/* * Copyright (C) 2013, 2015 Apple Inc. All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ #include "config.h" #include "DFGLoopPreHeaderCreationPhase.h" #if ENABLE(DFG_JIT) #include "DFGBasicBlockInlines.h" #include "DFGBlockInsertionSet.h" #include "DFGDominators.h" #include "DFGGraph.h" #include "DFGNaturalLoops.h" #include "DFGPhase.h" #include "JSCInlines.h" #include <wtf/HashMap.h> namespace JSC { namespace DFG { BasicBlock* createPreHeader(Graph& graph, BlockInsertionSet& insertionSet, BasicBlock* block) { ASSERT_WITH_MESSAGE(!graph.isRoot(block), "A CFG root should not be in a loop"); // FIXME: If we run this utility on SSA IR, then we may end up with a bizarre arrangement of // Upsilons and Phis, like: // // BB#1: // Upsilon(@a, ^p) // Jump(#3) // // BB#2: // Upsilon(@b, ^p) // Jump(#3) // // BB#3: // Jump(#4) // // BB#4: // p: Phi() // // Notice how the Upsilons are not in the predecessor of the Phi anymore. It's not clear if this // would be bad. Probably not, but it's weird anyway. We should add a validation rule, and we // should implement a Upsilon/Phi canonicalization that handles this by calling into the // SSACalculator and treating the Upsilons as Defs and rebuilding the Phis from scratch. // // https://bugs.webkit.org/show_bug.cgi?id=148587 // Determine a good frequency for the pre-header. It's definitely not the frequency of the loop body. // Instead, we use the max of the frequencies of the loop body's non-loop predecessors. float frequency = 0; for (BasicBlock* predecessor : block->predecessors) { ASSERT(graph.m_form != SSA); if (graph.m_cpsDominators->dominates(block, predecessor)) continue; frequency = std::max(frequency, predecessor->executionCount); } BasicBlock* preHeader = insertionSet.insertBefore(block, frequency); // FIXME: It would be great if we put some effort into enabling exitOK at this origin, if it // happens to be unset. It might not be set because the loop header (i.e. "block") has Phis in it. // Phis have to have exitOK=false. There are a few ways to try to set exitOK: // // - Regenerate an exit origin by proving that we are at an exit origin boundary. If all of the // predecessors' terminals have different exit origins than the exit origin of head of block, // then we can leverage the assumption that exit origin boundaries can always exit. We could // extend this further, and say that we will set exitOK even if a predecessor's terminal has the // same exit origin, but so long it hadn't done anything that clobbers exit since the start of // the origin. // // - Analyze the Phi's and MovHint's at the head of block. If prior to the ExitOK there are only // Phi's and MovHint's, we could "roll them back" by proving that for each of the MovHints, the // referenced Phi has a child that dominates the pre-header, and that child is the node that is // OSR-available at the local being MovHinted. // // Note that there are some obviously wrong ways to try to set exitOK. For example, we cannot // simply use the origin of our predecessors, since in bytecode that could be *any* kind of // instruction. It may not even be a control flow construct, if we had lowered some non-control // bytecode operation into DFG IR that has control flow. Hence, we really do need to try to use the // origin of the head of the loop header. // // https://bugs.webkit.org/show_bug.cgi?id=148586 preHeader->appendNode( graph, SpecNone, Jump, block->at(0)->origin, OpInfo(block)); for (unsigned predecessorIndex = 0; predecessorIndex < block->predecessors.size(); predecessorIndex++) { BasicBlock* predecessor = block->predecessors[predecessorIndex]; if (graph.m_cpsDominators->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.ensureCPSDominators(); m_graph.ensureCPSNaturalLoops(); for (unsigned loopIndex = m_graph.m_cpsNaturalLoops->numLoops(); loopIndex--;) { const CPSNaturalLoop& loop = m_graph.m_cpsNaturalLoops->loop(loopIndex); BasicBlock* existingPreHeader = nullptr; bool needsNewPreHeader = false; for (unsigned predecessorIndex = loop.header().node()->predecessors.size(); predecessorIndex--;) { BasicBlock* predecessor = loop.header().node()->predecessors[predecessorIndex]; if (m_graph.m_cpsDominators->dominates(loop.header().node(), 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, existingPreHeader->terminal()->op()); // A pre-header is most useful if it's possible to exit from its terminal. Hence // if the terminal of the existing pre-header doesn't allow for exit, but the first // origin of the loop header does, then we should create a new pre-header. if (!needsNewPreHeader && loop.header().node()->at(0)->origin.exitOK && !existingPreHeader->terminal()->origin.exitOK) needsNewPreHeader = true; if (!needsNewPreHeader) continue; createPreHeader(m_graph, m_insertionSet, loop.header().node()); } return m_insertionSet.execute(); } BlockInsertionSet m_insertionSet; }; bool performLoopPreHeaderCreation(Graph& graph) { return runPhase<LoopPreHeaderCreationPhase>(graph); } } } // namespace JSC::DFG #endif // ENABLE(DFG_JIT)