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Swift Optimizer: add a pass to cleanup debug_step instructions
If a `debug_step` has the same debug location as a previous or succeeding instruction it is removed. It's just important that there is at least one instruction for a certain debug location so that single stepping on that location will work.
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‎SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CMakeLists.txt

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swift_compiler_sources(Optimizer
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AssumeSingleThreaded.swift
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CleanupDebugSteps.swift
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ComputeEscapeEffects.swift
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ComputeSideEffects.swift
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ObjCBridgingOptimization.swift

‎SwiftCompilerSources/Sources/Optimizer/FunctionPasses/CleanupDebugSteps.swift

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//===--- SimplificationPasses.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 - 2023 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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/// Removes redundant `debug_step` instructions.
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/// If a `debug_step` has the same debug location as a previous or succeeding instruction
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/// it is removed. It's just important that there is at least one instruction for a
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/// certain debug location so that single stepping on that location will work.
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let cleanupDebugStepsPass = FunctionPass(name: "cleanup-debug-steps") {
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(function: Function, context: FunctionPassContext) in
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for block in function.blocks {
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cleanupDebugSteps(in: block, context)
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}
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}
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private func cleanupDebugSteps(in block: BasicBlock, _ context: FunctionPassContext) {
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var lastInstWithSameLocation: Instruction?
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for inst in block.instructions {
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if !inst.location.isDebugSteppable {
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if inst is DebugStepInst && !inst.location.isDebugSteppable {
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// First case: the instruction which is replaced by the debug_step didn't have a valid
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// location itself. Then we don't need the debug_step either.
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context.erase(instruction: inst)
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}
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continue
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}
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if let li = lastInstWithSameLocation,
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!inst.location.hasSameSourceLocation(as: li.location) {
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lastInstWithSameLocation = nil
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}
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// Only instructions which are really compiled down to some machine instructions can be
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// single stepped on.
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if !inst.producesMachineCode {
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continue
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}
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if let li = lastInstWithSameLocation {
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if inst is DebugStepInst {
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// Second case:
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// %li = some_instruction, loc "l"
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// debug_step, loc "l" // current inst -> erase
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context.erase(instruction: inst)
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continue
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} else if li is DebugStepInst {
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// Third case:
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// debug_step, loc "l" // li -> erase
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// %inst = some_instruction, loc "l" // current inst
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context.erase(instruction: li)
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}
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}
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lastInstWithSameLocation = inst
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}
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}
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private extension Instruction {
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var producesMachineCode: Bool {
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switch self {
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// We could include more instructions here.
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// In worst case a debug_step instruction remains in the code although it's not needed.
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// This is harmless.
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case is DebugStepInst, is ApplySite, is LoadInst, is StoreInst, is TermInst:
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return location.isDebugSteppable
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default:
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return false
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}
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}
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}

‎SwiftCompilerSources/Sources/Optimizer/PassManager/PassRegistration.swift

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@@ -75,6 +75,7 @@ private func registerSwiftPasses() {
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registerPass(simplificationPass, { simplificationPass.run($0) })
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registerPass(ononeSimplificationPass, { ononeSimplificationPass.run($0) })
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registerPass(lateOnoneSimplificationPass, { lateOnoneSimplificationPass.run($0) })
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registerPass(cleanupDebugStepsPass, { cleanupDebugStepsPass.run($0) })
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// Instruction passes
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registerForSILCombine(BeginCOWMutationInst.self, { run(BeginCOWMutationInst.self, $0) })

‎include/swift/SILOptimizer/PassManager/Passes.def

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@@ -395,6 +395,8 @@ SWIFT_FUNCTION_PASS(OnoneSimplification, "onone-simplification",
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"Peephole simplifications which runs at -Onone")
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SWIFT_FUNCTION_PASS(LateOnoneSimplification, "late-onone-simplification",
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"Peephole simplifications which can only run late in the -Onone pipeline")
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SWIFT_FUNCTION_PASS(CleanupDebugSteps, "cleanup-debug-steps",
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"Cleanup debug_step instructions for Onone")
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PASS(SimplifyBBArgs, "simplify-bb-args",
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"SIL Block Argument Simplification")
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PASS(SimplifyCFG, "simplify-cfg",

‎test/SILOptimizer/cleanup_debugstep.sil

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// RUN: %target-sil-opt -enable-sil-verify-all %s -cleanup-debug-steps -sil-print-debuginfo | %FileCheck %s
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// REQUIRES: swift_in_compiler
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import Swift
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import Builtin
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sil @f : $@convention(thin) () -> ()
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// CHECK-LABEL: sil @remove_after_previous_location
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// CHECK-NOT: debug_step
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// CHECK: } // end sil function 'remove_after_previous_location'
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sil @remove_after_previous_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> (), loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil @dont_remove_after_previous_location
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// CHECK: debug_step
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// CHECK: } // end sil function 'dont_remove_after_previous_location'
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sil @dont_remove_after_previous_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> (), loc "a.swift":1:2
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%r = tuple (), loc "a.swift":27:2
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debug_step, loc "a.swift":1:2
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return %r : $()
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}
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// CHECK-LABEL: sil @remove_two_after_previous_location
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// CHECK-NOT: debug_step
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// CHECK: } // end sil function 'remove_two_after_previous_location'
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sil @remove_two_after_previous_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> (), loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil @remove_before_next_location
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// CHECK-NOT: debug_step
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// CHECK: } // end sil function 'remove_before_next_location'
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sil @remove_before_next_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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%r = tuple (), loc "<compiler-generated>":0:0
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return %r : $(), loc "a.swift":1:2
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}
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// CHECK-LABEL: sil @dont_remove_before_next_location
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// CHECK: debug_step
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// CHECK: } // end sil function 'dont_remove_before_next_location'
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sil @dont_remove_before_next_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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%r = tuple (), loc "a.swift":27:2
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return %r : $(), loc "a.swift":1:2
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}
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// CHECK-LABEL: sil @remove_two_before_next_location
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// CHECK-NOT: debug_step
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// CHECK: } // end sil function 'remove_two_before_next_location'
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sil @remove_two_before_next_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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%r = tuple (), loc "<compiler-generated>":0:0
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return %r : $(), loc "a.swift":1:2
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}
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// CHECK-LABEL: sil @remove_null_location
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// CHECK-NOT: debug_step
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// CHECK: } // end sil function 'remove_null_location'
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sil @remove_null_location : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "<compiler-generated>":0:0
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%r = tuple ()
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return %r : $(), loc "a.swift":1:2
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}
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// CHECK-LABEL: sil @dont_remove_one
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// CHECK: debug_step
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// CHECK: } // end sil function 'dont_remove_one'
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sil @dont_remove_one : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil @replace_three_with_one
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// CHECK: apply
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// CHECK-NEXT: debug_step , loc "a.swift":1:2
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// CHECK-NEXT: tuple
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// CHECK: } // end sil function 'replace_three_with_one'
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sil @replace_three_with_one : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":1:2
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil @replace_three_with_two
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// CHECK: apply
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// CHECK-NEXT: debug_step , loc "a.swift":1:2
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// CHECK-NEXT: debug_step , loc "a.swift":2:3
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// CHECK-NEXT: tuple
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// CHECK: } // end sil function 'replace_three_with_two'
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sil @replace_three_with_two : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":2:3
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debug_step, loc "a.swift":2:3
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%r = tuple ()
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return %r : $()
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}
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// CHECK-LABEL: sil @dont_remove_two
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// CHECK: debug_step
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// CHECK-NEXT: debug_step
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// CHECK: } // end sil function 'dont_remove_two'
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sil @dont_remove_two : $@convention(thin) () -> () {
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bb0:
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%f = function_ref @f : $@convention(thin) () -> ()
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%a = apply %f() : $@convention(thin) () -> ()
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debug_step, loc "a.swift":1:2
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debug_step, loc "a.swift":2:3
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%r = tuple ()
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return %r : $()
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}
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