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ARC operations don't have an effect on immortal objects, like the empty array singleton or statically allocated arrays. Therefore we can freely remove and retain/release instructions on such objects, even if there is no paired balanced ARC operation. This optimization can only be done with a minimum deployment target of Swift 5.1, because in that version we added immortal ref count bits. The optimization is implemented in libswift. Additionally, the remaining logic of simplifying strong_retain and strong_release is also ported to libswift. rdar://81482156
59 lines
1.9 KiB
Swift
59 lines
1.9 KiB
Swift
//===--- SimplifyGlobalValue.swift - Simplify global_value instruction ----===//
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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 - 2021 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 all reference counting instructions of a `global_value` instruction
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// if it does not escape.
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//
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// Note that `simplifyStrongRetainPass` and `simplifyStrongReleasePass` can
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// even remove "unbalanced" retains/releases of a `global_value`, but this
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// requires a minimum deployment target.
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let simplifyGlobalValuePass = InstructionPass<GlobalValueInst>(
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name: "simplify-global_value", {
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(globalValue: GlobalValueInst, context: PassContext) in
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var users = StackList<Instruction>(context)
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if checkUsers(of: globalValue, users: &users) {
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while let inst = users.pop() {
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context.erase(instruction: inst)
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}
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} else {
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users.removeAll()
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}
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})
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/// Returns true if reference counting and debug_value users of a global_value
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/// can be deleted.
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private func checkUsers(of val: Value, users: inout StackList<Instruction>) -> Bool {
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for use in val.uses {
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let user = use.instruction
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if user is RefCountingInst || user is DebugValueInst {
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users.push(user)
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continue
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}
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if let upCast = user as? UpcastInst {
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if !checkUsers(of: upCast, users: &users) {
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return false
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}
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continue
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}
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// Projection instructions don't access the object header, so they don't
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// prevent deleting reference counting instructions.
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if user is RefElementAddrInst || user is RefTailAddrInst {
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continue
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}
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return false
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}
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return true
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}
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