (fixed by this
patch):
- test/SILOptimizer/closure_specialization_recurse1.sil
- test/SILOptimizer/closure_specialization_recurse2.sil

The proper fix is to drop the `|| fun.thunkKind == .thunk` condition and
only allow reabstraction thunks. Additionally, allow AutoDiff parameters
subset thunks for AutoDiff closure specialization.

This by itself prevents some legal `partial_apply`'s to be identified as
conversions/reabstractions, which harms optimization (though only
AutoDiff-specific tests show that). In order to get resolve this
regression, the patch enhances `checkRecursivelyIfClosureIsApplied`
logic so not only direct applies are detected, but also applies after
conversions or applies of `partial_apply` which consumes the closure and
applies it. This way, the iterative logic of closure specialization
introduced in #88920 safely does the job done by the unsafe `||
fun.thunkKind == .thunk` check previously.

Note that
test/AutoDiff/SILOptimizer/closure_specialization/single_bb.sil was
changed, and now the specialization is "less deep" than before.
Particularly, the following are no longer treated as behavior-preserving
conversions or reabstractions: `[thunk] @pullback_f_specialized` and
`[thunk] @pullback_y_specialized`. Given that the test does not have
these function definitions, specialization stops there, yielding a lower
optimization level than previously. Previous higher optimization level
was a result of unsafe miscompile. With function bodies present, further
optimization via iterative closure specialization would become possible:
#88920.

See also
https://github.com/swiftlang/swift/pull/88920#issuecomment-5069600329
2026-08-05 10:00:13 +00:00

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// RUN: %target-sil-opt -sil-print-types -enable-sil-verify-all -closure-specialization %s | %FileCheck %s
// Fails on wasm32 because of 32/64 mismatch on stdlib function operating on String
// REQUIRES: PTRSIZE=64
// The SIL corresponds to the following Swift source:
//
// @inline(never)
// func rec<T>(_ v: T, depth: Int, f: (T) -> String) -> String {
// if depth <= 0 { return f(v) }
// return rec(v, depth: depth - 1) { x in return f(x) + "-t" }
// }
// @inline(never)
// func caller() -> String {
// return rec("X", depth: 3) { "[\($0)]" }
// }
sil_stage canonical
import Builtin
import Swift
sil @$ss12_SmallStringV15_invariantCheckyyF : $@convention(method) (_SmallString) -> ()
sil @$ss13_StringObjectV15_invariantCheckyyF : $@convention(method) (@guaranteed _StringObject) -> ()
sil @$ss11_StringGutsV15_invariantCheckyyF : $@convention(method) (@guaranteed _StringGuts) -> ()
sil @$sSS15_invariantCheckyyF : $@convention(method) (@guaranteed String) -> ()
sil @$s4main6callerSSyFS2SXEfU_ : $@convention(thin) @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
sil [transparent] [thunk] [ossa] @$s4main3rec_5depth1fSSx_SiSSxXEtlFSSxXEfU_SS_TG5 : $@convention(thin) (@in_guaranteed String, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: sil shared [noinline] [ossa] @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg526$s4main6callerSSyFS2SXEfU_Tf1nnc_n : $@convention(thin) (@guaranteed String, Int) -> @owned String {
// CHECK: bb0(%0 : @guaranteed $String, %1 : $Int):
// CHECK: // function_ref closure #1 in caller()
// CHECK: %[[#A2:]] = function_ref @$s4main6callerSSyFS2SXEfU_ : $@convention(thin) @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: %[[#A3:]] = thin_to_thick_function %[[#A2]] : $@convention(thin) @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String> to $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: bb1:
// CHECK: %[[#A13:]] = apply %[[#A3]](%[[#]]) : $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: br bb3(%[[#A13]] : $String)
// CHECK: bb2:
// CHECK: %[[#A22:]] = struct $Int (%[[#]] : $Builtin.Int64)
// CHECK: %[[#A27:]] = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5026$s4main3rec_5depth1fSSx_SiF19XEtlFSSxXEfU_SS_TG5xSSRi_zRi0_zlySSIsgno_Tf1nnc_n : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: %[[#A28:]] = apply %[[#A27]](%0, %[[#A22]], %[[#A3]]) : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: br bb3(%[[#A28]] : $String)
// CHECK: bb3(%[[#A32:]] : @owned $String):
// CHECK: return %[[#A32]] : $String
// } // end sil function '$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg526$s4main6callerSSyFS2SXEfU_Tf1nnc_n'
// CHECK: sil shared [noinline] [ossa] @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5026$s4main3rec_5depth1fSSx_SiF19XEtlFSSxXEfU_SS_TG5xSSRi_zRi0_zlySSIsgno_Tf1nnc_n : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String {
// CHECK: bb0(%0 : @guaranteed $String, %1 : $Int, %2 : @guaranteed $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>):
// CHECK: // function_ref specialized closure #1 in rec<A>(_:depth:f:)
// CHECK: %[[#B3:]] = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSSxXEfU_SS_TG5 : $@convention(thin) (@in_guaranteed String, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: %[[#B4:]] = partial_apply [callee_guaranteed] [on_stack] %[[#B3]](%2) : $@convention(thin) (@in_guaranteed String, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: %[[#B5:]] = convert_function %[[#B4]] : $@noescape @callee_guaranteed (@in_guaranteed String) -> @owned String to $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: %[[#B6:]] = begin_borrow %[[#B5]] : $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: bb1: // Preds: bb0
// CHECK: %[[#B16:]] = apply %6(%[[#]]) : $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>
// CHECK: br bb3(%[[#B16]] : $String)
// CHECK: bb2: // Preds: bb0
// CHECK: %[[#B25:]] = struct $Int (%[[#]] : $Builtin.Int64)
// CHECK: // function_ref specialized rec<A>(_:depth:f:)
// CHECK: %[[#B30:]] = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5026$s4main3rec_5depth1fSSx_SiF19XEtlFSSxXEfU_SS_TG5xSSRi_zRi0_zlySSIsgno_Tf1nnc_n : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: %[[#B31:]] = apply %[[#B30]](%0, %[[#B25]], %[[#B6]]) : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String
// CHECK: br bb3(%[[#B31]] : $String)
// CHECK: bb3(%[[#B35:]] : @owned $String): // Preds: bb2 bb1
// CHECK: return %[[#B35]] : $String
// } // end sil function '$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5026$s4main3rec_5depth1fSSx_SiF19XEtlFSSxXEfU_SS_TG5xSSRi_zRi0_zlySSIsgno_Tf1nnc_n'
// specialized rec<A>(_:depth:f:)
sil shared [noinline] [ossa] @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5 : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String {
bb0(%0 : @guaranteed $String, %1 : $Int, %2 : @guaranteed $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>):
%3 = alloc_stack $String // users: %31, %4
%4 = store_borrow %0 to %3 // users: %27, %13, %12, %5
%8 = integer_literal $Builtin.Int64, 0 // user: %10
%9 = struct_extract %1, #Int._value // users: %17, %10
%10 = builtin "cmp_sge_Int64"(%8, %9) : $Builtin.Int1 // user: %11
cond_br %10, bb1, bb2 // id: %11
bb1: // Preds: bb0
%12 = apply %2(%4) : $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String> // user: %14
end_borrow %4 // id: %13
br bb3(%12) // id: %14
bb2: // Preds: bb0
%15 = integer_literal $Builtin.Int64, 1 // user: %17
%16 = integer_literal $Builtin.Int1, -1 // user: %17
%17 = builtin "ssub_with_overflow_Int64"(%9, %15, %16) : $(Builtin.Int64, Builtin.Int1) // users: %19, %18
%18 = tuple_extract %17, 0 // user: %21
%19 = tuple_extract %17, 1 // user: %20
cond_fail %19, "arithmetic overflow" // id: %20
%21 = struct $Int (%18) // user: %26
// function_ref specialized closure #1 in rec<A>(_:depth:f:)
%22 = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSSxXEfU_SS_TG5 : $@convention(thin) (@in_guaranteed String, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %23
%23 = partial_apply [callee_guaranteed] [on_stack] %22(%2) : $@convention(thin) (@in_guaranteed String, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %24
%24 = convert_function %23 to $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String> // users: %26, %28
// function_ref specialized rec<A>(_:depth:f:)
%25 = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5 : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %26
%26 = apply %25(%0, %21, %24) : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %29
end_borrow %4 // id: %27
destroy_value %24 // id: %28
br bb3(%26) // id: %29
// %30 // user: %32
bb3(%30 : @owned $String): // Preds: bb2 bb1
dealloc_stack %3 // id: %31
return %30 // id: %32
} // end sil function '$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5'
// caller()
sil hidden [noinline] [ossa] @$s4main6callerSSyF : $@convention(thin) () -> @owned String {
// CHECK: sil hidden [noinline] [ossa] @$s4main6callerSSyF : $@convention(thin) () -> @owned String {
bb0:
%0 = integer_literal $Builtin.Int8, 2 // users: %42, %50, %20, %6
%1 = integer_literal $Builtin.Int32, 0 // users: %38, %46
%2 = integer_literal $Builtin.Word, 24 // user: %20
%3 = string_literal utf8 "" // user: %5
%4 = integer_literal $Builtin.Word, 0 // user: %6
%5 = builtin "ptrtoint_Word"(%3) : $Builtin.Word // user: %6
%6 = struct $StaticString (%5, %4, %0) // users: %44, %52
%7 = integer_literal $Builtin.Int64, 0 // user: %35
%8 = integer_literal $Builtin.Int1, -1 // users: %36, %26
%9 = integer_literal $Builtin.Int64, 88 // user: %10
%10 = struct $UInt64 (%9) // users: %28, %13
%11 = integer_literal $Builtin.Int64, -2233785415175766016 // users: %12, %25, %17
%12 = struct $UInt64 (%11) // user: %13
%13 = tuple (%10, %12) // user: %14
%14 = struct $_SmallString (%13) // user: %16
// function_ref _SmallString._invariantCheck()
%15 = function_ref @$ss12_SmallStringV15_invariantCheckyyF : $@convention(method) (_SmallString) -> () // user: %16
%16 = apply %15(%14) : $@convention(method) (_SmallString) -> ()
%17 = value_to_bridge_object %11 // users: %28, %23; ownership: none
%18 = string_literal utf8 "Swift/StringObject.swift" // user: %19
%19 = builtin "ptrtoint_Word"(%18) : $Builtin.Word // user: %20
%20 = struct $StaticString (%19, %2, %0) // users: %44, %52
%21 = integer_literal $Builtin.Int64, 251 // user: %22
%22 = struct $UInt (%21) // user: %44
%23 = unchecked_trivial_bit_cast %17 to $UInt64 // user: %24
%24 = struct_extract %23, #UInt64._value // users: %34, %25
%25 = builtin "cmp_eq_Int64"(%24, %11) : $Builtin.Int1 // user: %26
%26 = builtin "int_expect_Int1"(%25, %8) : $Builtin.Int1 // user: %27
cond_br %26, bb1, bb2 // id: %27
bb1: // Preds: bb0
%28 = struct $_StringObject (%10, %17) // users: %54, %30; ownership: none
// function_ref _StringObject._invariantCheck()
%29 = function_ref @$ss13_StringObjectV15_invariantCheckyyF : $@convention(method) (@guaranteed _StringObject) -> () // user: %30
%30 = apply %29(%28) : $@convention(method) (@guaranteed _StringObject) -> ()
%31 = integer_literal $Builtin.Int64, 635 // user: %32
%32 = struct $UInt (%31) // user: %52
%33 = integer_literal $Builtin.Int64, 2305843009213693952 // user: %34
%34 = builtin "and_Int64"(%24, %33) : $Builtin.Int64 // user: %35
%35 = builtin "cmp_ne_Int64"(%34, %7) : $Builtin.Int1 // user: %36
%36 = builtin "int_expect_Int1"(%35, %8) : $Builtin.Int1 // user: %37
cond_br %36, bb4, bb3 // id: %37
bb2: // Preds: bb0
unreachable // id: %45
bb3: // Preds: bb1
unreachable // id: %53
bb4: // Preds: bb1
// CHECK: bb4: // Preds: bb1
// CHECK: %[[#C43:]] = struct $String (%[[#]] : $_StringGuts), forwarding: @owned
// CHECK: %[[#C48:]] = struct $Int (%[[#]] : $Builtin.Int64)
// CHECK: // function_ref specialized rec<A>(_:depth:f:)
// CHECK: %[[#C51:]] = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg526$s4main6callerSSyFS2SXEfU_Tf1nnc_n : $@convention(thin) (@guaranteed String, Int) -> @owned String
// CHECK: %[[#C52:]] = apply %[[#C51]](%[[#C43]], %[[#C48]]) : $@convention(thin) (@guaranteed String, Int) -> @owned String
// CHECK: return %[[#C52]] : $String
%54 = struct $_StringGuts (%28) // users: %58, %57, %56; ownership: none
// function_ref _StringGuts._invariantCheck()
%55 = function_ref @$ss11_StringGutsV15_invariantCheckyyF : $@convention(method) (@guaranteed _StringGuts) -> () // user: %56
%56 = apply %55(%54) : $@convention(method) (@guaranteed _StringGuts) -> ()
%57 = struct $String (%54), forwarding: @owned // users: %67, %66
%58 = struct $String (%54) // user: %60; ownership: none
// function_ref String._invariantCheck()
%59 = function_ref @$sSS15_invariantCheckyyF : $@convention(method) (@guaranteed String) -> () // user: %60
%60 = apply %59(%58) : $@convention(method) (@guaranteed String) -> ()
%61 = integer_literal $Builtin.Int64, 3 // user: %62
%62 = struct $Int (%61) // user: %66
// function_ref closure #1 in caller()
%63 = function_ref @$s4main6callerSSyFS2SXEfU_ : $@convention(thin) @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String> // user: %64
%64 = thin_to_thick_function %63 to $@noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String> // user: %66; ownership: none
// function_ref specialized rec<A>(_:depth:f:)
%65 = function_ref @$s4main3rec_5depth1fSSx_SiSSxXEtlFSS_Tg5 : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %66
%66 = apply %65(%57, %62, %64) : $@convention(thin) (@guaranteed String, Int, @guaranteed @noescape @callee_guaranteed @substituted <τ_0_0> (@in_guaranteed τ_0_0) -> @owned String for <String>) -> @owned String // user: %68
destroy_value %57 // id: %67
return %66 // id: %68
} // end sil function '$s4main6callerSSyF'
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