DivergenceAnalysis::block_set
DivergenceAnalysis::_getDivergentBlocksInPostdominanceFrontier(
const DataflowGraph::iterator &block) {
const Analysis* dfgAnalysis = getAnalysis("DataflowGraphAnalysis");
assert(dfgAnalysis != 0);
const DataflowGraph &cdfg =
static_cast<const DataflowGraph&>(*dfgAnalysis);
DataflowGraph &dfg = const_cast<DataflowGraph&>(cdfg);
PostdominatorTree* dtree = (PostdominatorTree*)
(getAnalysis("PostDominatorTreeAnalysis"));
auto postDominator = dfg.getCFGtoDFGMap()[
dtree->getPostDominator(block->block())];
block_set divergentBlocks;
for (auto successor = block->successors().begin();
successor != block->successors().end(); ++successor) {
if (*successor == postDominator) continue;
block_set allDivergentPaths;
buildDivergentSubgraph(allDivergentPaths, *successor, postDominator);
divergentBlocks.insert(allDivergentPaths.begin(),
allDivergentPaths.end());
}
return divergentBlocks;
}
/*! \brief Tests if a block ends with a divergent branch
instruction (isDivBranchInstr) */
bool DivergenceAnalysis::isDivBlock(const DataflowGraph::iterator &block) const
{
if (block->instructions().size() == 0) {
return false;
}
return isDivBranch(*--block->instructions().end());
}
static bool buildDivergentSubgraph(
DivergenceAnalysis::block_set& graph,
const DataflowGraph::iterator &block,
const DataflowGraph::iterator &postDominator) {
bool hitPostDominator = false;
// don't include blocks with barriers
if(hasBarrier(block)) return false;
// skip loops
if(!graph.insert(block).second) return false;
for (auto successor = block->successors().begin();
successor != block->successors().end(); ++successor) {
// stop at the post dominator
if (*successor == postDominator)
{
hitPostDominator = true;
continue;
}
hitPostDominator |= buildDivergentSubgraph(graph,
*successor, postDominator);
}
return hitPostDominator;
}
static bool hasBarrier(const DataflowGraph::iterator &block) {
for (auto instruction = block->instructions().begin();
instruction != block->instructions().end(); ++instruction) {
if (typeid(ir::PTXInstruction) == typeid(*(instruction->i))) {
auto ptxInstruction =
static_cast<ir::PTXInstruction*>(instruction->i);
if (ptxInstruction->opcode == ir::PTXInstruction::Bar) return true;
// texture instruction intrinsics
if (ptxInstruction->isCall()) {
if (ptxInstruction->a.addressMode !=
ir::PTXOperand::FunctionName) {
continue;
}
if (ptxInstruction->a.identifier.find(
"_Z_intrinsic_pseudo_tex") != 0) {
continue;
}
return true;
}
}
}
return false;
}
bool DivergenceAnalysis::_hasTrivialPathToExit(
const DataflowGraph::iterator &block) const {
// We can ignore divergent threads that immediately exit
unsigned int exitingPaths = 0;
const Analysis* dfgAnalysis = getAnalysis("DataflowGraphAnalysis");
assert(dfgAnalysis != 0);
const DataflowGraph &dfg =
static_cast<const DataflowGraph&>(*dfgAnalysis);
auto exit = --dfg.end();
for (auto successor = block->successors().begin();
successor != block->successors().end(); ++successor) {
auto path = *successor;
while (true) {
if (path == exit) {
++exitingPaths;
break;
}
if (path->successors().size() != 1) {
break;
}
if (!path->instructions().empty()) {
if (path->instructions().size() == 1) {
const ir::PTXInstruction &ptxI =
*(static_cast<ir::PTXInstruction *> (
path->instructions().back().i));
if (ptxI.isExit()) {
++exitingPaths;
}
}
break;
}
path = *path->successors().begin();
}
}
if (block->successors().size() - exitingPaths < 2) {
return true;
}
return false;
}
void ConvertPredicationToSelectPass::_replacePredicate(
DataflowGraph::iterator block, unsigned int id )
{
typedef DataflowGraph::Block::RegisterSet RegisterSet;
DataflowGraph::InstructionVector::const_iterator
instruction( block->instructions().begin() );
std::advance( instruction, id );
report( " Converting instruction " << instruction->i->toString() );
ir::PTXInstruction select( ir::PTXInstruction::SelP );
ir::PTXInstruction& ptx = static_cast< ir::PTXInstruction& >(
*instruction->i );
select.d = ptx.d;
select.b = select.d;
select.a = select.b;
select.a.reg = _tempRegister();
select.c = ptx.pg;
ptx.pg.condition = ir::PTXOperand::PT;
ptx.d.reg = select.a.reg;
_kernel->dfg()->insert( block, select, id + 1 );
}
void ConvertPredicationToSelectPass::_runOnBlock(
DataflowGraph::iterator block )
{
for( DataflowGraph::InstructionVector::const_iterator
instruction = block->instructions().begin();
instruction != block->instructions().end(); ++instruction )
{
ir::PTXInstruction& ptx = static_cast< ir::PTXInstruction& >(
*instruction->i );
if( ptx.opcode != ir::PTXInstruction::Bra
&& ptx.opcode != ir::PTXInstruction::Call
&& ptx.opcode != ir::PTXInstruction::Ret )
{
if( ptx.pg.condition != ir::PTXOperand::PT )
{
_replacePredicate( block, std::distance(
block->instructions().begin(), instruction ) );
}
}
}
}
unsigned int DivergenceAnalysis::_numberOfDivergentPathsToPostDominator(
const DataflowGraph::iterator &block) const {
const Analysis* dfgAnalysis = getAnalysis("DataflowGraphAnalysis");
assert(dfgAnalysis != 0);
const DataflowGraph &cdfg =
static_cast<const DataflowGraph&>(*dfgAnalysis);
DataflowGraph &dfg = const_cast<DataflowGraph&>(cdfg);
PostdominatorTree* dtree = (PostdominatorTree*)
(getAnalysis("PostDominatorTreeAnalysis"));
auto postDominator = dfg.getCFGtoDFGMap()[
dtree->getPostDominator(block->block())];
unsigned int divergentPaths = 0;
for (auto successor = block->successors().begin();
successor != block->successors().end(); ++successor) {
if (*successor == postDominator) {
++divergentPaths;
continue;
}
block_set allDivergentPaths;
if (doAnyDivergentPathsReachThePostDominator(allDivergentPaths,
*successor, postDominator)) {
++divergentPaths;
}
}
report(" There are " << divergentPaths << " divergent paths from "
<< block->label() << " to post-dominator " << postDominator->label());
return divergentPaths;
}
void BlockUnificationPass::runOnKernel( ir::Kernel& k )
{
InstructionConverter instConv;
ir::PTXInstruction::ComputeCapability deviceCapability = ir::PTXInstruction::Cap_2_0;
DataflowGraph::iterator unificationBranch;
DataflowGraph::iterator unificationTarget1;
DataflowGraph::iterator unificationTarget2;
BlockMatcher::MatrixPath bestPath;
float largestGain = 0.0;
// analyze kernel for divergence
DivergenceAnalysis divAnalysis;
divAnalysis.runOnKernel(k);
do {
largestGain = 0.0;
DataflowGraph::iterator block = k.dfg()->begin();
for (; block != k.dfg()->end(); ++block) {
ir::ControlFlowGraph::const_iterator irBlock = block->block();
DataflowGraph::const_iterator constBlock = block;
if (irBlock->endsWithConditionalBranch() &&
divAnalysis.isDivBlock(constBlock)
) {
// get the fallthrough block
DataflowGraph::iterator fallthroughBlock = block->fallthrough();
// get the branch block
DataflowGraph::iterator branchBlock = fallthroughBlock;
DataflowGraph::BlockPointerSet branchTargets = block->targets();
DataflowGraph::BlockPointerSet::const_iterator it =
branchTargets.begin();
for (; it != branchTargets.end(); ++it) {
if (*it != fallthroughBlock) {
branchBlock = *it;
break;
}
}
assertM(branchBlock != fallthroughBlock,
"Block unification pass error: could not find fallthrough");
ir::PostdominatorTree* pdomTree = k.pdom_tree();
ir::ControlFlowGraph::const_iterator postDomBlk =
pdomTree->getPostDominator(block->block());
bool haveBranch2FallthroughPath = thereIsPathFromB1toB2(
branchBlock->block(), fallthroughBlock->block(),
postDomBlk, new std::set<ir::ControlFlowGraph::BasicBlock*>);
bool haveFallthrough2BranchPath = thereIsPathFromB1toB2(
fallthroughBlock->block(), branchBlock->block(),
postDomBlk, new std::set<ir::ControlFlowGraph::BasicBlock*>);
if (!haveBranch2FallthroughPath && !haveFallthrough2BranchPath) {
// Calculate branch targets' unification gain
BlockMatcher::MatrixPath path;
float gain = BlockMatcher::calculateUnificationGain(
k.dfg(), *fallthroughBlock, *branchBlock, path,
instConv, deviceCapability);
if (gain > largestGain) {
largestGain = gain;
unificationBranch = block;
unificationTarget1 = fallthroughBlock;
unificationTarget2 = branchBlock;
bestPath = path;
}
}
}
}
if (largestGain > 10.0) {
// Unify the basic block pair with biggest gain (if there's one)
cout << ">>>>> unifying blocks: " << unificationTarget1->block()->label
<< " and " << unificationTarget2->block()->label << std::endl;
weaveBlocks(unificationBranch, unificationTarget1, unificationTarget2, bestPath, k.dfg());
}
// refresh divergence analysis data
divAnalysis.run();
} while (false); //(largestGain > 0.0);
}
void BlockUnificationPass::weaveBlocks(DataflowGraph::iterator branchBlock, DataflowGraph::iterator target1, DataflowGraph::iterator target2, BlockMatcher::MatrixPath& extractionPath, DataflowGraph* dfg)
{
DataflowGraph::iterator oldFallthroughBlock = branchBlock;
DataflowGraph::iterator oldBranchBlock = branchBlock;
// get branch predicate
ir::ControlFlowGraph::const_iterator irBlock = branchBlock->block();
ir::Instruction* branchInst = irBlock->getTerminator();
ir::PTXInstruction* branchInstPtx = static_cast<ir::PTXInstruction*>(branchInst);
ir::PTXOperand* branchPredicate = &(branchInstPtx->pg);
std::string labelPrefix = "$BBweave_" + target1->block()->label + "_" + target2->block()->label;
int blockNum = 0;
// while not consumed path, generate basic blocks
BlockExtractor extractor(dfg, target1, target2, extractionPath, *branchPredicate);
while (extractor.hasNext()) {
if (extractor.nextStep() == BlockMatcher::Match ||
extractor.nextStep() == BlockMatcher::Substitution) {
// block label
std::stringstream blockLabel;
blockLabel << labelPrefix << "_uni_" << blockNum++;
// create block
DataflowGraph::iterator newUnifiedBlock = dfg->insert(oldFallthroughBlock, target1, blockLabel.str());
extractor.extractUnifiedBlock(newUnifiedBlock);
// link blocks
dfg->addEdge(newUnifiedBlock, target2, ir::ControlFlowGraph::Edge::Branch);
if (oldFallthroughBlock == oldBranchBlock) {
// oldFallthroughBlock and oldBranchBlock all point to branchBlock.
// remove oldBranchBlock -> target2 because there's
// already a edge from branchBlock to newUnifiedBlock
dfg->removeEdge(oldBranchBlock, target2);
} else {
dfg->removeEdge(oldFallthroughBlock, newUnifiedBlock);
dfg->redirect(oldBranchBlock, target2, newUnifiedBlock);
dfg->addEdge(oldFallthroughBlock, newUnifiedBlock, ir::ControlFlowGraph::Edge::Branch);
// add goto in oldFallthroughBlock to newUnifiedBlock
ir::PTXInstruction gotoPtx(ir::PTXInstruction::Bra);
ir::PTXOperand gotoLabelOperand(blockLabel.str(), ir::PTXOperand::Label, ir::PTXOperand::s32);
gotoPtx.setDestination(gotoLabelOperand);
gotoPtx.uni = true;
ir::Instruction& gotoInst = gotoPtx;
dfg->insert(oldFallthroughBlock, gotoInst);
}
oldFallthroughBlock = newUnifiedBlock;
oldBranchBlock = newUnifiedBlock;
} else {
// create fallthrough block
std::stringstream fallthroughLabel;
fallthroughLabel << labelPrefix << "_ft_" << blockNum++;
DataflowGraph::iterator newFallthoughBlock = dfg->insert(oldFallthroughBlock, target1, fallthroughLabel.str());
// create branch block
std::stringstream branchLabel;
branchLabel << labelPrefix << "_bra_" << blockNum++;
DataflowGraph::iterator newBranchBlock = dfg->insert(oldBranchBlock, target2, branchLabel.str());
// fill blocks
extractor.extractDivergentBlocks(newFallthoughBlock, newBranchBlock);
if (oldFallthroughBlock == branchBlock) {
const ir::PTXOperand braLabelOperand(branchLabel.str(), ir::PTXOperand::Label, ir::PTXOperand::s32);
branchInstPtx->setDestination(braLabelOperand);
} else {
ir::PTXInstruction braPtx(ir::PTXInstruction::Bra);
ir::PTXOperand braLabelOperand(branchLabel.str(), ir::PTXOperand::Label, ir::PTXOperand::s32);
braPtx.setDestination(braLabelOperand);
braPtx.setPredicate(*branchPredicate);
ir::Instruction& bra = braPtx;
dfg->insert(oldFallthroughBlock, bra);
}
// all needed edges were already created in block creation,
// no more edge manipulation needed
oldFallthroughBlock = newFallthoughBlock;
oldBranchBlock = newBranchBlock;
}
}
// remove branch instruction on BranchBlock if needed
if (branchBlock->targets().size() == 0) {
// branchBlock does not have branch at end, remove that instruction
unsigned int branchInstPos = branchBlock->instructions().size() - 1;
dfg->erase(branchBlock, branchInstPos);
}
if (oldFallthroughBlock == oldBranchBlock) {
// If target1 has no fall-through, then
// switch branch and fall-through edges.
if ( !(target1->block()->has_fallthrough_edge()) ) {
dfg->setEdgeType(oldFallthroughBlock, target1, ir::ControlFlowGraph::BasicBlock::Edge::Branch);
dfg->setEdgeType(oldFallthroughBlock, target2, ir::ControlFlowGraph::BasicBlock::Edge::FallThrough);
}
}
// weaved block finishes with a divergent section
// copy target2 targets to a ending divergent fallthrough block
dfg->copyOutgoingBranchEdges(target1, oldFallthroughBlock);
// remove target1
dfg->erase(target1);
// copy target2 targets to a ending unified block or divergent branch block
dfg->copyOutgoingBranchEdges(target2, oldBranchBlock);
// remove target2
dfg->erase(target2);
// remove empty blocks or blocks with just a goto
// TODO: removing this blocks is not essential for correctness, yet it
// might increase performance.
// replaces all uses of old registers to use new ones
// in code after unified basic blocks
std::cerr << "\n\n\nCalling register replacement\n\n\n";
replaceRegisters(dfg, extractor);
// recalculate live in and live out
//dfg->compute();
//dfg->toSsa();
}
void DivergenceAnalysis::_findBranches(branch_set& branches)
{
Analysis* dfgAnalysis = getAnalysis("DataflowGraphAnalysis");
assert(dfgAnalysis != 0);
DataflowGraph &dfg = static_cast<DataflowGraph&>(*dfgAnalysis);
/* Create a list of branches that can be divergent, that is,
they are not bra.uni and have a predicate */
DataflowGraph::iterator block = dfg.begin();
DataflowGraph::iterator endBlock = dfg.end();
/* Post-dominator tree */
PostdominatorTree *dtree;
dtree = (PostdominatorTree*) (getAnalysis("PostDominatorTreeAnalysis"));
report(" Finding branches");
for (; block != endBlock; ++block) {
ir::PTXInstruction *ptxInstruction = NULL;
if (block->instructions().size() > 0) {
/* Branch instructions can only be the last
instruction of a basic block */
DataflowGraph::Instruction& lastInstruction =
*(--block->instructions().end());
if (typeid(ir::PTXInstruction) == typeid(*(lastInstruction.i))) {
ptxInstruction =
static_cast<ir::PTXInstruction*>(lastInstruction.i);
if ((ptxInstruction->opcode == ir::PTXInstruction::Bra)) {
report(" examining " << ptxInstruction->toString());
if(ptxInstruction->uni == true) {
report(" eliminated, uniform...");
continue;
}
if(lastInstruction.s.size() == 0) {
report(" eliminated, wrong source count ("
<< lastInstruction.s.size() << ")...");
continue;
}
assert(lastInstruction.s.size() == 1);
DataflowGraph::iterator postDomBlock =
dfg.getCFGtoDFGMap()[
dtree->getPostDominator(block->block())];
if (postDomBlock != dfg.end()) {
BranchInfo newBranch(&(*block), &(*postDomBlock),
lastInstruction, _divergGraph);
branches.insert(newBranch);
report(" is potentially divergent...");
}
else {
report(" eliminated, no post-dominator...");
}
}
}
}
}
}
