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();
}
