SPIRV/InReadableOrder.cpp

//
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// 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_set>

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 and continue-block processing until all
    // the branches have been completed.
    void visit(Block* block)
    {
        assert(block);
        if (visited_.count(block) || delayed_.count(block))
            return;
        callback_(block);
        visited_.insert(block);
        Block* mergeBlock = nullptr;
        Block* continueBlock = nullptr;
        auto mergeInst = block->getMergeInstruction();
        if (mergeInst) {
            Id mergeId = mergeInst->getIdOperand(0);
            mergeBlock = block->getParent().getParent().getInstruction(mergeId)->getBlock();
            delayed_.insert(mergeBlock);
            if (mergeInst->getOpCode() == spv::OpLoopMerge) {
                Id continueId = mergeInst->getIdOperand(1);
                continueBlock =
                    block->getParent().getParent().getInstruction(continueId)->getBlock();
                delayed_.insert(continueBlock);
            }
        }
        const auto successors = block->getSuccessors();
        for (auto it = successors.cbegin(); it != successors.cend(); ++it)
            visit(*it);
        if (continueBlock) {
            delayed_.erase(continueBlock);
            visit(continueBlock);
        }
        if (mergeBlock) {
            delayed_.erase(mergeBlock);
            visit(mergeBlock);
        }
    }

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

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