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Optimizer: eliminate struct_extracts of an owned struct where the struct_extracts are inside a borrow scope
This is done by splitting the `begin_borrow` of the whole struct into individual borrows of the fields (for trivial fields no borrow is needed). And then sinking the `struct` to it's consuming use(s). ``` %3 = struct $S(%nonTrivialField, %trivialField) // owned ... %4 = begin_borrow %3 %5 = struct_extract %4, #S.nonTrivialField %6 = struct_extract %4, #S.trivialField use %5, %6 end_borrow %4 ... end_of_lifetime %3 ``` -> ``` ... %5 = begin_borrow %nonTrivialField use %5, %trivialField end_borrow %5 ... %3 = struct $S(%nonTrivialField, %trivialField) end_of_lifetime %3 ``` This optimization is important for Array code where the Array buffer is constantly wrapped into structs and then extracted again to access the buffer.
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‎SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/CMakeLists.txt‎

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@@ -38,6 +38,7 @@ swift_compiler_sources(Optimizer
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SimplifyRefCasts.swift
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SimplifyRetainReleaseValue.swift
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SimplifyStrongRetainRelease.swift
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SimplifyStruct.swift
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SimplifyStructExtract.swift
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SimplifySwitchEnum.swift
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SimplifyTuple.swift

‎SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyStruct.swift‎

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//===--- SimplifyStruct.swift ---------------------------------------------===//
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//
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// This source file is part of the Swift.org open source project
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//
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// Copyright (c) 2014 - 2025 Apple Inc. and the Swift project authors
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// Licensed under Apache License v2.0 with Runtime Library Exception
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//
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// See https://swift.org/LICENSE.txt for license information
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// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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//
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//===----------------------------------------------------------------------===//
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import SIL
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extension StructInst : Simplifiable, SILCombineSimplifiable {
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/// Eliminates `struct_extract`s of an owned `struct` where the `struct_extract`s are inside a
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/// a borrow scope.
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/// This is done by splitting the `begin_borrow` of the whole struct into individual borrows of the fields
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/// (for trivial fields no borrow is needed). And then sinking the `struct` to it's consuming use(s).
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///
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/// ```
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/// %3 = struct $S(%nonTrivialField, %trivialField) // owned
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/// ...
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/// %4 = begin_borrow %3
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/// %5 = struct_extract %4, #S.nonTrivialField
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/// %6 = struct_extract %4, #S.trivialField
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/// use %5, %6
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/// end_borrow %4
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/// ...
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/// end_of_lifetime %3
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/// ```
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/// ->
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/// ```
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/// ...
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/// %5 = begin_borrow %nonTrivialField
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/// use %5, %trivialField
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/// end_borrow %5
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/// ...
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/// %3 = struct $S(%nonTrivialField, %trivialField)
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/// end_of_lifetime %3
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/// ```
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func simplify(_ context: SimplifyContext) {
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guard ownership == .owned,
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hasOnlyStructExtractUsesInBorrowScopes()
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else {
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return
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}
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for beginBorrow in uses.users(ofType: BeginBorrowInst.self) {
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splitAndRemoveStructExtracts(beginBorrow: beginBorrow, context)
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}
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self.sinkToEndOfLifetime(context)
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context.erase(instructionIncludingAllUsers: self)
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}
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private func hasOnlyStructExtractUsesInBorrowScopes() -> Bool {
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var hasStructExtract = false
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for use in uses.ignoreDebugUses {
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switch use.instruction {
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case let beginBorrow as BeginBorrowInst:
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for borrowUse in beginBorrow.uses.ignoreDebugUses {
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switch borrowUse.instruction {
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case is EndBorrowInst:
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break
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case is StructExtractInst:
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hasStructExtract = true
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default:
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return false
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}
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}
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default:
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guard use.endsLifetime else {
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return false
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}
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}
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}
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return hasStructExtract
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}
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private func splitAndRemoveStructExtracts(beginBorrow: BeginBorrowInst, _ context: SimplifyContext) {
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for structExtract in beginBorrow.uses.users(ofType: StructExtractInst.self) {
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let field = self.operands[structExtract.fieldIndex].value
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switch structExtract.ownership {
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case .none:
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structExtract.replace(with: field, context)
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case .guaranteed:
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let beginBuilder = Builder(before: beginBorrow, context)
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let borrowedField = beginBuilder.createBeginBorrow(of: field,
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isLexical: beginBorrow.isLexical,
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hasPointerEscape: beginBorrow.hasPointerEscape)
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structExtract.replace(with: borrowedField, context)
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for endBorrow in beginBorrow.endInstructions {
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let endBuilder = Builder(before: endBorrow, context)
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endBuilder.createEndBorrow(of: borrowedField)
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}
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case .owned, .unowned:
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fatalError("wrong ownership of struct_extract")
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}
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}
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}
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private func sinkToEndOfLifetime(_ context: SimplifyContext) {
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for use in uses where use.endsLifetime {
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let builder = Builder(before: use.instruction, context)
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let delayedStruct = builder.createStruct(type: type, elements: Array(operands.values))
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use.set(to: delayedStruct, context)
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}
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}
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}

‎SwiftCompilerSources/Sources/Optimizer/PassManager/PassRegistration.swift‎

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@@ -115,6 +115,7 @@ private func registerSwiftPasses() {
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registerForSILCombine(BuiltinInst.self, { run(BuiltinInst.self, $0) })
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registerForSILCombine(FixLifetimeInst.self, { run(FixLifetimeInst.self, $0) })
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registerForSILCombine(GlobalValueInst.self, { run(GlobalValueInst.self, $0) })
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registerForSILCombine(StructInst.self, { run(StructInst.self, $0) })
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registerForSILCombine(StrongRetainInst.self, { run(StrongRetainInst.self, $0) })
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registerForSILCombine(StrongReleaseInst.self, { run(StrongReleaseInst.self, $0) })
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registerForSILCombine(RetainValueInst.self, { run(RetainValueInst.self, $0) })

‎lib/SILOptimizer/SILCombiner/Simplifications.def‎

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@@ -36,6 +36,7 @@ INSTRUCTION_SIMPLIFICATION(ExplicitCopyAddrInst)
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INSTRUCTION_SIMPLIFICATION(ExplicitCopyValueInst)
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INSTRUCTION_SIMPLIFICATION(FixLifetimeInst)
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INSTRUCTION_SIMPLIFICATION(GlobalValueInst)
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INSTRUCTION_SIMPLIFICATION(StructInst)
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INSTRUCTION_SIMPLIFICATION(StrongRetainInst)
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INSTRUCTION_SIMPLIFICATION(StrongReleaseInst)
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INSTRUCTION_SIMPLIFICATION(RetainValueInst)

‎test/SILOptimizer/simplify_struct.sil‎

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// RUN: %target-sil-opt %s -simplification -simplify-instruction=struct | %FileCheck %s
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import Swift
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import Builtin
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struct S {
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let a: String
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let b: Int
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}
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struct Outer {
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let s: S
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}
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// CHECK-LABEL: sil [ossa] @simple_delay_struct :
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// CHECK: bb0
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// CHECK-NEXT: [[B:%.*]] = begin_borrow %0
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// CHECK-NEXT: fix_lifetime [[B]]
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// CHECK-NEXT: fix_lifetime %1
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// CHECK-NEXT: end_borrow [[B]]
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// CHECK-NEXT: [[S:%.*]] = struct $S (%0, %1)
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// CHECK-NEXT: destroy_value [[S]]
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// CHECK: } // end sil function 'simple_delay_struct'
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sil [ossa] @simple_delay_struct : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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debug_value %2, name "x"
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%3 = begin_borrow %2
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debug_value %3, name "x"
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%4 = struct_extract %3, #S.a
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%5 = struct_extract %3, #S.b
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fix_lifetime %4
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fix_lifetime %5
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end_borrow %3
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destroy_value %2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @test_nested :
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// CHECK: bb0
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// CHECK-NEXT: [[B:%.*]] = begin_borrow %0
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// CHECK-NEXT: fix_lifetime [[B]]
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// CHECK-NEXT: fix_lifetime %1
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// CHECK-NEXT: end_borrow [[B]]
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// CHECK-NEXT: [[S:%.*]] = struct $S (%0, %1)
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// CHECK-NEXT: [[OUTER:%.*]] = struct $Outer ([[S]])
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// CHECK-NEXT: destroy_value [[OUTER]]
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// CHECK: } // end sil function 'test_nested'
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sil [ossa] @test_nested : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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%3 = struct $Outer (%2)
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%4 = begin_borrow %3
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%5 = struct_extract %4, #Outer.s
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%6 = struct_extract %5, #S.a
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%7 = struct_extract %5, #S.b
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fix_lifetime %6
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fix_lifetime %7
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end_borrow %4
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destroy_value %3
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @multi_lifetime_ends :
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// CHECK: bb0
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// CHECK-NEXT: [[B:%.*]] = begin_borrow %0
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// CHECK-NEXT: fix_lifetime [[B]]
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// CHECK-NEXT: fix_lifetime %1
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// CHECK-NEXT: end_borrow [[B]]
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// CHECK: bb1:
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// CHECK-NEXT: [[S1:%.*]] = struct $S (%0, %1)
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// CHECK-NEXT: destroy_value [[S1]]
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// CHECK: bb2:
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// CHECK-NEXT: [[S2:%.*]] = struct $S (%0, %1)
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// CHECK-NEXT: destroy_value [[S2]]
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// CHECK: } // end sil function 'multi_lifetime_ends'
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sil [ossa] @multi_lifetime_ends : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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%3 = begin_borrow %2
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%4 = struct_extract %3, #S.a
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%5 = struct_extract %3, #S.b
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fix_lifetime %4
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fix_lifetime %5
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end_borrow %3
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cond_br undef, bb1, bb2
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bb1:
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destroy_value %2
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br bb3
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bb2:
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destroy_value %2
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br bb3
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bb3:
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @multi_borrows :
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// CHECK: bb1:
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// CHECK-NEXT: [[B:%.*]] = begin_borrow %0
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// CHECK-NEXT: fix_lifetime [[B]]
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// CHECK-NEXT: end_borrow [[B]]
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// CHECK: bb2:
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// CHECK-NEXT: fix_lifetime %1
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// CHECK: bb3:
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// CHECK-NEXT: [[S:%.*]] = struct $S (%0, %1)
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// CHECK-NEXT: destroy_value [[S]]
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// CHECK: } // end sil function 'multi_borrows'
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sil [ossa] @multi_borrows : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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cond_br undef, bb1, bb2
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bb1:
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%3 = begin_borrow %2
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%4 = struct_extract %3, #S.a
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fix_lifetime %4
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end_borrow %3
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br bb3
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bb2:
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%7 = begin_borrow %2
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%8 = struct_extract %7, #S.b
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fix_lifetime %8
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end_borrow %7
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br bb3
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bb3:
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destroy_value %2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @unknown_struct_use :
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// CHECK: struct_extract
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// CHECK: struct_extract
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// CHECK: } // end sil function 'unknown_struct_use'
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sil [ossa] @unknown_struct_use : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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fix_lifetime %2
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%3 = begin_borrow %2
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%4 = struct_extract %3, #S.a
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%5 = struct_extract %3, #S.b
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fix_lifetime %4
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fix_lifetime %5
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end_borrow %3
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destroy_value %2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @unknown_borrow_use :
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// CHECK: struct_extract
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// CHECK: struct_extract
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// CHECK: } // end sil function 'unknown_borrow_use'
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sil [ossa] @unknown_borrow_use : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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%3 = begin_borrow %2
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fix_lifetime %3
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%4 = struct_extract %3, #S.a
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%5 = struct_extract %3, #S.b
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fix_lifetime %4
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fix_lifetime %5
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end_borrow %3
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destroy_value %2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil [ossa] @no_borrows :
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// CHECK: %2 = struct
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// CHECK: destroy_value %2
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// CHECK: } // end sil function 'no_borrows'
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sil [ossa] @no_borrows : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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destroy_value %2
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%r = tuple ()
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return %r : $()
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}
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// TODO: remove this test once we require complete OSSA lifetimes
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// CHECK-LABEL: sil [ossa] @incomplete_lifetimes :
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// CHECK: bb0
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// CHECK-NEXT: [[B:%.*]] = begin_borrow %0
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// CHECK-NEXT: fix_lifetime [[B]]
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// CHECK-NEXT: fix_lifetime %1
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// CHECK-NEXT: unreachable
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// CHECK: } // end sil function 'incomplete_lifetimes'
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sil [ossa] @incomplete_lifetimes : $@convention(thin) (@owned String, Int) -> () {
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bb0(%0 : @owned $String, %1 : $Int):
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%2 = struct $S (%0, %1)
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%3 = begin_borrow %2
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%4 = struct_extract %3, #S.a
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%5 = struct_extract %3, #S.b
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fix_lifetime %4
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fix_lifetime %5
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unreachable
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}
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