106aecf262
Removes a branch that checks for a condition that has been checked on dominators of the branch. This introduces a new reducer that propagates the list of checked conditions (and their boolean values) through the control flow graph. If it encounters a branch checking a condition with a known value, the branch is eliminated. The analysis relies on loops being reducible: if a condition has been checked on all paths to loop entry, then it is checked in the loop (regardless what of the conditions checked inside the loop). The implementation is fairly naive and could be improved: - all the operation on the condition lists could be made allocation-free when revisited. - we could try to use a map structure rather than a linked list (to make lookups faster). - the merging of control flow could be changed to take into account conditions from non-dominating paths (as long as all paths check the condition). Review URL: https://codereview.chromium.org/1376293005 Cr-Commit-Position: refs/heads/master@{#31347}
205 lines
8.0 KiB
C++
205 lines
8.0 KiB
C++
// Copyright 2015 the V8 project authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "src/compiler/branch-elimination.h"
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#include "src/compiler/js-graph.h"
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#include "src/compiler/linkage.h"
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#include "src/compiler/node-properties.h"
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#include "test/unittests/compiler/compiler-test-utils.h"
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#include "test/unittests/compiler/graph-unittest.h"
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#include "test/unittests/compiler/node-test-utils.h"
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#include "testing/gmock-support.h"
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namespace v8 {
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namespace internal {
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namespace compiler {
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class BranchEliminationTest : public TypedGraphTest {
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public:
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BranchEliminationTest()
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: machine_(zone(), kMachPtr, MachineOperatorBuilder::kNoFlags) {}
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MachineOperatorBuilder* machine() { return &machine_; }
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void Reduce() {
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JSOperatorBuilder javascript(zone());
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JSGraph jsgraph(isolate(), graph(), common(), &javascript, nullptr,
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machine());
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GraphReducer graph_reducer(zone(), graph(), jsgraph.Dead());
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BranchElimination branch_condition_elimination(&graph_reducer, &jsgraph,
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zone());
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graph_reducer.AddReducer(&branch_condition_elimination);
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graph_reducer.ReduceGraph();
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}
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private:
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MachineOperatorBuilder machine_;
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};
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TEST_F(BranchEliminationTest, NestedBranchSameTrue) {
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// { return (x ? (x ? 1 : 2) : 3; }
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// should be reduced to
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// { return (x ? 1 : 3; }
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Node* condition = Parameter(0);
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Node* outer_branch =
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graph()->NewNode(common()->Branch(), condition, graph()->start());
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Node* outer_if_true = graph()->NewNode(common()->IfTrue(), outer_branch);
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Node* inner_branch =
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graph()->NewNode(common()->Branch(), condition, outer_if_true);
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Node* inner_if_true = graph()->NewNode(common()->IfTrue(), inner_branch);
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Node* inner_if_false = graph()->NewNode(common()->IfFalse(), inner_branch);
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Node* inner_merge =
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graph()->NewNode(common()->Merge(2), inner_if_true, inner_if_false);
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Node* inner_phi =
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graph()->NewNode(common()->Phi(kMachInt32, 2), Int32Constant(1),
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Int32Constant(2), inner_merge);
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Node* outer_if_false = graph()->NewNode(common()->IfFalse(), outer_branch);
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Node* outer_merge =
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graph()->NewNode(common()->Merge(2), inner_merge, outer_if_false);
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Node* outer_phi = graph()->NewNode(common()->Phi(kMachInt32, 2), inner_phi,
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Int32Constant(3), outer_merge);
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Node* ret = graph()->NewNode(common()->Return(), outer_phi, graph()->start(),
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outer_merge);
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graph()->SetEnd(graph()->NewNode(common()->End(1), ret));
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Reduce();
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// Outer branch should not be rewritten, the inner branch should be discarded.
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EXPECT_THAT(outer_branch, IsBranch(condition, graph()->start()));
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EXPECT_THAT(inner_phi,
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IsPhi(kMachInt32, IsInt32Constant(1), IsInt32Constant(2),
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IsMerge(outer_if_true, IsDead())));
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}
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TEST_F(BranchEliminationTest, NestedBranchSameFalse) {
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// { return (x ? 1 : (x ? 2 : 3); }
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// should be reduced to
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// { return (x ? 1 : 3; }
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Node* condition = Parameter(0);
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Node* outer_branch =
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graph()->NewNode(common()->Branch(), condition, graph()->start());
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Node* outer_if_true = graph()->NewNode(common()->IfTrue(), outer_branch);
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Node* outer_if_false = graph()->NewNode(common()->IfFalse(), outer_branch);
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Node* inner_branch =
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graph()->NewNode(common()->Branch(), condition, outer_if_false);
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Node* inner_if_true = graph()->NewNode(common()->IfTrue(), inner_branch);
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Node* inner_if_false = graph()->NewNode(common()->IfFalse(), inner_branch);
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Node* inner_merge =
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graph()->NewNode(common()->Merge(2), inner_if_true, inner_if_false);
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Node* inner_phi =
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graph()->NewNode(common()->Phi(kMachInt32, 2), Int32Constant(2),
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Int32Constant(3), inner_merge);
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Node* outer_merge =
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graph()->NewNode(common()->Merge(2), outer_if_true, inner_merge);
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Node* outer_phi = graph()->NewNode(common()->Phi(kMachInt32, 2),
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Int32Constant(1), inner_phi, outer_merge);
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Node* ret = graph()->NewNode(common()->Return(), outer_phi, graph()->start(),
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outer_merge);
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graph()->SetEnd(graph()->NewNode(common()->End(1), ret));
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Reduce();
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// Outer branch should not be rewritten, the inner branch should be discarded.
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EXPECT_THAT(outer_branch, IsBranch(condition, graph()->start()));
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EXPECT_THAT(inner_phi,
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IsPhi(kMachInt32, IsInt32Constant(2), IsInt32Constant(3),
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IsMerge(IsDead(), outer_if_false)));
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}
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TEST_F(BranchEliminationTest, BranchAfterDiamond) {
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// { var y = x ? 1 : 2; return y + x ? 3 : 4; }
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// should not be reduced.
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Node* condition = Parameter(0);
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Node* branch1 =
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graph()->NewNode(common()->Branch(), condition, graph()->start());
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Node* if_true1 = graph()->NewNode(common()->IfTrue(), branch1);
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Node* if_false1 = graph()->NewNode(common()->IfFalse(), branch1);
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Node* merge1 = graph()->NewNode(common()->Merge(2), if_true1, if_false1);
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Node* phi1 = graph()->NewNode(common()->Phi(kMachInt32, 2), Int32Constant(1),
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Int32Constant(2), merge1);
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Node* branch2 = graph()->NewNode(common()->Branch(), condition, merge1);
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Node* if_true2 = graph()->NewNode(common()->IfTrue(), branch2);
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Node* if_false2 = graph()->NewNode(common()->IfFalse(), branch2);
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Node* merge2 = graph()->NewNode(common()->Merge(2), if_true2, if_false2);
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Node* phi2 = graph()->NewNode(common()->Phi(kMachInt32, 2), Int32Constant(3),
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Int32Constant(4), merge1);
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Node* add = graph()->NewNode(machine()->Int32Add(), phi1, phi2);
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Node* ret =
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graph()->NewNode(common()->Return(), add, graph()->start(), merge2);
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graph()->SetEnd(graph()->NewNode(common()->End(1), ret));
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Reduce();
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// Outer branch should not be rewritten, the inner branch condition should
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// be true.
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EXPECT_THAT(branch1, IsBranch(condition, graph()->start()));
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EXPECT_THAT(branch2, IsBranch(condition, merge1));
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}
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TEST_F(BranchEliminationTest, BranchInsideLoopSame) {
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// if (x) while (x) { return 2; } else { return 1; }
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// should be rewritten to
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// if (x) while (true) { return 2; } else { return 1; }
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Node* condition = Parameter(0);
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Node* outer_branch =
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graph()->NewNode(common()->Branch(), condition, graph()->start());
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Node* outer_if_true = graph()->NewNode(common()->IfTrue(), outer_branch);
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Node* loop = graph()->NewNode(common()->Loop(1), outer_if_true);
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Node* effect =
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graph()->NewNode(common()->EffectPhi(1), graph()->start(), loop);
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Node* inner_branch = graph()->NewNode(common()->Branch(), condition, loop);
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Node* inner_if_true = graph()->NewNode(common()->IfTrue(), inner_branch);
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Node* ret1 = graph()->NewNode(common()->Return(), Int32Constant(2), effect,
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inner_if_true);
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Node* inner_if_false = graph()->NewNode(common()->IfFalse(), inner_branch);
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loop->AppendInput(zone(), inner_if_false);
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NodeProperties::ChangeOp(loop, common()->Loop(2));
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effect->InsertInput(zone(), 1, effect);
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NodeProperties::ChangeOp(effect, common()->EffectPhi(2));
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Node* outer_if_false = graph()->NewNode(common()->IfFalse(), outer_branch);
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Node* outer_merge =
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graph()->NewNode(common()->Merge(2), loop, outer_if_false);
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Node* outer_ephi = graph()->NewNode(common()->EffectPhi(2), effect,
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graph()->start(), outer_merge);
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Node* ret2 = graph()->NewNode(common()->Return(), Int32Constant(1),
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outer_ephi, outer_merge);
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Node* terminate = graph()->NewNode(common()->Terminate(), effect, loop);
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graph()->SetEnd(graph()->NewNode(common()->End(3), ret1, ret2, terminate));
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Reduce();
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// Outer branch should not be rewritten, the inner branch should be discarded.
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EXPECT_THAT(outer_branch, IsBranch(condition, graph()->start()));
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EXPECT_THAT(ret1, IsReturn(IsInt32Constant(2), effect, loop));
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}
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} // namespace compiler
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} // namespace internal
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} // namespace v8
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