DFGLoopPreHeaderCreationPhase.cpp   [plain text]

/*
 * Copyright (C) 2013 Apple Inc. All rights reserved.
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#include "config.h"
#include "DFGLoopPreHeaderCreationPhase.h"

#if ENABLE(DFG_JIT)

#include "DFGBasicBlockInlines.h"
#include "DFGBlockInsertionSet.h"
#include "DFGGraph.h"
#include "DFGPhase.h"
#include "JSCInlines.h"
#include <wtf/HashMap.h>

namespace JSC { namespace DFG {

BasicBlock* createPreHeader(Graph& graph, BlockInsertionSet& insertionSet, BasicBlock* block)
{
    // Don't bother to preserve execution frequencies for now.
    BasicBlock* preHeader = insertionSet.insertBefore(block, PNaN);
    preHeader->appendNode(
        graph, SpecNone, Jump, block->firstOrigin(), OpInfo(block));
    
    for (unsigned predecessorIndex = 0; predecessorIndex < block->predecessors.size(); predecessorIndex++) {
        BasicBlock* predecessor = block->predecessors[predecessorIndex];
        if (graph.m_dominators.dominates(block, predecessor))
            continue;
        block->predecessors[predecessorIndex--] = block->predecessors.last();
        block->predecessors.removeLast();
        for (unsigned successorIndex = predecessor->numSuccessors(); successorIndex--;) {
            BasicBlock*& successor = predecessor->successor(successorIndex);
            if (successor != block)
                continue;
            successor = preHeader;
            preHeader->predecessors.append(predecessor);
        }
    }
    
    block->predecessors.append(preHeader);
    return preHeader;
}

class LoopPreHeaderCreationPhase : public Phase {
public:
    LoopPreHeaderCreationPhase(Graph& graph)
        : Phase(graph, "loop pre-header creation")
        , m_insertionSet(graph)
    {
    }
    
    bool run()
    {
        m_graph.m_dominators.computeIfNecessary(m_graph);
        m_graph.m_naturalLoops.computeIfNecessary(m_graph);
        
        for (unsigned loopIndex = m_graph.m_naturalLoops.numLoops(); loopIndex--;) {
            const NaturalLoop& loop = m_graph.m_naturalLoops.loop(loopIndex);
            BasicBlock* existingPreHeader = 0;
            bool needsNewPreHeader = false;
            for (unsigned predecessorIndex = loop.header()->predecessors.size(); predecessorIndex--;) {
                BasicBlock* predecessor = loop.header()->predecessors[predecessorIndex];
                if (m_graph.m_dominators.dominates(loop.header(), predecessor))
                    continue;
                if (!existingPreHeader) {
                    existingPreHeader = predecessor;
                    continue;
                }
                // We won't have duplicate entries in the predecessors list.
                DFG_ASSERT(m_graph, nullptr, existingPreHeader != predecessor);
                needsNewPreHeader = true;
                break;
            }
            
            // This phase should only be run on a DFG where unreachable blocks have been pruned.
            // We also don't allow loops back to root. This means that every loop header has got
            // to have a pre-header.
            DFG_ASSERT(m_graph, nullptr, existingPreHeader);
            
            // We are looking at the predecessors of a loop header. A loop header has to have
            // some predecessor other than the pre-header. We must have broken critical edges
            // because that is the DFG SSA convention. Therefore, each predecessor of the loop
            // header must have only one successor.
            DFG_ASSERT(m_graph, nullptr, existingPreHeader->terminal()->op() == Jump);
            
            if (!needsNewPreHeader)
                continue;
            
            createPreHeader(m_graph, m_insertionSet, loop.header());
        }
        
        return m_insertionSet.execute();
    }

    BlockInsertionSet m_insertionSet;
};

bool performLoopPreHeaderCreation(Graph& graph)
{
    SamplingRegion samplingRegion("DFG Loop Pre-Header Creation Phase");
    return runPhase<LoopPreHeaderCreationPhase>(graph);
}

} } // namespace JSC::DFG

#endif // ENABLE(DFG_JIT)