Line data    Source code

       1             : //
       2             : // Copyright (c) 2015 The ANGLE Project Authors. All rights reserved.
       3             : // Use of this source code is governed by a BSD-style license that can be
       4             : // found in the LICENSE file.
       5             : //
       6             : 
       7             : // RewriteDoWhile.cpp: rewrites do-while loops using another equivalent
       8             : // construct.
       9             : 
      10             : #include "compiler/translator/RewriteDoWhile.h"
      11             : 
      12             : #include "compiler/translator/IntermNode.h"
      13             : 
      14             : namespace sh
      15             : {
      16             : 
      17             : namespace
      18             : {
      19             : 
      20             : // An AST traverser that rewrites loops of the form
      21             : //   do {
      22             : //     CODE;
      23             : //   } while (CONDITION)
      24             : //
      25             : // to loops of the form
      26             : //   bool temp = false;
      27             : //   while (true) {
      28             : //     if (temp) {
      29             : //       if (!CONDITION) {
      30             : //         break;
      31             : //       }
      32             : //     }
      33             : //     temp = true;
      34             : //     CODE;
      35             : //   }
      36             : //
      37             : // The reason we don't use a simpler form, with for example just (temp && !CONDITION) in the
      38             : // while condition, is that short-circuit is often badly supported by driver shader compiler.
      39             : // The double if has the same effect, but forces shader compilers to behave.
      40             : //
      41             : // TODO(cwallez) when UnfoldShortCircuitIntoIf handles loops correctly, revisit this as we might
      42             : // be able to use while (temp || CONDITION) with temp initially set to true then run
      43             : // UnfoldShortCircuitIntoIf
      44           0 : class DoWhileRewriter : public TIntermTraverser
      45             : {
      46             :   public:
      47           0 :     DoWhileRewriter() : TIntermTraverser(true, false, false) {}
      48             : 
      49           0 :     bool visitBlock(Visit, TIntermBlock *node) override
      50             :     {
      51             :         // A well-formed AST can only have do-while inside TIntermBlock. By doing a prefix traversal
      52             :         // we are able to replace the do-while in the sequence directly as the content of the
      53             :         // do-while will be traversed later.
      54             : 
      55           0 :         TIntermSequence *statements = node->getSequence();
      56             : 
      57             :         // The statements vector will have new statements inserted when we encounter a do-while,
      58             :         // which prevents us from using a range-based for loop. Using the usual i++ works, as
      59             :         // the (two) new statements inserted replace the statement at the current position.
      60           0 :         for (size_t i = 0; i < statements->size(); i++)
      61             :         {
      62           0 :             TIntermNode *statement = (*statements)[i];
      63           0 :             TIntermLoop *loop      = statement->getAsLoopNode();
      64             : 
      65           0 :             if (loop == nullptr || loop->getType() != ELoopDoWhile)
      66             :             {
      67           0 :                 continue;
      68             :             }
      69             : 
      70           0 :             TType boolType = TType(EbtBool);
      71             : 
      72             :             // bool temp = false;
      73           0 :             TIntermDeclaration *tempDeclaration = nullptr;
      74             :             {
      75           0 :                 TConstantUnion *falseConstant = new TConstantUnion();
      76           0 :                 falseConstant->setBConst(false);
      77           0 :                 TIntermTyped *falseValue = new TIntermConstantUnion(falseConstant, boolType);
      78             : 
      79           0 :                 tempDeclaration = createTempInitDeclaration(falseValue);
      80             :             }
      81             : 
      82             :             // temp = true;
      83           0 :             TIntermBinary *assignTrue = nullptr;
      84             :             {
      85           0 :                 TConstantUnion *trueConstant = new TConstantUnion();
      86           0 :                 trueConstant->setBConst(true);
      87           0 :                 TIntermTyped *trueValue = new TIntermConstantUnion(trueConstant, boolType);
      88             : 
      89           0 :                 assignTrue = createTempAssignment(trueValue);
      90             :             }
      91             : 
      92             :             // if (temp) {
      93             :             //   if (!CONDITION) {
      94             :             //     break;
      95             :             //   }
      96             :             // }
      97           0 :             TIntermIfElse *breakIf = nullptr;
      98             :             {
      99           0 :                 TIntermBranch *breakStatement = new TIntermBranch(EOpBreak, nullptr);
     100             : 
     101           0 :                 TIntermBlock *breakBlock = new TIntermBlock();
     102           0 :                 breakBlock->getSequence()->push_back(breakStatement);
     103             : 
     104             :                 TIntermUnary *negatedCondition =
     105           0 :                     new TIntermUnary(EOpLogicalNot, loop->getCondition());
     106             : 
     107           0 :                 TIntermIfElse *innerIf = new TIntermIfElse(negatedCondition, breakBlock, nullptr);
     108             : 
     109           0 :                 TIntermBlock *innerIfBlock = new TIntermBlock();
     110           0 :                 innerIfBlock->getSequence()->push_back(innerIf);
     111             : 
     112           0 :                 breakIf = new TIntermIfElse(createTempSymbol(boolType), innerIfBlock, nullptr);
     113             :             }
     114             : 
     115             :             // Assemble the replacement loops, reusing the do-while loop's body and inserting our
     116             :             // statements at the front.
     117           0 :             TIntermLoop *newLoop = nullptr;
     118             :             {
     119           0 :                 TConstantUnion *trueConstant = new TConstantUnion();
     120           0 :                 trueConstant->setBConst(true);
     121           0 :                 TIntermTyped *trueValue = new TIntermConstantUnion(trueConstant, boolType);
     122             : 
     123           0 :                 TIntermBlock *body = loop->getBody();
     124           0 :                 if (body == nullptr)
     125             :                 {
     126           0 :                     body = new TIntermBlock();
     127             :                 }
     128           0 :                 auto sequence = body->getSequence();
     129           0 :                 sequence->insert(sequence->begin(), assignTrue);
     130           0 :                 sequence->insert(sequence->begin(), breakIf);
     131             : 
     132           0 :                 newLoop = new TIntermLoop(ELoopWhile, nullptr, trueValue, nullptr, body);
     133             :             }
     134             : 
     135           0 :             TIntermSequence replacement;
     136           0 :             replacement.push_back(tempDeclaration);
     137           0 :             replacement.push_back(newLoop);
     138             : 
     139           0 :             node->replaceChildNodeWithMultiple(loop, replacement);
     140             : 
     141           0 :             nextTemporaryIndex();
     142             :         }
     143           0 :         return true;
     144             :     }
     145             : };
     146             : 
     147             : }  // anonymous namespace
     148             : 
     149           0 : void RewriteDoWhile(TIntermNode *root, unsigned int *temporaryIndex)
     150             : {
     151           0 :     ASSERT(temporaryIndex != 0);
     152             : 
     153           0 :     DoWhileRewriter rewriter;
     154           0 :     rewriter.useTemporaryIndex(temporaryIndex);
     155             : 
     156           0 :     root->traverse(&rewriter);
     157           0 : }
     158             : 
     159             : }  // namespace sh