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//
// Copyright (c) 2015 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.
//
// RewriteDoWhile.cpp: rewrites do-while loops using another equivalent
// construct.
#include "compiler/translator/RewriteDoWhile.h"
#include "compiler/translator/IntermNode.h"
namespace
{
// An AST traverser that rewrites loops of the form
// do {
// CODE;
// } while (CONDITION)
//
// to loops of the form
// bool temp = false;
// while (true) {
// if (temp) {
// if (!CONDITION) {
// break;
// }
// }
// temp = true;
// CODE;
// }
//
// The reason we don't use a simpler form, with for example just (temp && !CONDITION) in the
// while condition, is that short-circuit is often badly supported by driver shader compiler.
// The double if has the same effect, but forces shader compilers to behave.
//
// TODO(cwallez) when UnfoldShortCircuitIntoIf handles loops correctly, revisit this as we might
// be able to use while (temp || CONDITION) with temp initially set to true then run
// UnfoldShortCircuitIntoIf
class DoWhileRewriter : public TIntermTraverser
{
public:
DoWhileRewriter() : TIntermTraverser(true, false, false) {}
bool visitAggregate(Visit, TIntermAggregate *node) override
{
// A well-formed AST can only have do-while in EOpSequence which represent lists of
// statements. By doing a prefix traversal we are able to replace the do-while in the
// sequence directly as the content of the do-while will be traversed later.
if (node->getOp() != EOpSequence)
{
return true;
}
TIntermSequence *statements = node->getSequence();
// The statements vector will have new statements inserted when we encounter a do-while,
// which prevents us from using a range-based for loop. Using the usual i++ works, as
// the (two) new statements inserted replace the statement at the current position.
for (size_t i = 0; i < statements->size(); i++)
{
TIntermNode *statement = (*statements)[i];
TIntermLoop *loop = statement->getAsLoopNode();
if (loop == nullptr || loop->getType() != ELoopDoWhile)
{
continue;
}
TType boolType = TType(EbtBool);
// bool temp = false;
TIntermAggregate *tempDeclaration = nullptr;
{
TConstantUnion *falseConstant = new TConstantUnion();
falseConstant->setBConst(false);
TIntermTyped *falseValue = new TIntermConstantUnion(falseConstant, boolType);
tempDeclaration = createTempInitDeclaration(falseValue);
}
// temp = true;
TIntermBinary *assignTrue = nullptr;
{
TConstantUnion *trueConstant = new TConstantUnion();
trueConstant->setBConst(true);
TIntermTyped *trueValue = new TIntermConstantUnion(trueConstant, boolType);
assignTrue = createTempAssignment(trueValue);
}
// if (temp) {
// if (!CONDITION) {
// break;
// }
// }
TIntermSelection *breakIf = nullptr;
{
TIntermBranch *breakStatement = new TIntermBranch(EOpBreak, nullptr);
TIntermAggregate *breakBlock = new TIntermAggregate(EOpSequence);
breakBlock->getSequence()->push_back(breakStatement);
TIntermUnary *negatedCondition = new TIntermUnary(EOpLogicalNot);
negatedCondition->setOperand(loop->getCondition());
TIntermSelection *innerIf =
new TIntermSelection(negatedCondition, breakBlock, nullptr);
TIntermAggregate *innerIfBlock = new TIntermAggregate(EOpSequence);
innerIfBlock->getSequence()->push_back(innerIf);
breakIf = new TIntermSelection(createTempSymbol(boolType), innerIfBlock, nullptr);
}
// Assemble the replacement loops, reusing the do-while loop's body and inserting our
// statements at the front.
TIntermLoop *newLoop = nullptr;
{
TConstantUnion *trueConstant = new TConstantUnion();
trueConstant->setBConst(true);
TIntermTyped *trueValue = new TIntermConstantUnion(trueConstant, boolType);
TIntermAggregate *body = nullptr;
if (loop->getBody() != nullptr)
{
body = loop->getBody()->getAsAggregate();
}
else
{
body = new TIntermAggregate(EOpSequence);
}
auto sequence = body->getSequence();
sequence->insert(sequence->begin(), assignTrue);
sequence->insert(sequence->begin(), breakIf);
newLoop = new TIntermLoop(ELoopWhile, nullptr, trueValue, nullptr, body);
}
TIntermSequence replacement;
replacement.push_back(tempDeclaration);
replacement.push_back(newLoop);
node->replaceChildNodeWithMultiple(loop, replacement);
nextTemporaryIndex();
}
return true;
}
};
} // anonymous namespace
void RewriteDoWhile(TIntermNode *root, unsigned int *temporaryIndex)
{
ASSERT(temporaryIndex != 0);
DoWhileRewriter rewriter;
rewriter.useTemporaryIndex(temporaryIndex);
root->traverse(&rewriter);
}
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