예제 #1
0
bool fixSSA(Procedure& proc)
{
    PhaseScope phaseScope(proc, "fixSSA");
    
    // Collect the stack "variables". If there aren't any, then we don't have anything to do.
    // That's a fairly common case.
    HashMap<StackSlotValue*, Type> stackVariable;
    for (Value* value : proc.values()) {
        if (StackSlotValue* stack = value->as<StackSlotValue>()) {
            if (stack->kind() == StackSlotKind::Anonymous)
                stackVariable.add(stack, Void);
        }
    }

    if (stackVariable.isEmpty())
        return false;

    // Make sure that we know how to optimize all of these. We only know how to handle Load and
    // Store on anonymous variables.
    for (Value* value : proc.values()) {
        auto reject = [&] (Value* value) {
            if (StackSlotValue* stack = value->as<StackSlotValue>())
                stackVariable.remove(stack);
        };
        
        auto handleAccess = [&] (Value* access, Type type) {
            StackSlotValue* stack = access->lastChild()->as<StackSlotValue>();
            if (!stack)
                return;
            
            if (value->as<MemoryValue>()->offset()) {
                stackVariable.remove(stack);
                return;
            }

            auto result = stackVariable.find(stack);
            if (result == stackVariable.end())
                return;
            if (result->value == Void) {
                result->value = type;
                return;
            }
            if (result->value == type)
                return;
            stackVariable.remove(result);
        };
        
        switch (value->opcode()) {
        case Load:
            // We're OK with loads from stack variables at an offset of zero.
            handleAccess(value, value->type());
            break;
        case Store:
            // We're OK with stores to stack variables, but not storing stack variables.
            reject(value->child(0));
            handleAccess(value, value->child(0)->type());
            break;
        default:
            for (Value* child : value->children())
                reject(child);
            break;
        }
    }

    Vector<StackSlotValue*> deadValues;
    for (auto& entry : stackVariable) {
        if (entry.value == Void)
            deadValues.append(entry.key);
    }

    for (StackSlotValue* deadValue : deadValues) {
        deadValue->replaceWithNop();
        stackVariable.remove(deadValue);
    }

    if (stackVariable.isEmpty())
        return false;

    // We know that we have variables to optimize, so do that now.
    breakCriticalEdges(proc);

    SSACalculator ssa(proc);

    // Create a SSACalculator::Variable for every stack variable.
    Vector<StackSlotValue*> variableToStack;
    HashMap<StackSlotValue*, SSACalculator::Variable*> stackToVariable;

    for (auto& entry : stackVariable) {
        StackSlotValue* stack = entry.key;
        SSACalculator::Variable* variable = ssa.newVariable();
        RELEASE_ASSERT(variable->index() == variableToStack.size());
        variableToStack.append(stack);
        stackToVariable.add(stack, variable);
    }

    // Create Defs for all of the stores to the stack variable.
    for (BasicBlock* block : proc) {
        for (Value* value : *block) {
            if (value->opcode() != Store)
                continue;

            StackSlotValue* stack = value->child(1)->as<StackSlotValue>();
            if (!stack)
                continue;

            if (SSACalculator::Variable* variable = stackToVariable.get(stack))
                ssa.newDef(variable, block, value->child(0));
        }
    }

    // Decide where Phis are to be inserted. This creates them but does not insert them.
    ssa.computePhis(
        [&] (SSACalculator::Variable* variable, BasicBlock* block) -> Value* {
            StackSlotValue* stack = variableToStack[variable->index()];
            Value* phi = proc.add<Value>(Phi, stackVariable.get(stack), stack->origin());
            if (verbose) {
                dataLog(
                    "Adding Phi for ", pointerDump(stack), " at ", *block, ": ",
                    deepDump(proc, phi), "\n");
            }
            return phi;
        });

    // Now perform the conversion.
    InsertionSet insertionSet(proc);
    HashMap<StackSlotValue*, Value*> mapping;
    for (BasicBlock* block : proc.blocksInPreOrder()) {
        mapping.clear();

        for (auto& entry : stackToVariable) {
            StackSlotValue* stack = entry.key;
            SSACalculator::Variable* variable = entry.value;

            SSACalculator::Def* def = ssa.reachingDefAtHead(block, variable);
            if (def)
                mapping.set(stack, def->value());
        }

        for (SSACalculator::Def* phiDef : ssa.phisForBlock(block)) {
            StackSlotValue* stack = variableToStack[phiDef->variable()->index()];

            insertionSet.insertValue(0, phiDef->value());
            mapping.set(stack, phiDef->value());
        }

        for (unsigned valueIndex = 0; valueIndex < block->size(); ++valueIndex) {
            Value* value = block->at(valueIndex);
            value->performSubstitution();

            switch (value->opcode()) {
            case Load: {
                if (StackSlotValue* stack = value->child(0)->as<StackSlotValue>()) {
                    if (Value* replacement = mapping.get(stack))
                        value->replaceWithIdentity(replacement);
                }
                break;
            }
                
            case Store: {
                if (StackSlotValue* stack = value->child(1)->as<StackSlotValue>()) {
                    if (stackToVariable.contains(stack)) {
                        mapping.set(stack, value->child(0));
                        value->replaceWithNop();
                    }
                }
                break;
            }

            default:
                break;
            }
        }

        unsigned upsilonInsertionPoint = block->size() - 1;
        Origin upsilonOrigin = block->last()->origin();
        for (BasicBlock* successorBlock : block->successorBlocks()) {
            for (SSACalculator::Def* phiDef : ssa.phisForBlock(successorBlock)) {
                Value* phi = phiDef->value();
                SSACalculator::Variable* variable = phiDef->variable();
                StackSlotValue* stack = variableToStack[variable->index()];

                Value* mappedValue = mapping.get(stack);
                if (verbose) {
                    dataLog(
                        "Mapped value for ", *stack, " with successor Phi ", *phi, " at end of ",
                        *block, ": ", pointerDump(mappedValue), "\n");
                }
                
                if (!mappedValue)
                    mappedValue = insertionSet.insertBottom(upsilonInsertionPoint, phi);
                
                insertionSet.insert<UpsilonValue>(
                    upsilonInsertionPoint, upsilonOrigin, mappedValue, phi);
            }
        }

        insertionSet.execute(block);
    }

    // Finally, kill the stack slots.
    for (StackSlotValue* stack : variableToStack)
        stack->replaceWithNop();

    if (verbose) {
        dataLog("B3 after SSA conversion:\n");
        dataLog(proc);
    }

    return true;
}
예제 #2
0
    bool run()
    {
        RELEASE_ASSERT(m_graph.m_form == ThreadedCPS);
        
        m_graph.clearReplacements();
        m_graph.m_dominators.computeIfNecessary(m_graph);
        
        if (verbose) {
            dataLog("Graph before SSA transformation:\n");
            m_graph.dump();
        }

        // Create a SSACalculator::Variable for every root VariableAccessData.
        for (VariableAccessData& variable : m_graph.m_variableAccessData) {
            if (!variable.isRoot() || variable.isCaptured())
                continue;
            
            SSACalculator::Variable* ssaVariable = m_calculator.newVariable();
            ASSERT(ssaVariable->index() == m_variableForSSAIndex.size());
            m_variableForSSAIndex.append(&variable);
            m_ssaVariableForVariable.add(&variable, ssaVariable);
        }
        
        // Find all SetLocals and create Defs for them. We handle SetArgument by creating a
        // GetLocal, and recording the flush format.
        for (BlockIndex blockIndex = m_graph.numBlocks(); blockIndex--;) {
            BasicBlock* block = m_graph.block(blockIndex);
            if (!block)
                continue;
            
            // Must process the block in forward direction because we want to see the last
            // assignment for every local.
            for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
                Node* node = block->at(nodeIndex);
                if (node->op() != SetLocal && node->op() != SetArgument)
                    continue;
                
                VariableAccessData* variable = node->variableAccessData();
                if (variable->isCaptured())
                    continue;
                
                Node* childNode;
                if (node->op() == SetLocal)
                    childNode = node->child1().node();
                else {
                    ASSERT(node->op() == SetArgument);
                    childNode = m_insertionSet.insertNode(
                        nodeIndex, node->variableAccessData()->prediction(),
                        GetStack, node->origin,
                        OpInfo(m_graph.m_stackAccessData.add(variable->local(), variable->flushFormat())));
                    if (!ASSERT_DISABLED)
                        m_argumentGetters.add(childNode);
                    m_argumentMapping.add(node, childNode);
                }
                
                m_calculator.newDef(
                    m_ssaVariableForVariable.get(variable), block, childNode);
            }
            
            m_insertionSet.execute(block);
        }
        
        // Decide where Phis are to be inserted. This creates the Phi's but doesn't insert them
        // yet. We will later know where to insert them because SSACalculator is such a bro.
        m_calculator.computePhis(
            [&] (SSACalculator::Variable* ssaVariable, BasicBlock* block) -> Node* {
                VariableAccessData* variable = m_variableForSSAIndex[ssaVariable->index()];
                
                // Prune by liveness. This doesn't buy us much other than compile times.
                Node* headNode = block->variablesAtHead.operand(variable->local());
                if (!headNode)
                    return nullptr;

                // There is the possibiltiy of "rebirths". The SSA calculator will already prune
                // rebirths for the same VariableAccessData. But it will not be able to prune
                // rebirths that arose from the same local variable number but a different
                // VariableAccessData. We do that pruning here.
                //
                // Here's an example of a rebirth that this would catch:
                //
                //     var x;
                //     if (foo) {
                //         if (bar) {
                //             x = 42;
                //         } else {
                //             x = 43;
                //         }
                //         print(x);
                //         x = 44;
                //     } else {
                //         x = 45;
                //     }
                //     print(x); // Without this check, we'd have a Phi for x = 42|43 here.
                //
                // FIXME: Consider feeding local variable numbers, not VariableAccessData*'s, as
                // the "variables" for SSACalculator. That would allow us to eliminate this
                // special case.
                // https://bugs.webkit.org/show_bug.cgi?id=136641
                if (headNode->variableAccessData() != variable)
                    return nullptr;
                
                Node* phiNode = m_graph.addNode(
                    variable->prediction(), Phi, NodeOrigin());
                FlushFormat format = variable->flushFormat();
                NodeFlags result = resultFor(format);
                phiNode->mergeFlags(result);
                return phiNode;
            });
        
        if (verbose) {
            dataLog("Computed Phis, about to transform the graph.\n");
            dataLog("\n");
            dataLog("Graph:\n");
            m_graph.dump();
            dataLog("\n");
            dataLog("Mappings:\n");
            for (unsigned i = 0; i < m_variableForSSAIndex.size(); ++i)
                dataLog("    ", i, ": ", VariableAccessDataDump(m_graph, m_variableForSSAIndex[i]), "\n");
            dataLog("\n");
            dataLog("SSA calculator: ", m_calculator, "\n");
        }
        
        // Do the bulk of the SSA conversion. For each block, this tracks the operand->Node
        // mapping based on a combination of what the SSACalculator tells us, and us walking over
        // the block in forward order. We use our own data structure, valueForOperand, for
        // determining the local mapping, but we rely on SSACalculator for the non-local mapping.
        //
        // This does three things at once:
        //
        // - Inserts the Phis in all of the places where they need to go. We've already created
        //   them and they are accounted for in the SSACalculator's data structures, but we
        //   haven't inserted them yet, mostly because we want to insert all of a block's Phis in
        //   one go to amortize the cost of node insertion.
        //
        // - Create and insert Upsilons.
        //
        // - Convert all of the preexisting SSA nodes (other than the old CPS Phi nodes) into SSA
        //   form by replacing as follows:
        //
        //   - MovHint has KillLocal prepended to it.
        //
        //   - GetLocal over captured variables lose their phis and become GetStack.
        //
        //   - GetLocal over uncaptured variables die and get replaced with references to the node
        //     specified by valueForOperand.
        //
        //   - SetLocal turns into PutStack if it's flushed, or turns into a Check otherwise.
        //
        //   - Flush loses its children and turns into a Phantom.
        //
        //   - PhantomLocal becomes Phantom, and its child is whatever is specified by
        //     valueForOperand.
        //
        //   - SetArgument is removed. Note that GetStack nodes have already been inserted.
        Operands<Node*> valueForOperand(OperandsLike, m_graph.block(0)->variablesAtHead);
        for (BasicBlock* block : m_graph.blocksInPreOrder()) {
            valueForOperand.clear();
            
            // CPS will claim that the root block has all arguments live. But we have already done
            // the first step of SSA conversion: argument locals are no longer live at head;
            // instead we have GetStack nodes for extracting the values of arguments. So, we
            // skip the at-head available value calculation for the root block.
            if (block != m_graph.block(0)) {
                for (size_t i = valueForOperand.size(); i--;) {
                    Node* nodeAtHead = block->variablesAtHead[i];
                    if (!nodeAtHead)
                        continue;
                    
                    VariableAccessData* variable = nodeAtHead->variableAccessData();
                    if (variable->isCaptured())
                        continue;
                    
                    if (verbose)
                        dataLog("Considering live variable ", VariableAccessDataDump(m_graph, variable), " at head of block ", *block, "\n");
                    
                    SSACalculator::Variable* ssaVariable = m_ssaVariableForVariable.get(variable);
                    SSACalculator::Def* def = m_calculator.reachingDefAtHead(block, ssaVariable);
                    if (!def) {
                        // If we are required to insert a Phi, then we won't have a reaching def
                        // at head.
                        continue;
                    }
                    
                    Node* node = def->value();
                    if (node->replacement) {
                        // This will occur when a SetLocal had a GetLocal as its source. The
                        // GetLocal would get replaced with an actual SSA value by the time we get
                        // here. Note that the SSA value with which the GetLocal got replaced
                        // would not in turn have a replacement.
                        node = node->replacement;
                        ASSERT(!node->replacement);
                    }
                    if (verbose)
                        dataLog("Mapping: ", VirtualRegister(valueForOperand.operandForIndex(i)), " -> ", node, "\n");
                    valueForOperand[i] = node;
                }
            }
            
            // Insert Phis by asking the calculator what phis there are in this block. Also update
            // valueForOperand with those Phis. For Phis associated with variables that are not
            // flushed, we also insert a MovHint.
            size_t phiInsertionPoint = 0;
            for (SSACalculator::Def* phiDef : m_calculator.phisForBlock(block)) {
                VariableAccessData* variable = m_variableForSSAIndex[phiDef->variable()->index()];
                
                m_insertionSet.insert(phiInsertionPoint, phiDef->value());
                valueForOperand.operand(variable->local()) = phiDef->value();
                
                m_insertionSet.insertNode(
                    phiInsertionPoint, SpecNone, MovHint, NodeOrigin(),
                    OpInfo(variable->local().offset()), phiDef->value()->defaultEdge());
            }
            
            for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
                Node* node = block->at(nodeIndex);
                
                if (verbose) {
                    dataLog("Processing node ", node, ":\n");
                    m_graph.dump(WTF::dataFile(), "    ", node);
                }
                
                m_graph.performSubstitution(node);
                
                switch (node->op()) {
                case MovHint: {
                    m_insertionSet.insertNode(
                        nodeIndex, SpecNone, KillStack, node->origin,
                        OpInfo(node->unlinkedLocal().offset()));
                    break;
                }
                    
                case SetLocal: {
                    VariableAccessData* variable = node->variableAccessData();
                    
                    if (variable->isCaptured() || !!(node->flags() & NodeIsFlushed)) {
                        node->convertToPutStack(
                            m_graph.m_stackAccessData.add(
                                variable->local(), variable->flushFormat()));
                    } else
                        node->setOpAndDefaultFlags(Check);
                    
                    if (!variable->isCaptured()) {
                        if (verbose)
                            dataLog("Mapping: ", variable->local(), " -> ", node->child1().node(), "\n");
                        valueForOperand.operand(variable->local()) = node->child1().node();
                    }
                    break;
                }
                    
                case GetStack: {
                    ASSERT(m_argumentGetters.contains(node));
                    valueForOperand.operand(node->stackAccessData()->local) = node;
                    break;
                }
                    
                case GetLocal: {
                    VariableAccessData* variable = node->variableAccessData();
                    node->children.reset();
                    
                    if (variable->isCaptured()) {
                        node->convertToGetStack(m_graph.m_stackAccessData.add(variable->local(), variable->flushFormat()));
                        break;
                    }
                    
                    node->convertToPhantom();
                    if (verbose)
                        dataLog("Replacing node ", node, " with ", valueForOperand.operand(variable->local()), "\n");
                    node->replacement = valueForOperand.operand(variable->local());
                    break;
                }
                    
                case Flush: {
                    node->children.reset();
                    node->convertToPhantom();
                    break;
                }
                    
                case PhantomLocal: {
                    ASSERT(node->child1().useKind() == UntypedUse);
                    VariableAccessData* variable = node->variableAccessData();
                    if (variable->isCaptured()) {
                        // This is a fun case. We could have a captured variable that had some
                        // or all of its uses strength reduced to phantoms rather than flushes.
                        // SSA conversion will currently still treat it as flushed, in the sense
                        // that it will just keep the SetLocal. Therefore, there is nothing that
                        // needs to be done here: we don't need to also keep the source value
                        // alive. And even if we did want to keep the source value alive, we
                        // wouldn't be able to, because the variablesAtHead value for a captured
                        // local wouldn't have been computed by the Phi reduction algorithm
                        // above.
                        node->children.reset();
                    } else
                        node->child1() = valueForOperand.operand(variable->local())->defaultEdge();
                    node->convertToPhantom();
                    break;
                }
                    
                case SetArgument: {
                    node->convertToPhantom();
                    break;
                }
                    
                default:
                    break;
                }
            }
            
            // We want to insert Upsilons just before the end of the block. On the surface this
            // seems dangerous because the Upsilon will have a checking UseKind. But, we will not
            // actually be performing the check at the point of the Upsilon; the check will
            // already have been performed at the point where the original SetLocal was.
            size_t upsilonInsertionPoint = block->size() - 1;
            NodeOrigin upsilonOrigin = block->last()->origin;
            for (unsigned successorIndex = block->numSuccessors(); successorIndex--;) {
                BasicBlock* successorBlock = block->successor(successorIndex);
                for (SSACalculator::Def* phiDef : m_calculator.phisForBlock(successorBlock)) {
                    Node* phiNode = phiDef->value();
                    SSACalculator::Variable* ssaVariable = phiDef->variable();
                    VariableAccessData* variable = m_variableForSSAIndex[ssaVariable->index()];
                    FlushFormat format = variable->flushFormat();
                    UseKind useKind = useKindFor(format);
                    
                    m_insertionSet.insertNode(
                        upsilonInsertionPoint, SpecNone, Upsilon, upsilonOrigin,
                        OpInfo(phiNode), Edge(
                            valueForOperand.operand(variable->local()),
                            useKind));
                }
            }
            
            m_insertionSet.execute(block);
        }
        
        // Free all CPS phis and reset variables vectors.
        for (BlockIndex blockIndex = m_graph.numBlocks(); blockIndex--;) {
            BasicBlock* block = m_graph.block(blockIndex);
            if (!block)
                continue;
            for (unsigned phiIndex = block->phis.size(); phiIndex--;)
                m_graph.m_allocator.free(block->phis[phiIndex]);
            block->phis.clear();
            block->variablesAtHead.clear();
            block->variablesAtTail.clear();
            block->valuesAtHead.clear();
            block->valuesAtHead.clear();
            block->ssa = std::make_unique<BasicBlock::SSAData>(block);
        }
        
        m_graph.m_argumentFormats.resize(m_graph.m_arguments.size());
        for (unsigned i = m_graph.m_arguments.size(); i--;) {
            FlushFormat format = FlushedJSValue;

            Node* node = m_argumentMapping.get(m_graph.m_arguments[i]);

            // m_argumentMapping.get could return null for a captured local. That's fine. We only
            // track the argument loads of those arguments for which we speculate type. We don't
            // speculate type for captured arguments.
            if (node)
                format = node->stackAccessData()->format;
            
            m_graph.m_argumentFormats[i] = format;
            m_graph.m_arguments[i] = node; // Record the load that loads the arguments for the benefit of exit profiling.
        }
        
        m_graph.m_form = SSA;

        if (verbose) {
            dataLog("Graph after SSA transformation:\n");
            m_graph.dump();
        }

        return true;
    }