src/third_party/angle/src/compiler/translator/tree_ops/RecordConstantPrecision.cpp

//
// Copyright 2002 The ANGLE Project Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
//
// During parsing, all constant expressions are folded to constant union nodes. The expressions that
// have been folded may have had precision qualifiers, which should affect the precision of the
// consuming operation. If the folded constant union nodes are written to output as such they won't
// have any precision qualifiers, and their effect on the precision of the consuming operation is
// lost.
//
// RecordConstantPrecision is an AST traverser that inspects the precision qualifiers of constants
// and hoists the constants outside the containing expression as precision qualified named variables
// in case that is required for correct precision propagation.
//

#include "compiler/translator/tree_ops/RecordConstantPrecision.h"

#include "compiler/translator/InfoSink.h"
#include "compiler/translator/tree_util/IntermNode_util.h"
#include "compiler/translator/tree_util/IntermTraverse.h"

namespace sh
{

namespace
{

class RecordConstantPrecisionTraverser : public TIntermTraverser
{
  public:
    RecordConstantPrecisionTraverser(TSymbolTable *symbolTable);

    void visitConstantUnion(TIntermConstantUnion *node) override;

    void nextIteration();

    bool foundHigherPrecisionConstant() const { return mFoundHigherPrecisionConstant; }

  protected:
    bool operandAffectsParentOperationPrecision(TIntermTyped *operand);

    bool mFoundHigherPrecisionConstant;
};

RecordConstantPrecisionTraverser::RecordConstantPrecisionTraverser(TSymbolTable *symbolTable)
    : TIntermTraverser(true, false, true, symbolTable), mFoundHigherPrecisionConstant(false)
{}

bool RecordConstantPrecisionTraverser::operandAffectsParentOperationPrecision(TIntermTyped *operand)
{
    if (getParentNode()->getAsCaseNode() || getParentNode()->getAsBlock())
    {
        return false;
    }

    const TIntermBinary *parentAsBinary = getParentNode()->getAsBinaryNode();
    if (parentAsBinary != nullptr)
    {
        // If the constant is assigned or is used to initialize a variable, or if it's an index,
        // its precision has no effect.
        switch (parentAsBinary->getOp())
        {
            case EOpInitialize:
            case EOpAssign:
            case EOpIndexDirect:
            case EOpIndexDirectStruct:
            case EOpIndexDirectInterfaceBlock:
            case EOpIndexIndirect:
                return false;
            default:
                break;
        }

        TIntermTyped *otherOperand = parentAsBinary->getRight();
        if (otherOperand == operand)
        {
            otherOperand = parentAsBinary->getLeft();
        }
        // If the precision of the other child is at least as high as the precision of the constant,
        // the precision of the constant has no effect.
        if (otherOperand->getAsConstantUnion() == nullptr &&
            otherOperand->getPrecision() >= operand->getPrecision())
        {
            return false;
        }
    }

    TIntermAggregate *parentAsAggregate = getParentNode()->getAsAggregate();
    if (parentAsAggregate != nullptr)
    {
        if (!parentAsAggregate->gotPrecisionFromChildren())
        {
            // This can be either:
            // * a call to an user-defined function
            // * a call to a texture function
            // * some other kind of aggregate
            // In any of these cases the constant precision has no effect.
            return false;
        }
        if (parentAsAggregate->isConstructor() && parentAsAggregate->getBasicType() == EbtBool)
        {
            return false;
        }
        // If the precision of operands does affect the result, but the precision of any of the
        // other children has a precision that's at least as high as the precision of the constant,
        // the precision of the constant has no effect.
        TIntermSequence *parameters = parentAsAggregate->getSequence();
        for (TIntermNode *parameter : *parameters)
        {
            const TIntermTyped *typedParameter = parameter->getAsTyped();
            if (parameter != operand && typedParameter != nullptr &&
                parameter->getAsConstantUnion() == nullptr &&
                typedParameter->getPrecision() >= operand->getPrecision())
            {
                return false;
            }
        }
    }
    return true;
}

void RecordConstantPrecisionTraverser::visitConstantUnion(TIntermConstantUnion *node)
{
    if (mFoundHigherPrecisionConstant)
        return;

    // If the constant has lowp or undefined precision, it can't increase the precision of consuming
    // operations.
    if (node->getPrecision() < EbpMedium)
        return;

    // It's possible the node has no effect on the precision of the consuming expression, depending
    // on the consuming expression, and the precision of the other parameters of the expression.
    if (!operandAffectsParentOperationPrecision(node))
        return;

    // Make the constant a precision-qualified named variable to make sure it affects the precision
    // of the consuming expression.
    TIntermDeclaration *variableDeclaration = nullptr;
    TVariable *variable = DeclareTempVariable(mSymbolTable, node, EvqConst, &variableDeclaration);
    insertStatementInParentBlock(variableDeclaration);
    queueReplacement(CreateTempSymbolNode(variable), OriginalNode::IS_DROPPED);
    mFoundHigherPrecisionConstant = true;
}

void RecordConstantPrecisionTraverser::nextIteration()
{
    mFoundHigherPrecisionConstant = false;
}

}  // namespace

bool RecordConstantPrecision(TCompiler *compiler, TIntermNode *root, TSymbolTable *symbolTable)
{
    RecordConstantPrecisionTraverser traverser(symbolTable);
    // Iterate as necessary, and reset the traverser between iterations.
    do
    {
        traverser.nextIteration();
        root->traverse(&traverser);
        if (traverser.foundHigherPrecisionConstant())
        {
            if (!traverser.updateTree(compiler, root))
            {
                return false;
            }
        }
    } while (traverser.foundHigherPrecisionConstant());

    return true;
}

}  // namespace sh