This file implements a pass that automatically parallelizes a program,
using the Cilk multi-threaded runtime system to execute parallel code.
The current version inserts too many sync() operations in the program
because it does not attempt to optimize their placement.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4962 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/DataStructure/Parallelize.cpp b/lib/Analysis/DataStructure/Parallelize.cpp
new file mode 100644
index 0000000..08c800e
--- /dev/null
+++ b/lib/Analysis/DataStructure/Parallelize.cpp
@@ -0,0 +1,548 @@
+//===- Parallelize.cpp - Auto parallelization using DS Graphs ---*- C++ -*-===//
+//
+// This file implements a pass that automatically parallelizes a program,
+// using the Cilk multi-threaded runtime system to execute parallel code.
+//
+// The pass uses the Program Dependence Graph (class PDGIterator) to
+// identify parallelizable function calls, i.e., calls whose instances
+// can be executed in parallel with instances of other function calls.
+// (In the future, this should also execute different instances of the same
+// function call in parallel, but that requires parallelizing across
+// loop iterations.)
+//
+// The output of the pass is LLVM code with:
+// (1) all parallelizable functions renamed to flag them as parallelizable;
+// (2) calls to a sync() function introduced at synchronization points.
+// The CWriter recognizes these functions and inserts the appropriate Cilk
+// keywords when writing out C code. This C code must be compiled with cilk2c.
+//
+// Current algorithmic limitations:
+// -- no array dependence analysis
+// -- no parallelization for function calls in different loop iterations
+// (except in unlikely trivial cases)
+//
+// Limitations of using Cilk:
+// -- No parallelism within a function body, e.g., in a loop;
+// -- Simplistic synchronization model requiring all parallel threads
+// created within a function to block at a sync().
+// -- Excessive overhead at "spawned" function calls, which has no benefit
+// once all threads are busy (especially common when the degree of
+// parallelism is low).
+//===----------------------------------------------------------------------===//
+
+
+#include "llvm/Transforms/Parallelize.h"
+#include "llvm/Transforms/Utils/DemoteRegToStack.h"
+#include "llvm/Analysis/PgmDependenceGraph.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/DataStructure.h"
+#include "llvm/Analysis/DSGraph.h"
+#include "llvm/Module.h"
+#include "llvm/Function.h"
+#include "llvm/iOther.h"
+#include "llvm/iPHINode.h"
+#include "llvm/iTerminators.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Support/InstVisitor.h"
+#include "llvm/Support/Cilkifier.h"
+#include "Support/NonCopyable.h"
+#include "Support/Statistic.h"
+#include "Support/STLExtras.h"
+#include "Support/hash_set"
+#include "Support/hash_map"
+#include <vector>
+#include <stack>
+#include <functional>
+#include <algorithm>
+
+
+
+#if 0
+void AddToDomSet(vector<BasicBlock*>& domSet, BasicBlock* bb,
+ const DominatorTree& domTree)
+{
+ DominatorTreeBase::Node* bbNode = domTree.getNode(bb);
+ const std::vector<Node*>& domKids = bbNode.getChildren();
+ domSet.insert(domSet.end(), domKids.begin(), domKids.end());
+ for (unsigned i = 0; i < domKids.size(); ++i)
+ AddToDomSet(domSet, domKids[i]->getNode(), domTree);
+}
+
+bool CheckDominance(Function& func,
+ const CallInst& callInst1,
+ const CallInst& callInst2)
+{
+ if (callInst1 == callInst2) // makes sense if this is in a loop but
+ return false; // we're not handling loops yet
+
+ // Check first if one call dominates the other
+ DominatorSet& domSet = getAnalysis<DominatorSet>(func);
+ if (domSet.dominates(callInst2, callInst1))
+ { // swap callInst1 and callInst2
+ const CallInst& tmp = callInst2; callInst2 = callInst1; callInst1 = tmp;
+ }
+ else if (! domSet.dominates(callInst1, callInst2))
+ return false; // neither dominates the other:
+
+ //
+ if (! AreIndependent(func, callInst1, callInst2))
+ return false;
+}
+
+#endif
+
+
+//----------------------------------------------------------------------------
+// class Cilkifier
+//
+// Code generation pass that transforms code to identify where Cilk keywords
+// should be inserted. This relies on dis -c to print out the keywords.
+//----------------------------------------------------------------------------
+
+
+class Cilkifier: public InstVisitor<Cilkifier>
+{
+ Function* DummySyncFunc;
+
+ // Data used when transforming each function.
+ hash_set<const Instruction*> stmtsVisited; // Flags for recursive DFS
+ hash_map<const CallInst*, hash_set<CallInst*> > spawnToSyncsMap;
+
+ // Input data for the transformation.
+ const hash_set<Function*>* cilkFunctions; // Set of parallel functions
+ PgmDependenceGraph* depGraph;
+
+ void DFSVisitInstr (Instruction* I,
+ Instruction* root,
+ hash_set<const Instruction*>& depsOfRoot);
+
+public:
+ /*ctor*/ Cilkifier (Module& M);
+
+ // Transform a single function including its name, its call sites, and syncs
+ //
+ void TransformFunc (Function* F,
+ const hash_set<Function*>& cilkFunctions,
+ PgmDependenceGraph& _depGraph);
+
+ // The visitor function that does most of the hard work, via DFSVisitInstr
+ //
+ void visitCallInst(CallInst& CI);
+};
+
+
+Cilkifier::Cilkifier(Module& M)
+{
+ // create the dummy Sync function and add it to the Module
+ DummySyncFunc = new Function(FunctionType::get( Type::VoidTy,
+ std::vector<const Type*>(),
+ /*isVararg*/ false),
+ /*isInternal*/ false, DummySyncFuncName, &M);
+}
+
+void Cilkifier::TransformFunc(Function* F,
+ const hash_set<Function*>& _cilkFunctions,
+ PgmDependenceGraph& _depGraph)
+{
+ // Memoize the information for this function
+ cilkFunctions = &_cilkFunctions;
+ depGraph = &_depGraph;
+
+ // Add the marker suffix to the Function name
+ // This should automatically mark all calls to the function also!
+ F->setName(F->getName() + CilkSuffix);
+
+ // Insert sync operations for each separate spawn
+ visit(*F);
+
+ // Now traverse the CFG in rPostorder and eliminate redundant syncs, i.e.,
+ // two consecutive sync's on a straight-line path with no intervening spawn.
+
+}
+
+
+void Cilkifier::DFSVisitInstr(Instruction* I,
+ Instruction* root,
+ hash_set<const Instruction*>& depsOfRoot)
+{
+ assert(stmtsVisited.find(I) == stmtsVisited.end());
+ stmtsVisited.insert(I);
+
+ // If there is a dependence from root to I, insert Sync and return
+ if (depsOfRoot.find(I) != depsOfRoot.end())
+ { // Insert a sync before I and stop searching along this path.
+ // If I is a Phi instruction, the dependence can only be an SSA dep.
+ // and we need to insert the sync in the predecessor on the appropriate
+ // incoming edge!
+ CallInst* syncI = 0;
+ if (PHINode* phiI = dyn_cast<PHINode>(I))
+ { // check all operands of the Phi and insert before each one
+ for (unsigned i = 0, N = phiI->getNumIncomingValues(); i < N; ++i)
+ if (phiI->getIncomingValue(i) == root)
+ syncI = new CallInst(DummySyncFunc, std::vector<Value*>(), "",
+ phiI->getIncomingBlock(i)->getTerminator());
+ }
+ else
+ syncI = new CallInst(DummySyncFunc, std::vector<Value*>(), "", I);
+
+ // Remember the sync for each spawn to eliminate rendundant ones later
+ spawnToSyncsMap[cast<CallInst>(root)].insert(syncI);
+
+ return;
+ }
+
+ // else visit unvisited successors
+ if (BranchInst* brI = dyn_cast<BranchInst>(I))
+ { // visit first instruction in each successor BB
+ for (unsigned i = 0, N = brI->getNumSuccessors(); i < N; ++i)
+ if (stmtsVisited.find(&brI->getSuccessor(i)->front())
+ == stmtsVisited.end())
+ DFSVisitInstr(&brI->getSuccessor(i)->front(), root, depsOfRoot);
+ }
+ else
+ if (Instruction* nextI = I->getNext())
+ if (stmtsVisited.find(nextI) == stmtsVisited.end())
+ DFSVisitInstr(nextI, root, depsOfRoot);
+}
+
+
+void Cilkifier::visitCallInst(CallInst& CI)
+{
+ assert(CI.getCalledFunction() != 0 && "Only direct calls can be spawned.");
+ if (cilkFunctions->find(CI.getCalledFunction()) == cilkFunctions->end())
+ return; // not a spawn
+
+ // Find all the outgoing memory dependences.
+ hash_set<const Instruction*> depsOfRoot;
+ for (PgmDependenceGraph::iterator DI =
+ depGraph->outDepBegin(CI, MemoryDeps); ! DI.fini(); ++DI)
+ depsOfRoot.insert(&DI->getSink()->getInstr());
+
+ // Now find all outgoing SSA dependences to the eventual non-Phi users of
+ // the call value (i.e., direct users that are not phis, and for any
+ // user that is a Phi, direct non-Phi users of that Phi, and recursively).
+ std::stack<const PHINode*> phiUsers;
+ hash_set<const PHINode*> phisSeen; // ensures we don't visit a phi twice
+ for (Value::use_iterator UI=CI.use_begin(), UE=CI.use_end(); UI != UE; ++UI)
+ if (const PHINode* phiUser = dyn_cast<PHINode>(*UI))
+ {
+ if (phisSeen.find(phiUser) == phisSeen.end())
+ {
+ phiUsers.push(phiUser);
+ phisSeen.insert(phiUser);
+ }
+ }
+ else
+ depsOfRoot.insert(cast<Instruction>(*UI));
+
+ // Now we've found the non-Phi users and immediate phi users.
+ // Recursively walk the phi users and add their non-phi users.
+ for (const PHINode* phiUser; !phiUsers.empty(); phiUsers.pop())
+ {
+ phiUser = phiUsers.top();
+ for (Value::use_const_iterator UI=phiUser->use_begin(),
+ UE=phiUser->use_end(); UI != UE; ++UI)
+ if (const PHINode* pn = dyn_cast<PHINode>(*UI))
+ {
+ if (phisSeen.find(pn) == phisSeen.end())
+ {
+ phiUsers.push(pn);
+ phisSeen.insert(pn);
+ }
+ }
+ else
+ depsOfRoot.insert(cast<Instruction>(*UI));
+ }
+
+ // Walk paths of the CFG starting at the call instruction and insert
+ // one sync before the first dependence on each path, if any.
+ if (! depsOfRoot.empty())
+ {
+ stmtsVisited.clear(); // start a new DFS for this CallInst
+ assert(CI.getNext() && "Call instruction cannot be a terminator!");
+ DFSVisitInstr(CI.getNext(), &CI, depsOfRoot);
+ }
+
+ // Now, eliminate all users of the SSA value of the CallInst, i.e.,
+ // if the call instruction returns a value, delete the return value
+ // register and replace it by a stack slot.
+ if (CI.getType() != Type::VoidTy)
+ DemoteRegToStack(CI);
+}
+
+
+//----------------------------------------------------------------------------
+// class FindParallelCalls
+//
+// Find all CallInst instructions that have at least one other CallInst
+// that is independent. These are the instructions that can produce
+// useful parallelism.
+//----------------------------------------------------------------------------
+
+class FindParallelCalls: public InstVisitor<FindParallelCalls>,
+ public NonCopyable
+{
+ typedef hash_set<CallInst*> DependentsSet;
+ typedef DependentsSet::iterator Dependents_iterator;
+ typedef DependentsSet::const_iterator Dependents_const_iterator;
+
+ PgmDependenceGraph& depGraph; // dependence graph for the function
+ hash_set<Instruction*> stmtsVisited; // flags for DFS walk of depGraph
+ hash_map<CallInst*, bool > completed; // flags marking if a CI is done
+ hash_map<CallInst*, DependentsSet> dependents; // dependent CIs for each CI
+
+ void VisitOutEdges(Instruction* I,
+ CallInst* root,
+ DependentsSet& depsOfRoot);
+
+public:
+ std::vector<CallInst*> parallelCalls;
+
+public:
+ /*ctor*/ FindParallelCalls (Function& F, PgmDependenceGraph& DG);
+ void visitCallInst (CallInst& CI);
+};
+
+
+FindParallelCalls::FindParallelCalls(Function& F,
+ PgmDependenceGraph& DG)
+ : depGraph(DG)
+{
+ // Find all CallInsts reachable from each CallInst using a recursive DFS
+ visit(F);
+
+ // Now we've found all CallInsts reachable from each CallInst.
+ // Find those CallInsts that are parallel with at least one other CallInst
+ // by counting total inEdges and outEdges.
+ //
+ unsigned long totalNumCalls = completed.size();
+
+ if (totalNumCalls == 1)
+ { // Check first for the special case of a single call instruction not
+ // in any loop. It is not parallel, even if it has no dependences
+ // (this is why it is a special case).
+ //
+ // FIXME:
+ // THIS CASE IS NOT HANDLED RIGHT NOW, I.E., THERE IS NO
+ // PARALLELISM FOR CALLS IN DIFFERENT ITERATIONS OF A LOOP.
+ //
+ return;
+ }
+
+ hash_map<CallInst*, unsigned long> numDeps;
+ for (hash_map<CallInst*, DependentsSet>::iterator II = dependents.begin(),
+ IE = dependents.end(); II != IE; ++II)
+ {
+ CallInst* fromCI = II->first;
+ numDeps[fromCI] += II->second.size();
+ for (Dependents_iterator DI = II->second.begin(), DE = II->second.end();
+ DI != DE; ++DI)
+ numDeps[*DI]++; // *DI can be reached from II->first
+ }
+
+ for (hash_map<CallInst*, DependentsSet>::iterator
+ II = dependents.begin(), IE = dependents.end(); II != IE; ++II)
+
+ // FIXME: Remove "- 1" when considering parallelism in loops
+ if (numDeps[II->first] < totalNumCalls - 1)
+ parallelCalls.push_back(II->first);
+}
+
+
+void FindParallelCalls::VisitOutEdges(Instruction* I,
+ CallInst* root,
+ DependentsSet& depsOfRoot)
+{
+ assert(stmtsVisited.find(I) == stmtsVisited.end() && "Stmt visited twice?");
+ stmtsVisited.insert(I);
+
+ if (CallInst* CI = dyn_cast<CallInst>(I))
+
+ // FIXME: Ignoring parallelism in a loop. Here we're actually *ignoring*
+ // a self-dependence in order to get the count comparison right above.
+ // When we include loop parallelism, self-dependences should be included.
+ //
+ if (CI != root)
+
+ { // CallInst root has a path to CallInst I and any calls reachable from I
+ depsOfRoot.insert(CI);
+ if (completed[CI])
+ { // We have already visited I so we know all nodes it can reach!
+ DependentsSet& depsOfI = dependents[CI];
+ depsOfRoot.insert(depsOfI.begin(), depsOfI.end());
+ return;
+ }
+ }
+
+ // If we reach here, we need to visit all children of I
+ for (PgmDependenceGraph::iterator DI = depGraph.outDepBegin(*I);
+ ! DI.fini(); ++DI)
+ {
+ Instruction* sink = &DI->getSink()->getInstr();
+ if (stmtsVisited.find(sink) == stmtsVisited.end())
+ VisitOutEdges(sink, root, depsOfRoot);
+ }
+}
+
+
+void FindParallelCalls::visitCallInst(CallInst& CI)
+{
+ if (completed[&CI])
+ return;
+ stmtsVisited.clear(); // clear flags to do a fresh DFS
+
+ // Visit all children of CI using a recursive walk through dep graph
+ DependentsSet& depsOfRoot = dependents[&CI];
+ for (PgmDependenceGraph::iterator DI = depGraph.outDepBegin(CI);
+ ! DI.fini(); ++DI)
+ {
+ Instruction* sink = &DI->getSink()->getInstr();
+ if (stmtsVisited.find(sink) == stmtsVisited.end())
+ VisitOutEdges(sink, &CI, depsOfRoot);
+ }
+
+ completed[&CI] = true;
+}
+
+
+//----------------------------------------------------------------------------
+// class Parallelize
+//
+// (1) Find candidate parallel functions: any function F s.t.
+// there is a call C1 to the function F that is followed or preceded
+// by at least one other call C2 that is independent of this one
+// (i.e., there is no dependence path from C1 to C2 or C2 to C1)
+// (2) Label such a function F as a cilk function.
+// (3) Convert every call to F to a spawn
+// (4) For every function X, insert sync statements so that
+// every spawn is postdominated by a sync before any statements
+// with a data dependence to/from the call site for the spawn
+//
+//----------------------------------------------------------------------------
+
+namespace {
+ class Parallelize: public Pass
+ {
+ public:
+ /// Driver functions to transform a program
+ ///
+ bool run(Module& M);
+
+ /// getAnalysisUsage - Modifies extensively so preserve nothing.
+ /// Uses the DependenceGraph and the Top-down DS Graph (only to find
+ /// all functions called via an indirect call).
+ ///
+ void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired<TDDataStructures>();
+ AU.addRequired<MemoryDepAnalysis>(); // force this not to be released
+ AU.addRequired<PgmDependenceGraph>(); // because it is needed by this
+ }
+ };
+
+ RegisterOpt<Parallelize> X("parallel", "Parallelize program using Cilk");
+}
+
+
+static Function* FindMain(Module& M)
+{
+ for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
+ if (FI->getName() == std::string("main"))
+ return FI;
+ return NULL;
+}
+
+
+bool Parallelize::run(Module& M)
+{
+ hash_set<Function*> parallelFunctions;
+ hash_set<Function*> safeParallelFunctions;
+ hash_set<const GlobalValue*> indirectlyCalled;
+
+ // If there is no main (i.e., for an incomplete program), we can do nothing.
+ // If there is a main, mark main as a parallel function.
+ //
+ Function* mainFunc = FindMain(M);
+ if (!mainFunc)
+ return false;
+
+ // (1) Find candidate parallel functions and mark them as Cilk functions
+ //
+ for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
+ if (! FI->isExternal())
+ {
+ Function* F = FI;
+ DSGraph& tdg = getAnalysis<TDDataStructures>().getDSGraph(*F);
+
+ // All the hard analysis work gets done here!
+ //
+ FindParallelCalls finder(*F,
+ getAnalysis<PgmDependenceGraph>().getGraph(*F));
+ /* getAnalysis<MemoryDepAnalysis>().getGraph(*F)); */
+
+ // Now we know which call instructions are useful to parallelize.
+ // Remember those callee functions.
+ //
+ for (std::vector<CallInst*>::iterator
+ CII = finder.parallelCalls.begin(),
+ CIE = finder.parallelCalls.end(); CII != CIE; ++CII)
+ {
+ // Check if this is a direct call...
+ if ((*CII)->getCalledFunction() != NULL)
+ { // direct call: if this is to a non-external function,
+ // mark it as a parallelizable function
+ if (! (*CII)->getCalledFunction()->isExternal())
+ parallelFunctions.insert((*CII)->getCalledFunction());
+ }
+ else
+ { // Indirect call: mark all potential callees as bad
+ std::vector<GlobalValue*> callees =
+ tdg.getNodeForValue((*CII)->getCalledValue())
+ .getNode()->getGlobals();
+ indirectlyCalled.insert(callees.begin(), callees.end());
+ }
+ }
+ }
+
+ // Remove all indirectly called functions from the list of Cilk functions.
+ //
+ for (hash_set<Function*>::iterator PFI = parallelFunctions.begin(),
+ PFE = parallelFunctions.end(); PFI != PFE; ++PFI)
+ if (indirectlyCalled.count(*PFI) == 0)
+ safeParallelFunctions.insert(*PFI);
+
+#undef CAN_USE_BIND1ST_ON_REFERENCE_TYPE_ARGS
+#ifdef CAN_USE_BIND1ST_ON_REFERENCE_TYPE_ARGS
+ // Use this undecipherable STLese because erase invalidates iterators.
+ // Otherwise we have to copy sets as above.
+ hash_set<Function*>::iterator extrasBegin =
+ std::remove_if(parallelFunctions.begin(), parallelFunctions.end(),
+ compose1(std::bind2nd(std::greater<int>(), 0),
+ bind_obj(&indirectlyCalled,
+ &hash_set<const GlobalValue*>::count)));
+ parallelFunctions.erase(extrasBegin, parallelFunctions.end());
+#endif
+
+ // If there are no parallel functions, we can just give up.
+ if (safeParallelFunctions.empty())
+ return false;
+
+ // Add main as a parallel function since Cilk requires this.
+ safeParallelFunctions.insert(mainFunc);
+
+ // (2,3) Transform each Cilk function and all its calls simply by
+ // adding a unique suffix to the function name.
+ // This should identify both functions and calls to such functions
+ // to the code generator.
+ // (4) Also, insert calls to sync at appropriate points.
+ //
+ Cilkifier cilkifier(M);
+ for (hash_set<Function*>::iterator CFI = safeParallelFunctions.begin(),
+ CFE = safeParallelFunctions.end(); CFI != CFE; ++CFI)
+ {
+ cilkifier.TransformFunc(*CFI, safeParallelFunctions,
+ getAnalysis<PgmDependenceGraph>().getGraph(**CFI));
+ /* getAnalysis<MemoryDepAnalysis>().getGraph(**CFI)); */
+ }
+
+ return true;
+}