js/src/jit/FoldLinearArithConstants.cpp

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #include "jit/FoldLinearArithConstants.h"
   8
   9 #include "jit/IonAnalysis.h"
  10 #include "jit/MIR.h"
  11 #include "jit/MIRGenerator.h"
  12 #include "jit/MIRGraph.h"
  13
  14 using namespace js;
  15 using namespace jit;
  16
  17 namespace js {
  18 namespace jit {
  19
  20 // Mark this node and its children as RecoveredOnBailout when they are not used.
  21 // The marked nodes will be removed during DCE. Marking as RecoveredOnBailout is
  22 // necessary because the Sink pass is ran before this pass.
  23 static void markNodesAsRecoveredOnBailout(MDefinition* def) {
  24   if (def->hasLiveDefUses() || !DeadIfUnused(def) ||
  25       !def->canRecoverOnBailout()) {
  26     return;
  27   }
  28
  29   JitSpew(JitSpew_FLAC, "mark as recovered on bailout: %s%u", def->opName(),
  30           def->id());
  31   def->setRecoveredOnBailoutUnchecked();
  32
  33   // Recursively mark nodes that do not have multiple uses. This loop is
  34   // necessary because a node could be an unused right shift zero or an
  35   // unused add, and both need to be marked as RecoveredOnBailout.
  36   for (size_t i = 0; i < def->numOperands(); i++) {
  37     markNodesAsRecoveredOnBailout(def->getOperand(i));
  38   }
  39 }
  40
  41 // Fold AddIs with one variable and two or more constants into one AddI.
  42 static void AnalyzeAdd(TempAllocator& alloc, MAdd* add) {
  43   if (add->type() != MIRType::Int32 || add->isRecoveredOnBailout()) {
  44     return;
  45   }
  46
  47   if (!add->hasUses()) {
  48     return;
  49   }
  50
  51   JitSpew(JitSpew_FLAC, "analyze add: %s%u", add->opName(), add->id());
  52
  53   SimpleLinearSum sum = ExtractLinearSum(add);
  54   if (sum.constant == 0 || !sum.term) {
  55     return;
  56   }
  57
  58   // Determine which operand is the constant.
  59   int idx = add->getOperand(0)->isConstant() ? 0 : 1;
  60   if (add->getOperand(idx)->isConstant()) {
  61     // Do not replace an add where the outcome is the same add instruction.
  62     MOZ_ASSERT(add->getOperand(idx)->toConstant()->type() == MIRType::Int32);
  63     if (sum.term == add->getOperand(1 - idx) ||
  64         sum.constant == add->getOperand(idx)->toConstant()->toInt32()) {
  65       return;
  66     }
  67   }
  68
  69   MInstruction* rhs = MConstant::New(alloc, Int32Value(sum.constant));
  70   add->block()->insertBefore(add, rhs);
  71
  72   MAdd* addNew = MAdd::New(alloc, sum.term, rhs, add->truncateKind());
  73   addNew->setBailoutKind(add->bailoutKind());
  74
  75   add->replaceAllLiveUsesWith(addNew);
  76   add->block()->insertBefore(add, addNew);
  77   JitSpew(JitSpew_FLAC, "replaced with: %s%u", addNew->opName(), addNew->id());
  78   JitSpew(JitSpew_FLAC, "and constant: %s%u (%d)", rhs->opName(), rhs->id(),
  79           sum.constant);
  80
  81   // Mark the stale nodes as RecoveredOnBailout since the Sink pass has
  82   // been run before this pass. DCE will then remove the unused nodes.
  83   markNodesAsRecoveredOnBailout(add);
  84 }
  85
  86 bool FoldLinearArithConstants(MIRGenerator* mir, MIRGraph& graph) {
  87   JitSpew(JitSpew_FLAC, "Begin");
  88   for (PostorderIterator block(graph.poBegin()); block != graph.poEnd();
  89        block++) {
  90     if (mir->shouldCancel("Fold Linear Arithmetic Constants (main loop)")) {
  91       return false;
  92     }
  93
  94     for (MInstructionIterator i = block->begin(); i != block->end(); i++) {
  95       if (!graph.alloc().ensureBallast()) {
  96         return false;
  97       }
  98
  99       if (mir->shouldCancel("Fold Linear Arithmetic Constants (inner loop)")) {
 100         return false;
 101       }
 102
 103       if (i->isAdd()) {
 104         AnalyzeAdd(graph.alloc(), i->toAdd());
 105       }
 106     }
 107   }
 108   return true;
 109 }
 110
 111 } /* namespace jit */
 112 } /* namespace js */