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Swift Optimizer: add Onone simplification of branch instructions
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‎SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/CMakeLists.txt

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swift_compiler_sources(Optimizer
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SimplifyApply.swift
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SimplifyBeginCOWMutation.swift
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SimplifyBranch.swift
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SimplifyGlobalValue.swift
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SimplifyStrongRetainRelease.swift)

‎SwiftCompilerSources/Sources/Optimizer/InstructionSimplification/SimplifyBranch.swift

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//===--- SimplifyBranch.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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extension BranchInst : OnoneSimplifyable {
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func simplify(_ context: SimplifyContext) {
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tryMergeWithTargetBlock(context)
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}
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}
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private extension BranchInst {
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func tryMergeWithTargetBlock(_ context: SimplifyContext) {
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if canMergeWithTargetBlock {
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mergeWithTargetBlock(context)
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}
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}
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var canMergeWithTargetBlock: Bool {
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// We can only merge if there is a 1:1 relation to the target block.
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guard let pred = targetBlock.singlePredecessor else {
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return false
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}
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assert(pred == parentBlock)
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// Ignore self cycles
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if targetBlock == parentBlock {
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return false
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}
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if hasInvalidDominanceCycle {
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return false
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}
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return true
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}
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func mergeWithTargetBlock(_ context: SimplifyContext) {
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let targetBB = targetBlock
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let parentBB = parentBlock
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for (argIdx, op) in operands.enumerated() {
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targetBB.arguments[argIdx].uses.replaceAll(with: op.value, context)
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}
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targetBB.eraseAllArguments(context)
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if context.preserveDebugInfo {
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let builder = Builder(before: self, context)
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builder.createDebugStep()
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}
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context.erase(instruction: self)
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// Move instruction from the smaller block to the larger block.
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// The order is essential because if many blocks are merged and this is done
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// in the wrong order, we end up with quadratic complexity.
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//
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if parentBB.hasLessInstructions(than: targetBB) &&
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parentBB != parentBB.parentFunction.entryBlock {
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for pred in parentBB.predecessors {
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pred.terminator.replaceBranchTarget(from: parentBB, to: targetBB, context)
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}
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parentBB.moveAllInstructions(toBeginOf: targetBB, context)
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parentBB.moveAllArguments(to: targetBB, context)
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} else {
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targetBB.moveAllInstructions(toEndOf: parentBB, context)
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}
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}
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}
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private extension BasicBlock {
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func hasLessInstructions(than otherBlock: BasicBlock) -> Bool {
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var insts = instructions
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var otherInsts = otherBlock.instructions
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while true {
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if otherInsts.next() == nil {
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return false
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}
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if insts.next() == nil {
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return true
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}
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}
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}
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}
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private extension BranchInst {
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// True if this block is part of an unreachable cfg cycle, where an argument dominates itself.
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// For example:
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// ```
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// bb1(arg1): // preds: bb2
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// br bb2
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//
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// bb2: // preds: bb1
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// br bb1(arg1)
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// ```
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var hasInvalidDominanceCycle: Bool {
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for (argIdx, op) in operands.enumerated() {
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if targetBlock.arguments[argIdx] == op.value {
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return true
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}
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}
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return false
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}
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}

‎test/SILOptimizer/simplify_branch.sil

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// RUN: %target-sil-opt -enable-sil-verify-all %s -simplification -simplify-instruction=br | %FileCheck %s
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// RUN: %target-sil-opt -enable-sil-verify-all %s -onone-simplification -simplify-instruction=br -sil-print-debuginfo | %FileCheck %s --check-prefix=CHECK --check-prefix=CHECK-ONONE
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// REQUIRES: swift_in_compiler
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import Swift
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import Builtin
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// CHECK-LABEL: sil @merge_blocks_same_location1
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// CHECK: %0 = integer_literal $Builtin.Int64, 0
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// CHECK: fix_lifetime %0 : $Builtin.Int64
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// CHECK: return %0 : $Builtin.Int64
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// CHECK: } // end sil function 'merge_blocks_same_location1'
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sil @merge_blocks_same_location1 : $@convention(thin) () -> Builtin.Int64 {
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bb0:
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%0 = integer_literal $Builtin.Int64, 0
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br bb1(%0 : $Builtin.Int64), loc "a.swift":1:1
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bb1(%2 : $Builtin.Int64):
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fix_lifetime %2 : $Builtin.Int64, loc "a.swift":1:1
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return %2 : $Builtin.Int64
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}
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// CHECK-LABEL: sil @merge_blocks_same_location2
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// CHECK: %0 = integer_literal $Builtin.Int64, 0
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// CHECK: fix_lifetime %0 : $Builtin.Int64
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// CHECK: return %0 : $Builtin.Int64
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// CHECK: } // end sil function 'merge_blocks_same_location2'
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sil @merge_blocks_same_location2 : $@convention(thin) () -> Builtin.Int64 {
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bb0:
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%0 = integer_literal $Builtin.Int64, 0, loc "a.swift":1:1
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br bb1(%0 : $Builtin.Int64), loc "a.swift":1:1
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bb1(%2 : $Builtin.Int64):
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fix_lifetime %2 : $Builtin.Int64
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return %2 : $Builtin.Int64
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}
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// CHECK-LABEL: sil @merge_blocks_different_locations
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// CHECK: %0 = integer_literal $Builtin.Int64, 0
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// CHECK-ONONE: debug_step , loc "a.swift":2:1
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// CHECK: fix_lifetime %0 : $Builtin.Int64
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// CHECK: return %0 : $Builtin.Int64
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// CHECK: } // end sil function 'merge_blocks_different_locations'
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sil @merge_blocks_different_locations : $@convention(thin) () -> Builtin.Int64 {
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bb0:
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%0 = integer_literal $Builtin.Int64, 0, loc "a.swift":1:1
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br bb1(%0 : $Builtin.Int64), loc "a.swift":2:1
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bb1(%2 : $Builtin.Int64):
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fix_lifetime %2 : $Builtin.Int64, loc "a.swift":3:1
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return %2 : $Builtin.Int64
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}
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// CHECK-LABEL: sil @dont_merge_blocks
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// CHECK: bb1:
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// CHECK-NEXT: br bb3
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// CHECK: bb2:
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// CHECK-NEXT: br bb3
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// CHECK: } // end sil function 'dont_merge_blocks'
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sil @dont_merge_blocks : $@convention(thin) () -> () {
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bb0:
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cond_br undef, bb1, bb2
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bb1:
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br bb3
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bb2:
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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 @dont_merge_block_in_single_cycle
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// CHECK: bb1:
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// CHECK-NEXT: br bb1
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// CHECK: } // end sil function 'dont_merge_block_in_single_cycle'
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sil @dont_merge_block_in_single_cycle : $@convention(thin) () -> () {
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bb0:
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br bb1
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bb1:
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br bb1
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}
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// CHECK-LABEL: sil @dont_crash_with_unreachable_invalid_dominance
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// CHECK-NOT: bb1
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// CHECK: } // end sil function 'dont_crash_with_unreachable_invalid_dominance'
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sil @dont_crash_with_unreachable_invalid_dominance : $@convention(thin) (Builtin.Int64) -> Builtin.Int64 {
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bb0(%0 : $Builtin.Int64):
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return %0 : $Builtin.Int64
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bb1(%2 : $Builtin.Int64):
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br bb2
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bb2:
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br bb1(%2 : $Builtin.Int64)
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}
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