kc3-lang/angle/src/compiler/translator/RewriteDoWhile.cpp

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• Author : Corentin Wallez
  Date : 2015-09-28 12:19:26
  Hash : d4b5054d
  Message : compiler: Rewrite do-while loops as while loops This works around a Mac driver shader compiler bug that makes many do-while loops cause GPU-hangs when ran. BUG=angleproject:891 Change-Id: I29828d6ea9e887ad0ed0c577f1deb41fb632a900 Reviewed-on: https://chromium-review.googlesource.com/302465 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>

• src/compiler/translator/RewriteDoWhile.cpp

• //
  // 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);
  }