| /* |
| * 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 "JSCJSValueInlines.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) |
| |
| |
