src/compiler/translator/SimplifyLoopConditions.cpp

//
// Copyright (c) 2016 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.
//
// SimplifyLoopConditions is an AST traverser that converts loop conditions and loop expressions
// to regular statements inside the loop. This way further transformations that generate statements
// from loop conditions and loop expressions work correctly.
//

#include "compiler/translator/SimplifyLoopConditions.h"

#include "compiler/translator/IntermNode.h"
#include "compiler/translator/IntermNodePatternMatcher.h"

namespace
{

TIntermConstantUnion *CreateBoolConstantNode(bool value)
{
    TConstantUnion *u = new TConstantUnion;
    u->setBConst(value);
    TIntermConstantUnion *node =
        new TIntermConstantUnion(u, TType(EbtBool, EbpUndefined, EvqConst, 1));
    return node;
}

class SimplifyLoopConditionsTraverser : public TLValueTrackingTraverser
{
  public:
    SimplifyLoopConditionsTraverser(unsigned int conditionsToSimplifyMask,
                                    const TSymbolTable &symbolTable,
                                    int shaderVersion);

    void traverseLoop(TIntermLoop *node) override;

    bool visitBinary(Visit visit, TIntermBinary *node) override;
    bool visitAggregate(Visit visit, TIntermAggregate *node) override;
    bool visitTernary(Visit visit, TIntermTernary *node) override;

    void nextIteration();
    bool foundLoopToChange() const { return mFoundLoopToChange; }

  protected:
    // Marked to true once an operation that needs to be hoisted out of the expression has been
    // found. After that, no more AST updates are performed on that traversal.
    bool mFoundLoopToChange;
    bool mInsideLoopConditionOrExpression;
    IntermNodePatternMatcher mConditionsToSimplify;
};

SimplifyLoopConditionsTraverser::SimplifyLoopConditionsTraverser(
    unsigned int conditionsToSimplifyMask,
    const TSymbolTable &symbolTable,
    int shaderVersion)
    : TLValueTrackingTraverser(true, false, false, symbolTable, shaderVersion),
      mFoundLoopToChange(false),
      mInsideLoopConditionOrExpression(false),
      mConditionsToSimplify(conditionsToSimplifyMask)
{
}

void SimplifyLoopConditionsTraverser::nextIteration()
{
    mFoundLoopToChange               = false;
    mInsideLoopConditionOrExpression = false;
    nextTemporaryIndex();
}

bool SimplifyLoopConditionsTraverser::visitBinary(Visit visit, TIntermBinary *node)
{
    // The visit functions operate in three modes:
    // 1. If a matching expression has already been found, we return early since only one loop can
    //    be transformed on one traversal.
    // 2. We try to find loops. In case a node is not inside a loop and can not contain loops, we
    //    stop traversing the subtree.
    // 3. If we're inside a loop condition or expression, we check for expressions that should be
    //    moved out of the loop condition or expression. If one is found, the loop is processed.

    if (mFoundLoopToChange)
        return false;

    if (!mInsideLoopConditionOrExpression)
        return false;

    mFoundLoopToChange = mConditionsToSimplify.match(node, getParentNode(), isLValueRequiredHere());
    return !mFoundLoopToChange;
}

bool SimplifyLoopConditionsTraverser::visitAggregate(Visit visit, TIntermAggregate *node)
{
    if (mFoundLoopToChange)
        return false;

    // If we're outside a loop condition, we only need to traverse nodes that may contain loops.
    if (!mInsideLoopConditionOrExpression)
        return false;

    mFoundLoopToChange = mConditionsToSimplify.match(node, getParentNode());
    return !mFoundLoopToChange;
}

bool SimplifyLoopConditionsTraverser::visitTernary(Visit visit, TIntermTernary *node)
{
    if (mFoundLoopToChange)
        return false;

    // Don't traverse ternary operators outside loop conditions.
    if (!mInsideLoopConditionOrExpression)
        return false;

    mFoundLoopToChange = mConditionsToSimplify.match(node);
    return !mFoundLoopToChange;
}

void SimplifyLoopConditionsTraverser::traverseLoop(TIntermLoop *node)
{
    if (mFoundLoopToChange)
        return;

    // Mark that we're inside a loop condition or expression, and transform the loop if needed.

    incrementDepth(node);

    // Note: No need to traverse the loop init node.

    mInsideLoopConditionOrExpression = true;
    TLoopType loopType               = node->getType();

    if (node->getCondition())
    {
        node->getCondition()->traverse(this);

        if (mFoundLoopToChange)
        {
            // Replace the loop condition with a boolean variable that's updated on each iteration.
            if (loopType == ELoopWhile)
            {
                // Transform:
                //   while (expr) { body; }
                // into
                //   bool s0 = expr;
                //   while (s0) { { body; } s0 = expr; }
                TIntermSequence tempInitSeq;
                tempInitSeq.push_back(createTempInitDeclaration(node->getCondition()->deepCopy()));
                insertStatementsInParentBlock(tempInitSeq);

                TIntermBlock *newBody = new TIntermBlock();
                if (node->getBody())
                {
                    newBody->getSequence()->push_back(node->getBody());
                }
                newBody->getSequence()->push_back(
                    createTempAssignment(node->getCondition()->deepCopy()));

                // Can't use queueReplacement to replace old body, since it may have been nullptr.
                // It's safe to do the replacements in place here - this node won't be traversed
                // further.
                node->setBody(newBody);
                node->setCondition(createTempSymbol(node->getCondition()->getType()));
            }
            else if (loopType == ELoopDoWhile)
            {
                // Transform:
                //   do {
                //     body;
                //   } while (expr);
                // into
                //   bool s0 = true;
                //   do {
                //     { body; }
                //     s0 = expr;
                //   while (s0);
                TIntermSequence tempInitSeq;
                tempInitSeq.push_back(createTempInitDeclaration(CreateBoolConstantNode(true)));
                insertStatementsInParentBlock(tempInitSeq);

                TIntermBlock *newBody = new TIntermBlock();
                if (node->getBody())
                {
                    newBody->getSequence()->push_back(node->getBody());
                }
                newBody->getSequence()->push_back(
                    createTempAssignment(node->getCondition()->deepCopy()));

                // Can't use queueReplacement to replace old body, since it may have been nullptr.
                // It's safe to do the replacements in place here - this node won't be traversed
                // further.
                node->setBody(newBody);
                node->setCondition(createTempSymbol(node->getCondition()->getType()));
            }
            else if (loopType == ELoopFor)
            {
                // Move the loop condition inside the loop.
                // Transform:
                //   for (init; expr; exprB) { body; }
                // into
                //   {
                //     init;
                //     bool s0 = expr;
                //     while (s0) { { body; } exprB; s0 = expr; }
                //   }
                TIntermBlock *loopScope = new TIntermBlock();
                if (node->getInit())
                {
                    loopScope->getSequence()->push_back(node->getInit());
                }
                loopScope->getSequence()->push_back(
                    createTempInitDeclaration(node->getCondition()->deepCopy()));

                TIntermBlock *whileLoopBody = new TIntermBlock();
                if (node->getBody())
                {
                    whileLoopBody->getSequence()->push_back(node->getBody());
                }
                whileLoopBody->getSequence()->push_back(node->getExpression());
                whileLoopBody->getSequence()->push_back(
                    createTempAssignment(node->getCondition()->deepCopy()));
                TIntermLoop *whileLoop = new TIntermLoop(
                    ELoopWhile, nullptr, createTempSymbol(node->getCondition()->getType()), nullptr,
                    whileLoopBody);
                loopScope->getSequence()->push_back(whileLoop);
                queueReplacementWithParent(getAncestorNode(1), node, loopScope,
                                           OriginalNode::IS_DROPPED);
            }
        }
    }

    if (!mFoundLoopToChange && node->getExpression())
    {
        node->getExpression()->traverse(this);

        if (mFoundLoopToChange)
        {
            ASSERT(loopType == ELoopFor);
            // Move the loop expression to inside the loop.
            // Transform:
            //   for (init; expr; exprB) { body; }
            // into
            //   for (init; expr; ) { { body; } exprB; }
            TIntermTyped *loopExpression = node->getExpression();
            node->setExpression(nullptr);
            TIntermBlock *oldBody = node->getBody();
            node->setBody(new TIntermBlock());
            if (oldBody != nullptr)
            {
                node->getBody()->getSequence()->push_back(oldBody);
            }
            node->getBody()->getSequence()->push_back(loopExpression);
        }
    }

    mInsideLoopConditionOrExpression = false;

    if (!mFoundLoopToChange && node->getBody())
        node->getBody()->traverse(this);

    decrementDepth();
}

}  // namespace

void SimplifyLoopConditions(TIntermNode *root,
                            unsigned int conditionsToSimplifyMask,
                            unsigned int *temporaryIndex,
                            const TSymbolTable &symbolTable,
                            int shaderVersion)
{
    SimplifyLoopConditionsTraverser traverser(conditionsToSimplifyMask, symbolTable, shaderVersion);
    ASSERT(temporaryIndex != nullptr);
    traverser.useTemporaryIndex(temporaryIndex);
    // Process one loop at a time, and reset the traverser between iterations.
    do
    {
        traverser.nextIteration();
        root->traverse(&traverser);
        if (traverser.foundLoopToChange())
            traverser.updateTree();
    } while (traverser.foundLoopToChange());
}