dfg/DFGLICMPhase.cpp

   1 /*
   2  * Copyright (C) 2013, 2014 Apple Inc. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions
   6  * are met:
   7  * 1. Redistributions of source code must retain the above copyright
   8  *    notice, this list of conditions and the following disclaimer.
   9  * 2. Redistributions in binary form must reproduce the above copyright
  10  *    notice, this list of conditions and the following disclaimer in the
  11  *    documentation and/or other materials provided with the distribution.
  12  *
  13  * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
  14  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  16  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
  17  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  18  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  19  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  20  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
  21  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  22  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  23  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  24  */
  25
  26 #include "config.h"
  27 #include "DFGLICMPhase.h"
  28
  29 #if ENABLE(DFG_JIT)
  30
  31 #include "DFGAbstractInterpreterInlines.h"
  32 #include "DFGAtTailAbstractState.h"
  33 #include "DFGBasicBlockInlines.h"
  34 #include "DFGClobberSet.h"
  35 #include "DFGClobberize.h"
  36 #include "DFGEdgeDominates.h"
  37 #include "DFGGraph.h"
  38 #include "DFGInsertionSet.h"
  39 #include "DFGPhase.h"
  40 #include "DFGSafeToExecute.h"
  41 #include "JSCInlines.h"
  42
  43 namespace JSC { namespace DFG {
  44
  45 namespace {
  46
  47 struct LoopData {
  48     LoopData()
  49         : preHeader(0)
  50     {
  51     }
  52
  53     ClobberSet writes;
  54     BasicBlock* preHeader;
  55 };
  56
  57 } // anonymous namespace
  58
  59 class LICMPhase : public Phase {
  60     static const bool verbose = false;
  61
  62 public:
  63     LICMPhase(Graph& graph)
  64         : Phase(graph, "LICM")
  65         , m_interpreter(graph, m_state)
  66     {
  67     }
  68
  69     bool run()
  70     {
  71         ASSERT(m_graph.m_form == SSA);
  72
  73         m_graph.m_dominators.computeIfNecessary(m_graph);
  74         m_graph.m_naturalLoops.computeIfNecessary(m_graph);
  75
  76         m_data.resize(m_graph.m_naturalLoops.numLoops());
  77
  78         // Figure out the set of things each loop writes to, not including blocks that
  79         // belong to inner loops. We fix this later.
  80         for (BlockIndex blockIndex = m_graph.numBlocks(); blockIndex--;) {
  81             BasicBlock* block = m_graph.block(blockIndex);
  82             if (!block)
  83                 continue;
  84
  85             // Skip blocks that are proved to not execute.
  86             // FIXME: This shouldn't be needed.
  87             // https://bugs.webkit.org/show_bug.cgi?id=128584
  88             if (!block->cfaHasVisited)
  89                 continue;
  90
  91             const NaturalLoop* loop = m_graph.m_naturalLoops.innerMostLoopOf(block);
  92             if (!loop)
  93                 continue;
  94             LoopData& data = m_data[loop->index()];
  95             for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
  96                 Node* node = block->at(nodeIndex);
  97
  98                 // Don't look beyond parts of the code that definitely always exit.
  99                 // FIXME: This shouldn't be needed.
 100                 // https://bugs.webkit.org/show_bug.cgi?id=128584
 101                 if (node->op() == ForceOSRExit)
 102                     break;
 103
 104                 addWrites(m_graph, node, data.writes);
 105             }
 106         }
 107
 108         // For each loop:
 109         // - Identify its pre-header.
 110         // - Make sure its outer loops know what it clobbers.
 111         for (unsigned loopIndex = m_graph.m_naturalLoops.numLoops(); loopIndex--;) {
 112             const NaturalLoop& loop = m_graph.m_naturalLoops.loop(loopIndex);
 113             LoopData& data = m_data[loop.index()];
 114             for (
 115                 const NaturalLoop* outerLoop = m_graph.m_naturalLoops.innerMostOuterLoop(loop);
 116                 outerLoop;
 117                 outerLoop = m_graph.m_naturalLoops.innerMostOuterLoop(*outerLoop))
 118                 m_data[outerLoop->index()].writes.addAll(data.writes);
 119
 120             BasicBlock* header = loop.header();
 121             BasicBlock* preHeader = 0;
 122             for (unsigned i = header->predecessors.size(); i--;) {
 123                 BasicBlock* predecessor = header->predecessors[i];
 124                 if (m_graph.m_dominators.dominates(header, predecessor))
 125                     continue;
 126                 RELEASE_ASSERT(!preHeader || preHeader == predecessor);
 127                 preHeader = predecessor;
 128             }
 129
 130             RELEASE_ASSERT(preHeader->last()->op() == Jump);
 131
 132             data.preHeader = preHeader;
 133         }
 134
 135         m_graph.initializeNodeOwners();
 136
 137         // Walk all basic blocks that belong to loops, looking for hoisting opportunities.
 138         // We try to hoist to the outer-most loop that permits it. Hoisting is valid if:
 139         // - The node doesn't write anything.
 140         // - The node doesn't read anything that the loop writes.
 141         // - The preHeader's state at tail makes the node safe to execute.
 142         // - The loop's children all belong to nodes that strictly dominate the loop header.
 143         // - The preHeader's state at tail is still valid. This is mostly to save compile
 144         //   time and preserve some kind of sanity, if we hoist something that must exit.
 145         //
 146         // Also, we need to remember to:
 147         // - Update the state-at-tail with the node we hoisted, so future hoist candidates
 148         //   know about any type checks we hoisted.
 149         //
 150         // For maximum profit, we walk blocks in DFS order to ensure that we generally
 151         // tend to hoist dominators before dominatees.
 152         Vector<BasicBlock*> depthFirst;
 153         m_graph.getBlocksInDepthFirstOrder(depthFirst);
 154         Vector<const NaturalLoop*> loopStack;
 155         bool changed = false;
 156         for (
 157             unsigned depthFirstIndex = 0;
 158             depthFirstIndex < depthFirst.size();
 159             ++depthFirstIndex) {
 160
 161             BasicBlock* block = depthFirst[depthFirstIndex];
 162             const NaturalLoop* loop = m_graph.m_naturalLoops.innerMostLoopOf(block);
 163             if (!loop)
 164                 continue;
 165
 166             loopStack.resize(0);
 167             for (
 168                 const NaturalLoop* current = loop;
 169                 current;
 170                 current = m_graph.m_naturalLoops.innerMostOuterLoop(*current))
 171                 loopStack.append(current);
 172
 173             // Remember: the loop stack has the inner-most loop at index 0, so if we want
 174             // to bias hoisting to outer loops then we need to use a reverse loop.
 175
 176             if (verbose) {
 177                 dataLog(
 178                     "Attempting to hoist out of block ", *block, " in loops:\n");
 179                 for (unsigned stackIndex = loopStack.size(); stackIndex--;) {
 180                     dataLog(
 181                         "        ", *loopStack[stackIndex], ", which writes ",
 182                         m_data[loopStack[stackIndex]->index()].writes, "\n");
 183                 }
 184             }
 185
 186             for (unsigned nodeIndex = 0; nodeIndex < block->size(); ++nodeIndex) {
 187                 Node*& nodeRef = block->at(nodeIndex);
 188                 if (doesWrites(m_graph, nodeRef)) {
 189                     if (verbose)
 190                         dataLog("    Not hoisting ", nodeRef, " because it writes things.\n");
 191                     continue;
 192                 }
 193
 194                 for (unsigned stackIndex = loopStack.size(); stackIndex--;)
 195                     changed |= attemptHoist(block, nodeRef, loopStack[stackIndex]);
 196             }
 197         }
 198
 199         return changed;
 200     }
 201
 202 private:
 203     bool attemptHoist(BasicBlock* fromBlock, Node*& nodeRef, const NaturalLoop* loop)
 204     {
 205         Node* node = nodeRef;
 206         LoopData& data = m_data[loop->index()];
 207
 208         if (!data.preHeader->cfaDidFinish) {
 209             if (verbose)
 210                 dataLog("    Not hoisting ", node, " because CFA is invalid.\n");
 211             return false;
 212         }
 213
 214         if (!edgesDominate(m_graph, node, data.preHeader)) {
 215             if (verbose) {
 216                 dataLog(
 217                     "    Not hoisting ", node, " because it isn't loop invariant.\n");
 218             }
 219             return false;
 220         }
 221
 222         if (readsOverlap(m_graph, node, data.writes)) {
 223             if (verbose) {
 224                 dataLog(
 225                     "    Not hoisting ", node,
 226                     " because it reads things that the loop writes.\n");
 227             }
 228             return false;
 229         }
 230
 231         m_state.initializeTo(data.preHeader);
 232         if (!safeToExecute(m_state, m_graph, node)) {
 233             if (verbose) {
 234                 dataLog(
 235                     "    Not hoisting ", node, " because it isn't safe to execute.\n");
 236             }
 237             return false;
 238         }
 239
 240         if (verbose) {
 241             dataLog(
 242                 "    Hoisting ", node, " from ", *fromBlock, " to ", *data.preHeader,
 243                 "\n");
 244         }
 245
 246         data.preHeader->insertBeforeLast(node);
 247         node->misc.owner = data.preHeader;
 248         NodeOrigin originalOrigin = node->origin;
 249         node->origin.forExit = data.preHeader->last()->origin.forExit;
 250
 251         // Modify the states at the end of the preHeader of the loop we hoisted to,
 252         // and all pre-headers inside the loop.
 253         // FIXME: This could become a scalability bottleneck. Fortunately, most loops
 254         // are small and anyway we rapidly skip over basic blocks here.
 255         for (unsigned bodyIndex = loop->size(); bodyIndex--;) {
 256             BasicBlock* subBlock = loop->at(bodyIndex);
 257             const NaturalLoop* subLoop = m_graph.m_naturalLoops.headerOf(subBlock);
 258             if (!subLoop)
 259                 continue;
 260             BasicBlock* subPreHeader = m_data[subLoop->index()].preHeader;
 261             if (!subPreHeader->cfaDidFinish)
 262                 continue;
 263             m_state.initializeTo(subPreHeader);
 264             m_interpreter.execute(node);
 265         }
 266
 267         // It just so happens that all of the nodes we currently know how to hoist
 268         // don't have var-arg children. That may change and then we can fix this
 269         // code. But for now we just assert that's the case.
 270         RELEASE_ASSERT(!(node->flags() & NodeHasVarArgs));
 271
 272         nodeRef = m_graph.addNode(SpecNone, Phantom, originalOrigin, node->children);
 273
 274         return true;
 275     }
 276
 277     AtTailAbstractState m_state;
 278     AbstractInterpreter<AtTailAbstractState> m_interpreter;
 279     Vector<LoopData> m_data;
 280 };
 281
 282 bool performLICM(Graph& graph)
 283 {
 284     SamplingRegion samplingRegion("DFG LICM Phase");
 285     return runPhase<LICMPhase>(graph);
 286 }
 287
 288 } } // namespace JSC::DFG
 289
 290 #endif // ENABLE(DFG_JIT)
 291