path: root/js/src/jit/LICM.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * 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->isConstantElements())
        return true;

    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;
}