SPIRV/InReadableOrder.cpp

//
//Copyright (C) 2016 LunarG, Inc.
//
//All rights reserved.
//
//Redistribution and use in source and binary forms, with or without
//modification, are permitted provided that the following conditions
//are met:
//
//    Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
//
//    Redistributions in binary form must reproduce the above
//    copyright notice, this list of conditions and the following
//    disclaimer in the documentation and/or other materials provided
//    with the distribution.
//
//    Neither the name of 3Dlabs Inc. Ltd. nor the names of its
//    contributors may be used to endorse or promote products derived
//    from this software without specific prior written permission.
//
//THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
//"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
//LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
//FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
//COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
//INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
//BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
//LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
//CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
//LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
//ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
//POSSIBILITY OF SUCH DAMAGE.

//
// Author: Dejan Mircevski, Google
//

// The SPIR-V spec requires code blocks to appear in an order satisfying the
// dominator-tree direction (ie, dominator before the dominated).  This is,
// actually, easy to achieve: any pre-order CFG traversal algorithm will do it.
// Because such algorithms visit a block only after traversing some path to it
// from the root, they necessarily visit the block's idom first.
//
// But not every graph-traversal algorithm outputs blocks in an order that
// appears logical to human readers.  The problem is that unrelated branches may
// be interspersed with each other, and merge blocks may come before some of the
// branches being merged.
//
// A good, human-readable order of blocks may be achieved by performing
// depth-first search but delaying merge nodes until after all their branches
// have been visited.  This is implemented below by the inReadableOrder()
// function.

#include "spvIR.h"

#include <cassert>
#include <unordered_map>

using spv::Block;
using spv::Id;

namespace {
// Traverses CFG in a readable order, invoking a pre-set callback on each block.
// Use by calling visit() on the root block.
class ReadableOrderTraverser {
 public:
  explicit ReadableOrderTraverser(std::function<void(Block*)> callback)
      : callback_(callback) {}

  // Visits the block if it hasn't been visited already and isn't currently
  // being delayed.  Invokes callback(block), then descends into its successors.
  // Delays merge-block processing until all the branches have been completed.
  void visit(Block* block) {
    assert(block);
    if (visited_[block] || delayed_[block]) return;
    callback_(block);
    visited_[block] = true;
    Block* mergeBlock = nullptr;
    auto mergeInst = block->getMergeInstruction();
    if (mergeInst) {
      Id mergeId = mergeInst->getIdOperand(0);
      mergeBlock =
          block->getParent().getParent().getInstruction(mergeId)->getBlock();
      delayed_[mergeBlock] = true;
    }
    for (const auto succ : block->getSuccessors())
      if (succ != mergeBlock) visit(succ);
    if (mergeBlock) {
      delayed_[mergeBlock] = false;
      visit(mergeBlock);
    }
  }

 private:
  std::function<void(Block*)> callback_;
  // Whether a block has already been visited or is being delayed.
  std::unordered_map<Block*, bool> visited_, delayed_;
};
}

void spv::inReadableOrder(Block* root, std::function<void(Block*)> callback) {
  ReadableOrderTraverser(callback).visit(root);
}