DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Header

Mercurial (b6d82b1a6b02)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "jit/LICM.h"

#include "jit/IonAnalysis.h"
#include "jit/JitSpewer.h"
#include "jit/MIRGenerator.h"
#include "jit/MIRGraph.h"

using namespace js;
using namespace js::jit;

// Test whether any instruction in the loop possiblyCalls().
static bool LoopContainsPossibleCall(MIRGraph& graph, MBasicBlock* header,
                                     MBasicBlock* backedge) {
  for (auto i(graph.rpoBegin(header));; ++i) {
    MOZ_ASSERT(i != graph.rpoEnd(),
               "Reached end of graph searching for blocks in loop");
    MBasicBlock* block = *i;
    if (!block->isMarked()) {
      continue;
    }

    for (auto insIter(block->begin()), insEnd(block->end()); insIter != insEnd;
         ++insIter) {
      MInstruction* ins = *insIter;
      if (ins->possiblyCalls()) {
#ifdef JS_JITSPEW
        JitSpew(JitSpew_LICM, "    Possile call found at %s%u", ins->opName(),
                ins->id());
#endif
        return true;
      }
    }

    if (block == backedge) {
      break;
    }
  }
  return false;
}

// When a nested loop has no exits back into what would be its parent loop,
// MarkLoopBlocks on the parent loop doesn't mark the blocks of the nested
// loop, since they technically aren't part of the loop. However, AliasAnalysis
// currently does consider such nested loops to be part of their parent
// loops. Consequently, we can't use IsInLoop on dependency() values; we must
// test whether a dependency() is *before* the loop, even if it is not
// technically in the loop.
static bool IsBeforeLoop(MDefinition* ins, MBasicBlock* header) {
  return ins->block()->id() < header->id();
}

// Test whether the given instruction is inside the loop (and thus not
// loop-invariant).
static bool IsInLoop(MDefinition* ins) { return ins->block()->isMarked(); }

// Test whether the given instruction is cheap and not worth hoisting unless
// one of its users will be hoisted as well.
static bool RequiresHoistedUse(const MDefinition* ins, bool hasCalls) {
  if (ins->isBox()) {
    MOZ_ASSERT(!ins->toBox()->input()->isBox(),
               "Box of a box could lead to unbounded recursion");
    return true;
  }

  // Integer constants are usually cheap and aren't worth hoisting on their
  // own, in general. Floating-point constants typically are worth hoisting,
  // unless they'll end up being spilled (eg. due to a call).
  if (ins->isConstant() && (!IsFloatingPointType(ins->type()) || hasCalls)) {
    return true;
  }

  return false;
}

// Test whether the given instruction has any operands defined within the loop.
static bool HasOperandInLoop(MInstruction* ins, bool hasCalls) {
  // An instruction is only loop invariant if it and all of its operands can
  // be safely hoisted into the loop preheader.
  for (size_t i = 0, e = ins->numOperands(); i != e; ++i) {
    MDefinition* op = ins->getOperand(i);

    if (!IsInLoop(op)) {
      continue;
    }

    if (RequiresHoistedUse(op, hasCalls)) {
      // Recursively test for loop invariance. Note that the recursion is
      // bounded because we require RequiresHoistedUse to be set at each
      // level.
      if (!HasOperandInLoop(op->toInstruction(), hasCalls)) {
        continue;
      }
    }

    return true;
  }
  return false;
}

// Test whether the given instruction is hoistable, ignoring memory
// dependencies.
static bool IsHoistableIgnoringDependency(MInstruction* ins, bool hasCalls) {
  return ins->isMovable() && !ins->isEffectful() && !ins->neverHoist() &&
         !HasOperandInLoop(ins, hasCalls);
}

// Test whether the given instruction has a memory dependency inside the loop.
static bool HasDependencyInLoop(MInstruction* ins, MBasicBlock* header) {
  // Don't hoist if this instruction depends on a store inside the loop.
  if (MDefinition* dep = ins->dependency()) {
    return !IsBeforeLoop(dep, header);
  }
  return false;
}

// Test whether the given instruction is hoistable.
static bool IsHoistable(MInstruction* ins, MBasicBlock* header, bool hasCalls) {
  return IsHoistableIgnoringDependency(ins, hasCalls) &&
         !HasDependencyInLoop(ins, header);
}

// In preparation for hoisting an instruction, hoist any of its operands which
// were too cheap to hoist on their own.
static void MoveDeferredOperands(MInstruction* ins, MInstruction* hoistPoint,
                                 bool hasCalls) {
  // If any of our operands were waiting for a user to be hoisted, make a note
  // to hoist them.
  for (size_t i = 0, e = ins->numOperands(); i != e; ++i) {
    MDefinition* op = ins->getOperand(i);
    if (!IsInLoop(op)) {
      continue;
    }
    MOZ_ASSERT(RequiresHoistedUse(op, hasCalls),
               "Deferred loop-invariant operand is not cheap");
    MInstruction* opIns = op->toInstruction();

    // Recursively move the operands. Note that the recursion is bounded
    // because we require RequiresHoistedUse to be set at each level.
    MoveDeferredOperands(opIns, hoistPoint, hasCalls);

#ifdef JS_JITSPEW
    JitSpew(JitSpew_LICM, "    Hoisting %s%u (now that a user will be hoisted)",
            opIns->opName(), opIns->id());
#endif

    opIns->block()->moveBefore(hoistPoint, opIns);
  }
}

static void VisitLoopBlock(MBasicBlock* block, MBasicBlock* header,
                           MInstruction* hoistPoint, bool hasCalls) {
  for (auto insIter(block->begin()), insEnd(block->end()); insIter != insEnd;) {
    MInstruction* ins = *insIter++;

    if (!IsHoistable(ins, header, hasCalls)) {
#ifdef JS_JITSPEW
      if (IsHoistableIgnoringDependency(ins, hasCalls)) {
        JitSpew(JitSpew_LICM,
                "    %s%u isn't hoistable due to dependency on %s%u",
                ins->opName(), ins->id(), ins->dependency()->opName(),
                ins->dependency()->id());
      }
#endif
      continue;
    }

    // Don't hoist a cheap constant if it doesn't enable us to hoist one of
    // its uses. We want those instructions as close as possible to their
    // use, to minimize register pressure.
    if (RequiresHoistedUse(ins, hasCalls)) {
#ifdef JS_JITSPEW
      JitSpew(JitSpew_LICM, "    %s%u will be hoisted only if its users are",
              ins->opName(), ins->id());
#endif
      continue;
    }

    // Hoist operands which were too cheap to hoist on their own.
    MoveDeferredOperands(ins, hoistPoint, hasCalls);

#ifdef JS_JITSPEW
    JitSpew(JitSpew_LICM, "    Hoisting %s%u", ins->opName(), ins->id());
#endif

    // Move the instruction to the hoistPoint.
    block->moveBefore(hoistPoint, ins);
  }
}

static void VisitLoop(MIRGraph& graph, MBasicBlock* header) {
  MInstruction* hoistPoint = header->loopPredecessor()->lastIns();

#ifdef JS_JITSPEW
  JitSpew(JitSpew_LICM, "  Visiting loop with header block%u, hoisting to %s%u",
          header->id(), hoistPoint->opName(), hoistPoint->id());
#endif

  MBasicBlock* backedge = header->backedge();

  // This indicates whether the loop contains calls or other things which
  // clobber most or all floating-point registers. In such loops,
  // floating-point constants should not be hoisted unless it enables further
  // hoisting.
  bool hasCalls = LoopContainsPossibleCall(graph, header, backedge);

  for (auto i(graph.rpoBegin(header));; ++i) {
    MOZ_ASSERT(i != graph.rpoEnd(),
               "Reached end of graph searching for blocks in loop");
    MBasicBlock* block = *i;
    if (!block->isMarked()) {
      continue;
    }

    VisitLoopBlock(block, header, hoistPoint, hasCalls);

    if (block == backedge) {
      break;
    }
  }
}

bool jit::LICM(MIRGenerator* mir, MIRGraph& graph) {
  JitSpew(JitSpew_LICM, "Beginning LICM pass");

  // Iterate in RPO to visit outer loops before inner loops. We'd hoist the
  // same things either way, but outer first means we do a little less work.
  for (auto i(graph.rpoBegin()), e(graph.rpoEnd()); i != e; ++i) {
    MBasicBlock* header = *i;
    if (!header->isLoopHeader()) {
      continue;
    }

    bool canOsr;
    size_t numBlocks = MarkLoopBlocks(graph, header, &canOsr);

    if (numBlocks == 0) {
      JitSpew(JitSpew_LICM, "  Loop with header block%u isn't actually a loop",
              header->id());
      continue;
    }

    // Hoisting out of a loop that has an entry from the OSR block in
    // addition to its normal entry is tricky. In theory we could clone
    // the instruction and insert phis.
    if (!canOsr) {
      VisitLoop(graph, header);
    } else {
      JitSpew(JitSpew_LICM, "  Skipping loop with header block%u due to OSR",
              header->id());
    }

    UnmarkLoopBlocks(graph, header);

    if (mir->shouldCancel("LICM (main loop)")) {
      return false;
    }
  }

  return true;
}