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Emil Pedersen
7bf4798e4e
A dead debug_value instruction is one where the variable can't be viewed from any possible breakpoint location. This means the scope associated with the variable is not visible from any real instruction. Visible scopes have associated instructions, are parents of associated instructions, or are in parent inlined frames. This pass is interesting after rounds of DCE and simplifications that happen after inlining, that can make whole scopes disappear.
59 lines
2.1 KiB
Swift
59 lines
2.1 KiB
Swift
//===--- DeadDebugVariableElimination.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 - 2026 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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/// Eliminates `debug_value` instructions whose scope is dead.
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///
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/// After optimization, all non-debug instructions from a scope may be eliminated,
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/// but `debug_value` instructions remain. Since there's no breakpoint location left in that
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/// scope, these debug values can never be observed by the debugger. Removing them saves memory.
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let deadDebugVariableEliminationPass = FunctionPass(name: "dead-debug-variable-elimination") {
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(function: Function, context: FunctionPassContext) in
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// Phase 1: Build the set of live scopes.
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// A scope is live if it is visible from setting a breakpoint on a real instruction.
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var liveScopes = Set<DebugScope>()
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for inst in function.instructions {
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if inst is MetaInstruction {
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// Breakpoints cannot be set on meta instructions.
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continue
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}
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guard let scope = inst.location.scope else {
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continue
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}
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// Each parent scope and parent frame is live.
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var currentFrame: DebugScope? = scope
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while let frame = currentFrame {
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var currentScope: DebugScope? = frame
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while let scope = currentScope {
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guard liveScopes.insert(scope).inserted else {
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break // Parents were already inserted.
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}
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currentScope = scope.parentScope
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}
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currentFrame = frame.inlinedCallSite
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}
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}
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// Phase 2: Remove debug_values whose variable's scope is not live.
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for case let inst as DebugValueInst in function.instructions {
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if let scope = inst.debugVariable?.scope, liveScopes.contains(scope) {
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// Keep live scopes.
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continue
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}
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context.erase(instruction: inst, salvageDebugInfo: false)
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}
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}
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