Source/JavaScriptCore/b3/B3ReduceDoubleToFloat.cpp

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 * Copyright (C) 2015 Apple Inc. All rights reserved.
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 * modification, are permitted provided that the following conditions
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 * 1. Redistributions of source code must retain the above copyright
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 *    notice, this list of conditions and the following disclaimer in the
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 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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#include "config.h"
#include "B3ReduceDoubleToFloat.h"

#if ENABLE(B3_JIT)

#include "B3BasicBlock.h"
#include "B3PhaseScope.h"
#include "B3ValueInlines.h"

namespace JSC { namespace B3 {

namespace {
void attemptSimplification(Value* candidate)
{
    switch (candidate->opcode()) {
    case Add:
    case Sub:
    case Mul:
    case Div:
        if (candidate->child(0)->opcode() == FloatToDouble && candidate->child(1)->opcode() == FloatToDouble) {
            candidate->child(0) = candidate->child(0)->child(0);
            candidate->child(1) = candidate->child(1)->child(0);
            candidate->setType(Float);
        }
        break;
    case Abs:
    case Ceil:
    case Sqrt:
        if (candidate->child(0)->opcode() == FloatToDouble) {
            candidate->child(0) = candidate->child(0)->child(0);
            candidate->setType(Float);
        }
        break;
    default:
        break;
    }
}

} // namespace

// The goal of this phase is to transform Double operations
// into float if the Double result is never used as Double.
//
// In C, that would be something like:
//     float add(float a, float b) {
//         return (double)a + (double)b;
//     }
//
// Such operation arise in JS because there are is no Float type
// and float operations are generated by adding double-to-float rounding.
//
// The operations can be done entirely without Double conversion.
// Using float in place remove the useless conversion, and the float
// ops is sometime massively cheaper (SQRT for example).
//
// If the Double value is used as Double, we do not do the conversion.
// It is cheaper to do a conversion than repeat any floating-point operation.
void reduceDoubleToFloat(Procedure& procedure)
{
    // FIXME: We should tune this phase for performance and make it part of ReduceStrength.
    // ReduceStrength can eliminate nodes that prevents us from simplifying operations.
    PhaseScope phaseScope(procedure, "reduceDoubleToFloat");

    HashSet<Value*> candidates;

    // First, we find any value that is converted to float
    // and only used as float.
    // We also simplify comparisons since that's always safe and we
    // don't want them to appear in the next loop.
    for (BasicBlock* block : procedure) {
        for (Value* value : *block) {
            value->performSubstitution();

            switch (value->opcode()) {
            case DoubleToFloat:
                candidates.add(value->child(0));
                break;
            case Equal:
            case NotEqual:
            case LessThan:
            case GreaterThan:
            case LessEqual:
            case GreaterEqual:
            case EqualOrUnordered:
                if (value->child(0)->opcode() == FloatToDouble && value->child(1)->opcode() == FloatToDouble) {
                    value->child(0) = value->child(0)->child(0);
                    value->child(1) = value->child(1)->child(0);
                }
                break;
            default:
                break;
            }
        }
    }

    for (BasicBlock* block : procedure) {
        for (Value* value : *block) {
            if (value->opcode() == DoubleToFloat)
                continue;

            for (Value* child : value->children()) {
                if (child->type() == Double)
                    candidates.remove(child);
            }
        }
    }

    if (candidates.isEmpty())
        return;

    // Second, we go over the candidates and attempt to simplify them.
    // This leaves the graph in an invalid state where Float Values are
    // used by DoubleToFloat Values. This is only temporary.
    for (Value* candidate : candidates)
        attemptSimplification(candidate);

    // Finally, remove the DoubleToFloat made useless by the simplifications.
    for (BasicBlock* block : procedure) {
        for (Value* value : *block) {
            if (value->opcode() == DoubleToFloat && value->child(0)->type() == Float)
                value->replaceWithIdentity(value->child(0));
        }
    }

    // We do not clean all the useless nodes and conversions. ReduceStrength does that better.
}

} } // namespace JSC::B3

#endif // ENABLE(B3_JIT)