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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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/ValueNumbering.h"

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

using namespace js;
using namespace js::jit;

/*
 * Some notes on the main algorithm here:
 *  - The SSA identifier id() is the value number. We do replaceAllUsesWith as
 *    we go, so there's always at most one visible value with a given number.
 *
 *  - Consequently, the GVN algorithm is effectively pessimistic. This means it
 *    is not as powerful as an optimistic GVN would be, but it is simpler and
 *    faster.
 *
 *  - We iterate in RPO, so that when visiting a block, we've already optimized
 *    and hashed all values in dominating blocks. With occasional exceptions,
 *    this allows us to do everything in a single pass.
 *
 *  - When we do use multiple passes, we just re-run the algorithm on the whole
 *    graph instead of doing sparse propagation. This is a tradeoff to keep the
 *    algorithm simpler and lighter on inputs that don't have a lot of
 *    interesting unreachable blocks or degenerate loop induction variables, at
 *    the expense of being slower on inputs that do. The loop for this always
 *    terminates, because it only iterates when code is or will be removed, so
 *    eventually it must stop iterating.
 *
 *  - Values are not immediately removed from the hash set when they go out of
 *    scope. Instead, we check for dominance after a lookup. If the dominance
 *    check fails, the value is removed.
 */

HashNumber
ValueNumberer::VisibleValues::ValueHasher::hash(Lookup ins)
{
    return ins->valueHash();
}

// Test whether two MDefinitions are congruent.
bool
ValueNumberer::VisibleValues::ValueHasher::match(Key k, Lookup l)
{
    // If one of the instructions depends on a store, and the other instruction
    // does not depend on the same store, the instructions are not congruent.
    if (k->dependency() != l->dependency())
        return false;

    bool congruent = k->congruentTo(l); // Ask the values themselves what they think.
#ifdef JS_JITSPEW
    if (congruent != l->congruentTo(k)) {
       JitSpew(JitSpew_GVN, "      congruentTo relation is not symmetric between %s%u and %s%u!!",
               k->opName(), k->id(),
               l->opName(), l->id());
    }
#endif
    return congruent;
}

void
ValueNumberer::VisibleValues::ValueHasher::rekey(Key& k, Key newKey)
{
    k = newKey;
}

ValueNumberer::VisibleValues::VisibleValues(TempAllocator& alloc)
  : set_(alloc)
{}

// Initialize the set.
bool
ValueNumberer::VisibleValues::init()
{
    return set_.init();
}

// Look up the first entry for |def|.
ValueNumberer::VisibleValues::Ptr
ValueNumberer::VisibleValues::findLeader(const MDefinition* def) const
{
    return set_.lookup(def);
}

// Look up the first entry for |def|.
ValueNumberer::VisibleValues::AddPtr
ValueNumberer::VisibleValues::findLeaderForAdd(MDefinition* def)
{
    return set_.lookupForAdd(def);
}

// Insert a value into the set.
bool
ValueNumberer::VisibleValues::add(AddPtr p, MDefinition* def)
{
    return set_.add(p, def);
}

// Insert a value onto the set overwriting any existing entry.
void
ValueNumberer::VisibleValues::overwrite(AddPtr p, MDefinition* def)
{
    set_.replaceKey(p, def);
}

// |def| will be discarded, so remove it from any sets.
void
ValueNumberer::VisibleValues::forget(const MDefinition* def)
{
    Ptr p = set_.lookup(def);
    if (p && *p == def)
        set_.remove(p);
}

// Clear all state.
void
ValueNumberer::VisibleValues::clear()
{
    set_.clear();
}

#ifdef DEBUG
// Test whether |def| is in the set.
bool
ValueNumberer::VisibleValues::has(const MDefinition* def) const
{
    Ptr p = set_.lookup(def);
    return p && *p == def;
}
#endif

// Call MDefinition::justReplaceAllUsesWith, and add some GVN-specific asserts.
static void
ReplaceAllUsesWith(MDefinition* from, MDefinition* to)
{
    MOZ_ASSERT(from != to, "GVN shouldn't try to replace a value with itself");
    MOZ_ASSERT(from->type() == to->type(), "Def replacement has different type");
    MOZ_ASSERT(!to->isDiscarded(), "GVN replaces an instruction by a removed instruction");

    // We don't need the extra setting of UseRemoved flags that the regular
    // replaceAllUsesWith does because we do it ourselves.
    from->justReplaceAllUsesWith(to);
}

// Test whether |succ| is a successor of |block|.
static bool
HasSuccessor(const MControlInstruction* block, const MBasicBlock* succ)
{
    for (size_t i = 0, e = block->numSuccessors(); i != e; ++i) {
        if (block->getSuccessor(i) == succ)
            return true;
    }
    return false;
}

// Given a block which has had predecessors removed but is still reachable, test
// whether the block's new dominator will be closer than its old one and whether
// it will expose potential optimization opportunities.
static MBasicBlock*
ComputeNewDominator(MBasicBlock* block, MBasicBlock* old)
{
    MBasicBlock* now = block->getPredecessor(0);
    for (size_t i = 1, e = block->numPredecessors(); i < e; ++i) {
        MBasicBlock* pred = block->getPredecessor(i);
        // Note that dominators haven't been recomputed yet, so we have to check
        // whether now dominates pred, not block.
        while (!now->dominates(pred)) {
            MBasicBlock* next = now->immediateDominator();
            if (next == old)
                return old;
            if (next == now) {
                MOZ_ASSERT(block == old, "Non-self-dominating block became self-dominating");
                return block;
            }
            now = next;
        }
    }
    MOZ_ASSERT(old != block || old != now, "Missed self-dominating block staying self-dominating");
    return now;
}

// Test for any defs which look potentially interesting to GVN.
static bool
BlockHasInterestingDefs(MBasicBlock* block)
{
    return !block->phisEmpty() || *block->begin() != block->lastIns();
}

// Walk up the dominator tree from |block| to the root and test for any defs
// which look potentially interesting to GVN.
static bool
ScanDominatorsForDefs(MBasicBlock* block)
{
    for (MBasicBlock* i = block;;) {
        if (BlockHasInterestingDefs(block))
            return true;

        MBasicBlock* immediateDominator = i->immediateDominator();
        if (immediateDominator == i)
            break;
        i = immediateDominator;
    }
    return false;
}

// Walk up the dominator tree from |now| to |old| and test for any defs which
// look potentially interesting to GVN.
static bool
ScanDominatorsForDefs(MBasicBlock* now, MBasicBlock* old)
{
    MOZ_ASSERT(old->dominates(now), "Refined dominator not dominated by old dominator");

    for (MBasicBlock* i = now; i != old; i = i->immediateDominator()) {
        if (BlockHasInterestingDefs(i))
            return true;
    }
    return false;
}

// Given a block which has had predecessors removed but is still reachable, test
// whether the block's new dominator will be closer than its old one and whether
// it will expose potential optimization opportunities.
static bool
IsDominatorRefined(MBasicBlock* block)
{
    MBasicBlock* old = block->immediateDominator();
    MBasicBlock* now = ComputeNewDominator(block, old);

    // If this block is just a goto and it doesn't dominate its destination,
    // removing its predecessors won't refine the dominators of anything
    // interesting.
    MControlInstruction* control = block->lastIns();
    if (*block->begin() == control && block->phisEmpty() && control->isGoto() &&
        !block->dominates(control->toGoto()->target()))
    {
        return false;
    }

    // We've computed block's new dominator. Test whether there are any
    // newly-dominating definitions which look interesting.
    if (block == old)
        return block != now && ScanDominatorsForDefs(now);
    MOZ_ASSERT(block != now, "Non-self-dominating block became self-dominating");
    return ScanDominatorsForDefs(now, old);
}

// |def| has just had one of its users release it. If it's now dead, enqueue it
// for discarding, otherwise just make note of it.
bool
ValueNumberer::handleUseReleased(MDefinition* def, UseRemovedOption useRemovedOption)
{
    if (IsDiscardable(def)) {
        values_.forget(def);
        if (!deadDefs_.append(def))
            return false;
    } else {
        if (useRemovedOption == SetUseRemoved)
            def->setUseRemovedUnchecked();
    }
    return true;
}

// Discard |def| and anything in its use-def subtree which is no longer needed.
bool
ValueNumberer::discardDefsRecursively(MDefinition* def)
{
    MOZ_ASSERT(deadDefs_.empty(), "deadDefs_ not cleared");

    return discardDef(def) && processDeadDefs();
}

// Assuming |resume| is unreachable, release its operands.
// It might be nice to integrate this code with prepareForDiscard, however GVN
// needs it to call handleUseReleased so that it can observe when a definition
// becomes unused, so it isn't trivial to do.
bool
ValueNumberer::releaseResumePointOperands(MResumePoint* resume)
{
    for (size_t i = 0, e = resume->numOperands(); i < e; ++i) {
        if (!resume->hasOperand(i))
            continue;
        MDefinition* op = resume->getOperand(i);
        resume->releaseOperand(i);

        // We set the UseRemoved flag when removing resume point operands,
        // because even though we may think we're certain that a particular
        // branch might not be taken, the type information might be incomplete.
        if (!handleUseReleased(op, SetUseRemoved))
            return false;
    }
    return true;
}

// Assuming |phi| is dead, release and remove its operands. If an operand
// becomes dead, push it to the discard worklist.
bool
ValueNumberer::releaseAndRemovePhiOperands(MPhi* phi)
{
    // MPhi saves operands in a vector so we iterate in reverse.
    for (int o = phi->numOperands() - 1; o >= 0; --o) {
        MDefinition* op = phi->getOperand(o);
        phi->removeOperand(o);
        if (!handleUseReleased(op, DontSetUseRemoved))
            return false;
    }
    return true;
}

// Assuming |def| is dead, release its operands. If an operand becomes dead,
// push it to the discard worklist.
bool
ValueNumberer::releaseOperands(MDefinition* def)
{
    for (size_t o = 0, e = def->numOperands(); o < e; ++o) {
        MDefinition* op = def->getOperand(o);
        def->releaseOperand(o);
        if (!handleUseReleased(op, DontSetUseRemoved))
            return false;
    }
    return true;
}

// Discard |def| and mine its operands for any subsequently dead defs.
bool
ValueNumberer::discardDef(MDefinition* def)
{
#ifdef JS_JITSPEW
    JitSpew(JitSpew_GVN, "      Discarding %s %s%u",
            def->block()->isMarked() ? "unreachable" : "dead",
            def->opName(), def->id());
#endif
#ifdef DEBUG
    MOZ_ASSERT(def != nextDef_, "Invalidating the MDefinition iterator");
    if (def->block()->isMarked()) {
        MOZ_ASSERT(!def->hasUses(), "Discarding def that still has uses");
    } else {
        MOZ_ASSERT(IsDiscardable(def), "Discarding non-discardable definition");
        MOZ_ASSERT(!values_.has(def), "Discarding a definition still in the set");
    }
#endif

    MBasicBlock* block = def->block();
    if (def->isPhi()) {
        MPhi* phi = def->toPhi();
        if (!releaseAndRemovePhiOperands(phi))
             return false;
        block->discardPhi(phi);
    } else {
        MInstruction* ins = def->toInstruction();
        if (MResumePoint* resume = ins->resumePoint()) {
            if (!releaseResumePointOperands(resume))
                return false;
        }
        if (!releaseOperands(ins))
             return false;
        block->discardIgnoreOperands(ins);
    }

    // If that was the last definition in the block, it can be safely removed
    // from the graph.
    if (block->phisEmpty() && block->begin() == block->end()) {
        MOZ_ASSERT(block->isMarked(), "Reachable block lacks at least a control instruction");

        // As a special case, don't remove a block which is a dominator tree
        // root so that we don't invalidate the iterator in visitGraph. We'll
        // check for this and remove it later.
        if (block->immediateDominator() != block) {
            JitSpew(JitSpew_GVN, "      Block block%u is now empty; discarding", block->id());
            graph_.removeBlock(block);
            blocksRemoved_ = true;
        } else {
            JitSpew(JitSpew_GVN, "      Dominator root block%u is now empty; will discard later",
                    block->id());
        }
    }

    return true;
}

// Recursively discard all the defs on the deadDefs_ worklist.
bool
ValueNumberer::processDeadDefs()
{
    MDefinition* nextDef = nextDef_;
    while (!deadDefs_.empty()) {
        MDefinition* def = deadDefs_.popCopy();

        // Don't invalidate the MDefinition iterator. This is what we're going
        // to visit next, so we won't miss anything.
        if (def == nextDef)
            continue;

        if (!discardDef(def))
            return false;
    }
    return true;
}

// Test whether |block|, which is a loop header, has any predecessors other than
// |loopPred|, the loop predecessor, which it doesn't dominate.
static bool
hasNonDominatingPredecessor(MBasicBlock* block, MBasicBlock* loopPred)
{
    MOZ_ASSERT(block->isLoopHeader());
    MOZ_ASSERT(block->loopPredecessor() == loopPred);

    for (uint32_t i = 0, e = block->numPredecessors(); i < e; ++i) {
        MBasicBlock* pred = block->getPredecessor(i);
        if (pred != loopPred && !block->dominates(pred))
            return true;
    }
    return false;
}

// A loop is about to be made reachable only through an OSR entry into one of
// its nested loops. Fix everything up.
bool
ValueNumberer::fixupOSROnlyLoop(MBasicBlock* block, MBasicBlock* backedge)
{
    // Create an empty and unreachable(!) block which jumps to |block|. This
    // allows |block| to remain marked as a loop header, so we don't have to
    // worry about moving a different block into place as the new loop header,
    // which is hard, especially if the OSR is into a nested loop. Doing all
    // that would produce slightly more optimal code, but this is so
    // extraordinarily rare that it isn't worth the complexity.
    MBasicBlock* fake = MBasicBlock::New(graph_, block->info(), nullptr, MBasicBlock::NORMAL);
    if (fake == nullptr)
        return false;

    graph_.insertBlockBefore(block, fake);
    fake->setImmediateDominator(fake);
    fake->addNumDominated(1);
    fake->setDomIndex(fake->id());
    fake->setUnreachable();

    // Create zero-input phis to use as inputs for any phis in |block|.
    // Again, this is a little odd, but it's the least-odd thing we can do
    // without significant complexity.
    for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd()); iter != end; ++iter) {
        MPhi* phi = *iter;
        MPhi* fakePhi = MPhi::New(graph_.alloc(), phi->type());
        fake->addPhi(fakePhi);
        if (!phi->addInputSlow(fakePhi))
            return false;
    }

    fake->end(MGoto::New(graph_.alloc(), block));

    if (!block->addPredecessorWithoutPhis(fake))
        return false;

    // Restore |backedge| as |block|'s loop backedge.
    block->clearLoopHeader();
    block->setLoopHeader(backedge);

    JitSpew(JitSpew_GVN, "        Created fake block%u", fake->id());
    hasOSRFixups_ = true;
    return true;
}

// Remove the CFG edge between |pred| and |block|, after releasing the phi
// operands on that edge and discarding any definitions consequently made dead.
bool
ValueNumberer::removePredecessorAndDoDCE(MBasicBlock* block, MBasicBlock* pred, size_t predIndex)
{
    MOZ_ASSERT(!block->isMarked(),
               "Block marked unreachable should have predecessors removed already");

    // Before removing the predecessor edge, scan the phi operands for that edge
    // for dead code before they get removed.
    MOZ_ASSERT(nextDef_ == nullptr);
    for (MPhiIterator iter(block->phisBegin()), end(block->phisEnd()); iter != end; ) {
        MPhi* phi = *iter++;
        MOZ_ASSERT(!values_.has(phi), "Visited phi in block having predecessor removed");
        MOZ_ASSERT(!phi->isGuard());

        MDefinition* op = phi->getOperand(predIndex);
        phi->removeOperand(predIndex);

        nextDef_ = iter != end ? *iter : nullptr;
        if (!handleUseReleased(op, DontSetUseRemoved) || !processDeadDefs())
            return false;

        // If |nextDef_| became dead while we had it pinned, advance the
        // iterator and discard it now.
        while (nextDef_ && !nextDef_->hasUses() && !nextDef_->isGuardRangeBailouts()) {
            phi = nextDef_->toPhi();
            iter++;
            nextDef_ = iter != end ? *iter : nullptr;
            if (!discardDefsRecursively(phi))
                return false;
        }
    }
    nextDef_ = nullptr;

    block->removePredecessorWithoutPhiOperands(pred, predIndex);
    return true;
}

// Remove the CFG edge between |pred| and |block|, and if this makes |block|
// unreachable, mark it so, and remove the rest of its incoming edges too. And
// discard any instructions made dead by the entailed release of any phi
// operands.
bool
ValueNumberer::removePredecessorAndCleanUp(MBasicBlock* block, MBasicBlock* pred)
{
    MOZ_ASSERT(!block->isMarked(), "Removing predecessor on block already marked unreachable");

    // We'll be removing a predecessor, so anything we know about phis in this
    // block will be wrong.
    for (MPhiIterator iter(block->phisBegin()),