RegionDesc::BlockVec::iterator
RegionDesc::deleteBlock(RegionDesc::BlockVec::iterator it) {
const auto bid = (*it)->id();
for (auto pid : preds(bid)) removeArc(pid, bid);
for (auto sid : succs(bid)) removeArc(bid, sid);
if (auto nextR = nextRetrans(bid)) {
auto prevR = prevRetrans(bid);
clearPrevRetrans(nextR.value());
if (prevR) {
clearNextRetrans(prevR.value());
setNextRetrans(prevR.value(), nextR.value());
} else {
clearPrevRetrans(nextR.value());
}
} else if (auto prevR = prevRetrans(bid)) {
clearNextRetrans(prevR.value());
}
m_data.erase(bid);
return m_blocks.erase(it);
}
/**
* Chain the retranslation blocks. This method enforces that, for
* each region block, all its successor have distinct SrcKeys.
*/
void RegionDesc::chainRetransBlocks() {
jit::vector<Chain> chains;
BlockToChainMap block2chain;
// 1. Initially assign each region block to its own chain.
for (auto b : blocks()) {
auto bid = b->id();
auto cid = chains.size();
chains.push_back({cid, {bid}});
block2chain[bid] = cid;
}
// 2. For each block, if it has 2 successors with the same SrcKey,
// then merge the successors' chains into one.
for (auto b : blocks()) {
auto bid = b->id();
const auto& succSet = succs(bid);
for (auto it1 = succSet.begin(); it1 != succSet.end(); it1++) {
auto bid1 = *it1;
auto cid1 = block2chain[bid1];
for (auto it2 = it1 + 1; it2 != succSet.end(); it2++) {
auto bid2 = *it2;
auto cid2 = block2chain[bid2];
if (data(bid1).block->start() == data(bid2).block->start()) {
mergeChains(chains[cid1], chains[cid2], block2chain);
}
}
}
}
// 3. Sort each chain. In general, we want to sort each chain in
// decreasing order of profile weights. However, note that this
// transformation can turn acyclic graphs into cyclic ones (see
// example below). Therefore, if JitLoops are disabled, we
// instead sort each chain following the original block order,
// which prevents loops from being generated if the region was
// originally acyclic.
//
// Here's an example showing how an acyclic CFG can become cyclic
// by chaining its retranslation blocks:
//
// - Region before chaining retranslation blocks, where B2' and B2"
// are retranslations starting at the same SrcKey:
// B1 -> B2'
// B1 -> B2"
// B2' -> B3
// B3 -> B2"
//
// - Region after sorting the chain as B2" -R-> B2':
// B1 -> B2"
// B2" -R-> B2'
// B2' -> B3
// B3 -> B2"
// Note the cycle: B2" -R-> B2' -> B3 -> B2".
//
auto profData = mcg->tx().profData();
auto weight = [&](RegionDesc::BlockId bid) {
return hasTransID(bid) ? profData->absTransCounter(getTransID(bid)) : 0;
};
auto sortGeneral = [&](RegionDesc::BlockId bid1, RegionDesc::BlockId bid2) {
return weight(bid1) > weight(bid2);
};
using SortFun = std::function<bool(RegionDesc::BlockId, RegionDesc::BlockId)>;
SortFun sortFunc = sortGeneral;
hphp_hash_map<RegionDesc::BlockId, uint32_t> origBlockOrder;
if (!RuntimeOption::EvalJitLoops) {
for (uint32_t i = 0; i < m_blocks.size(); i++) {
origBlockOrder[m_blocks[i]->id()] = i;
}
auto sortAcyclic = [&](RegionDesc::BlockId bid1, RegionDesc::BlockId bid2) {
return origBlockOrder[bid1] < origBlockOrder[bid2];
};
sortFunc = sortAcyclic;
}
TRACE(1, "chainRetransBlocks: computed chains:\n");
for (auto& c : chains) {
std::sort(c.blocks.begin(), c.blocks.end(), sortFunc);
if (Trace::moduleEnabled(Trace::region, 1) && c.blocks.size() > 0) {
FTRACE(1, " -> {} (w={})", c.blocks[0], weight(c.blocks[0]));
for (size_t i = 1; i < c.blocks.size(); i++) {
FTRACE(1, ", {} (w={})", c.blocks[i], weight(c.blocks[i]));
}
FTRACE(1, "\n");
}
}
// 4. Set the nextRetrans blocks according to the computed chains.
for (auto& c : chains) {
if (c.blocks.size() == 0) continue;
for (size_t i = 0; i < c.blocks.size() - 1; i++) {
setNextRetrans(c.blocks[i], c.blocks[i + 1]);
}
}
// 5. For each block with multiple successors in the same chain,
// only keep the successor that first appears in the chain.
for (auto b : blocks()) {
auto& succSet = data(b->id()).succs;
for (auto s : succSet) {
auto& c = chains[block2chain[s]];
auto selectedSucc = findFirstInSet(c, succSet);
for (auto other : c.blocks) {
if (other == selectedSucc) continue;
succSet.erase(other);
}
}
}
// 6. Reorder the blocks in the region in topological order (if
// region is acyclic), since the previous steps may break it.
sortBlocks();
}
