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//===- DependenceGraphBuilder.cpp ------------------------------------------==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// This file implements common steps of the build algorithm for construction
// of dependence graphs such as DDG and PDG.
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/DependenceGraphBuilder.h"
#include "llvm/ADT/SCCIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DDG.h"
using namespace llvm;
#define DEBUG_TYPE "dgb"
STATISTIC(TotalGraphs, "Number of dependence graphs created.");
STATISTIC(TotalDefUseEdges, "Number of def-use edges created.");
STATISTIC(TotalMemoryEdges, "Number of memory dependence edges created.");
STATISTIC(TotalFineGrainedNodes, "Number of fine-grained nodes created.");
STATISTIC(TotalConfusedEdges,
"Number of confused memory dependencies between two nodes.");
STATISTIC(TotalEdgeReversals,
"Number of times the source and sink of dependence was reversed to "
"expose cycles in the graph.");
using InstructionListType = SmallVector<Instruction *, 2>;
//===--------------------------------------------------------------------===//
// AbstractDependenceGraphBuilder implementation
//===--------------------------------------------------------------------===//
template <class G>
void AbstractDependenceGraphBuilder<G>::createFineGrainedNodes() {
++TotalGraphs;
assert(IMap.empty() && "Expected empty instruction map at start");
for (BasicBlock *BB : BBList)
for (Instruction &I : *BB) {
auto &NewNode = createFineGrainedNode(I);
IMap.insert(std::make_pair(&I, &NewNode));
++TotalFineGrainedNodes;
}
}
template <class G> void AbstractDependenceGraphBuilder<G>::createDefUseEdges() {
for (NodeType *N : Graph) {
InstructionListType SrcIList;
N->collectInstructions([](const Instruction *I) { return true; }, SrcIList);
// Use a set to mark the targets that we link to N, so we don't add
// duplicate def-use edges when more than one instruction in a target node
// use results of instructions that are contained in N.
SmallPtrSet<NodeType *, 4> VisitedTargets;
for (Instruction *II : SrcIList) {
for (User *U : II->users()) {
Instruction *UI = dyn_cast<Instruction>(U);
if (!UI)
continue;
NodeType *DstNode = nullptr;
if (IMap.find(UI) != IMap.end())
DstNode = IMap.find(UI)->second;
// In the case of loops, the scope of the subgraph is all the
// basic blocks (and instructions within them) belonging to the loop. We
// simply ignore all the edges coming from (or going into) instructions
// or basic blocks outside of this range.
if (!DstNode) {
LLVM_DEBUG(
dbgs()
<< "skipped def-use edge since the sink" << *UI
<< " is outside the range of instructions being considered.\n");
continue;
}
// Self dependencies are ignored because they are redundant and
// uninteresting.
if (DstNode == N) {
LLVM_DEBUG(dbgs()
<< "skipped def-use edge since the sink and the source ("
<< N << ") are the same.\n");
continue;
}
if (VisitedTargets.insert(DstNode).second) {
createDefUseEdge(*N, *DstNode);
++TotalDefUseEdges;
}
}
}
}
}
template <class G>
void AbstractDependenceGraphBuilder<G>::createMemoryDependencyEdges() {
using DGIterator = typename G::iterator;
auto isMemoryAccess = [](const Instruction *I) {
return I->mayReadOrWriteMemory();
};
for (DGIterator SrcIt = Graph.begin(), E = Graph.end(); SrcIt != E; ++SrcIt) {
InstructionListType SrcIList;
(*SrcIt)->collectInstructions(isMemoryAccess, SrcIList);
if (SrcIList.empty())
continue;
for (DGIterator DstIt = SrcIt; DstIt != E; ++DstIt) {
if (**SrcIt == **DstIt)
continue;
InstructionListType DstIList;
(*DstIt)->collectInstructions(isMemoryAccess, DstIList);
if (DstIList.empty())
continue;
bool ForwardEdgeCreated = false;
bool BackwardEdgeCreated = false;
for (Instruction *ISrc : SrcIList) {
for (Instruction *IDst : DstIList) {
auto D = DI.depends(ISrc, IDst, true);
if (!D)
continue;
// If we have a dependence with its left-most non-'=' direction
// being '>' we need to reverse the direction of the edge, because
// the source of the dependence cannot occur after the sink. For
// confused dependencies, we will create edges in both directions to
// represent the possibility of a cycle.
auto createConfusedEdges = [&](NodeType &Src, NodeType &Dst) {
if (!ForwardEdgeCreated) {
createMemoryEdge(Src, Dst);
++TotalMemoryEdges;
}
if (!BackwardEdgeCreated) {
createMemoryEdge(Dst, Src);
++TotalMemoryEdges;
}
ForwardEdgeCreated = BackwardEdgeCreated = true;
++TotalConfusedEdges;
};
auto createForwardEdge = [&](NodeType &Src, NodeType &Dst) {
if (!ForwardEdgeCreated) {
createMemoryEdge(Src, Dst);
++TotalMemoryEdges;
}
ForwardEdgeCreated = true;
};
auto createBackwardEdge = [&](NodeType &Src, NodeType &Dst) {
if (!BackwardEdgeCreated) {
createMemoryEdge(Dst, Src);
++TotalMemoryEdges;
}
BackwardEdgeCreated = true;
};
if (D->isConfused())
createConfusedEdges(**SrcIt, **DstIt);
else if (D->isOrdered() && !D->isLoopIndependent()) {
bool ReversedEdge = false;
for (unsigned Level = 1; Level <= D->getLevels(); ++Level) {
if (D->getDirection(Level) == Dependence::DVEntry::EQ)
continue;
else if (D->getDirection(Level) == Dependence::DVEntry::GT) {
createBackwardEdge(**SrcIt, **DstIt);
ReversedEdge = true;
++TotalEdgeReversals;
break;
} else if (D->getDirection(Level) == Dependence::DVEntry::LT)
break;
else {
createConfusedEdges(**SrcIt, **DstIt);
break;
}
}
if (!ReversedEdge)
createForwardEdge(**SrcIt, **DstIt);
} else
createForwardEdge(**SrcIt, **DstIt);
// Avoid creating duplicate edges.
if (ForwardEdgeCreated && BackwardEdgeCreated)
break;
}
// If we've created edges in both directions, there is no more
// unique edge that we can create between these two nodes, so we
// can exit early.
if (ForwardEdgeCreated && BackwardEdgeCreated)
break;
}
}
}
}
template class llvm::AbstractDependenceGraphBuilder<DataDependenceGraph>;
template class llvm::DependenceGraphInfo<DDGNode>;
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