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849346ee23
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. llvm-svn: 4962
549 lines
20 KiB
C++
549 lines
20 KiB
C++
//===- Parallelize.cpp - Auto parallelization using DS Graphs ---*- C++ -*-===//
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//
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// This file implements a pass that automatically parallelizes a program,
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// using the Cilk multi-threaded runtime system to execute parallel code.
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//
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// The pass uses the Program Dependence Graph (class PDGIterator) to
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// identify parallelizable function calls, i.e., calls whose instances
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// can be executed in parallel with instances of other function calls.
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// (In the future, this should also execute different instances of the same
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// function call in parallel, but that requires parallelizing across
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// loop iterations.)
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//
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// The output of the pass is LLVM code with:
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// (1) all parallelizable functions renamed to flag them as parallelizable;
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// (2) calls to a sync() function introduced at synchronization points.
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// The CWriter recognizes these functions and inserts the appropriate Cilk
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// keywords when writing out C code. This C code must be compiled with cilk2c.
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//
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// Current algorithmic limitations:
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// -- no array dependence analysis
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// -- no parallelization for function calls in different loop iterations
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// (except in unlikely trivial cases)
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//
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// Limitations of using Cilk:
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// -- No parallelism within a function body, e.g., in a loop;
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// -- Simplistic synchronization model requiring all parallel threads
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// created within a function to block at a sync().
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// -- Excessive overhead at "spawned" function calls, which has no benefit
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// once all threads are busy (especially common when the degree of
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// parallelism is low).
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Parallelize.h"
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#include "llvm/Transforms/Utils/DemoteRegToStack.h"
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#include "llvm/Analysis/PgmDependenceGraph.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/DataStructure.h"
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#include "llvm/Analysis/DSGraph.h"
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#include "llvm/Module.h"
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#include "llvm/Function.h"
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#include "llvm/iOther.h"
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#include "llvm/iPHINode.h"
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#include "llvm/iTerminators.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Support/InstVisitor.h"
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#include "llvm/Support/Cilkifier.h"
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#include "Support/NonCopyable.h"
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#include "Support/Statistic.h"
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#include "Support/STLExtras.h"
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#include "Support/hash_set"
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#include "Support/hash_map"
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#include <vector>
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#include <stack>
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#include <functional>
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#include <algorithm>
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#if 0
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void AddToDomSet(vector<BasicBlock*>& domSet, BasicBlock* bb,
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const DominatorTree& domTree)
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{
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DominatorTreeBase::Node* bbNode = domTree.getNode(bb);
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const std::vector<Node*>& domKids = bbNode.getChildren();
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domSet.insert(domSet.end(), domKids.begin(), domKids.end());
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for (unsigned i = 0; i < domKids.size(); ++i)
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AddToDomSet(domSet, domKids[i]->getNode(), domTree);
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}
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bool CheckDominance(Function& func,
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const CallInst& callInst1,
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const CallInst& callInst2)
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{
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if (callInst1 == callInst2) // makes sense if this is in a loop but
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return false; // we're not handling loops yet
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// Check first if one call dominates the other
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DominatorSet& domSet = getAnalysis<DominatorSet>(func);
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if (domSet.dominates(callInst2, callInst1))
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{ // swap callInst1 and callInst2
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const CallInst& tmp = callInst2; callInst2 = callInst1; callInst1 = tmp;
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}
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else if (! domSet.dominates(callInst1, callInst2))
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return false; // neither dominates the other:
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//
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if (! AreIndependent(func, callInst1, callInst2))
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return false;
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}
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#endif
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//----------------------------------------------------------------------------
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// class Cilkifier
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//
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// Code generation pass that transforms code to identify where Cilk keywords
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// should be inserted. This relies on dis -c to print out the keywords.
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//----------------------------------------------------------------------------
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class Cilkifier: public InstVisitor<Cilkifier>
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{
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Function* DummySyncFunc;
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// Data used when transforming each function.
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hash_set<const Instruction*> stmtsVisited; // Flags for recursive DFS
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hash_map<const CallInst*, hash_set<CallInst*> > spawnToSyncsMap;
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// Input data for the transformation.
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const hash_set<Function*>* cilkFunctions; // Set of parallel functions
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PgmDependenceGraph* depGraph;
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void DFSVisitInstr (Instruction* I,
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Instruction* root,
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hash_set<const Instruction*>& depsOfRoot);
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public:
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/*ctor*/ Cilkifier (Module& M);
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// Transform a single function including its name, its call sites, and syncs
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//
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void TransformFunc (Function* F,
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const hash_set<Function*>& cilkFunctions,
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PgmDependenceGraph& _depGraph);
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// The visitor function that does most of the hard work, via DFSVisitInstr
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//
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void visitCallInst(CallInst& CI);
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};
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Cilkifier::Cilkifier(Module& M)
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{
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// create the dummy Sync function and add it to the Module
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DummySyncFunc = new Function(FunctionType::get( Type::VoidTy,
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std::vector<const Type*>(),
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/*isVararg*/ false),
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/*isInternal*/ false, DummySyncFuncName, &M);
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}
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void Cilkifier::TransformFunc(Function* F,
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const hash_set<Function*>& _cilkFunctions,
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PgmDependenceGraph& _depGraph)
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{
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// Memoize the information for this function
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cilkFunctions = &_cilkFunctions;
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depGraph = &_depGraph;
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// Add the marker suffix to the Function name
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// This should automatically mark all calls to the function also!
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F->setName(F->getName() + CilkSuffix);
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// Insert sync operations for each separate spawn
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visit(*F);
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// Now traverse the CFG in rPostorder and eliminate redundant syncs, i.e.,
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// two consecutive sync's on a straight-line path with no intervening spawn.
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}
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void Cilkifier::DFSVisitInstr(Instruction* I,
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Instruction* root,
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hash_set<const Instruction*>& depsOfRoot)
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{
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assert(stmtsVisited.find(I) == stmtsVisited.end());
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stmtsVisited.insert(I);
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// If there is a dependence from root to I, insert Sync and return
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if (depsOfRoot.find(I) != depsOfRoot.end())
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{ // Insert a sync before I and stop searching along this path.
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// If I is a Phi instruction, the dependence can only be an SSA dep.
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// and we need to insert the sync in the predecessor on the appropriate
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// incoming edge!
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CallInst* syncI = 0;
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if (PHINode* phiI = dyn_cast<PHINode>(I))
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{ // check all operands of the Phi and insert before each one
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for (unsigned i = 0, N = phiI->getNumIncomingValues(); i < N; ++i)
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if (phiI->getIncomingValue(i) == root)
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syncI = new CallInst(DummySyncFunc, std::vector<Value*>(), "",
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phiI->getIncomingBlock(i)->getTerminator());
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}
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else
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syncI = new CallInst(DummySyncFunc, std::vector<Value*>(), "", I);
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// Remember the sync for each spawn to eliminate rendundant ones later
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spawnToSyncsMap[cast<CallInst>(root)].insert(syncI);
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return;
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}
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// else visit unvisited successors
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if (BranchInst* brI = dyn_cast<BranchInst>(I))
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{ // visit first instruction in each successor BB
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for (unsigned i = 0, N = brI->getNumSuccessors(); i < N; ++i)
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if (stmtsVisited.find(&brI->getSuccessor(i)->front())
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== stmtsVisited.end())
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DFSVisitInstr(&brI->getSuccessor(i)->front(), root, depsOfRoot);
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}
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else
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if (Instruction* nextI = I->getNext())
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if (stmtsVisited.find(nextI) == stmtsVisited.end())
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DFSVisitInstr(nextI, root, depsOfRoot);
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}
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void Cilkifier::visitCallInst(CallInst& CI)
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{
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assert(CI.getCalledFunction() != 0 && "Only direct calls can be spawned.");
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if (cilkFunctions->find(CI.getCalledFunction()) == cilkFunctions->end())
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return; // not a spawn
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// Find all the outgoing memory dependences.
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hash_set<const Instruction*> depsOfRoot;
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for (PgmDependenceGraph::iterator DI =
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depGraph->outDepBegin(CI, MemoryDeps); ! DI.fini(); ++DI)
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depsOfRoot.insert(&DI->getSink()->getInstr());
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// Now find all outgoing SSA dependences to the eventual non-Phi users of
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// the call value (i.e., direct users that are not phis, and for any
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// user that is a Phi, direct non-Phi users of that Phi, and recursively).
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std::stack<const PHINode*> phiUsers;
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hash_set<const PHINode*> phisSeen; // ensures we don't visit a phi twice
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for (Value::use_iterator UI=CI.use_begin(), UE=CI.use_end(); UI != UE; ++UI)
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if (const PHINode* phiUser = dyn_cast<PHINode>(*UI))
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{
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if (phisSeen.find(phiUser) == phisSeen.end())
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{
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phiUsers.push(phiUser);
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phisSeen.insert(phiUser);
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}
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}
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else
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depsOfRoot.insert(cast<Instruction>(*UI));
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// Now we've found the non-Phi users and immediate phi users.
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// Recursively walk the phi users and add their non-phi users.
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for (const PHINode* phiUser; !phiUsers.empty(); phiUsers.pop())
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{
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phiUser = phiUsers.top();
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for (Value::use_const_iterator UI=phiUser->use_begin(),
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UE=phiUser->use_end(); UI != UE; ++UI)
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if (const PHINode* pn = dyn_cast<PHINode>(*UI))
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{
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if (phisSeen.find(pn) == phisSeen.end())
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{
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phiUsers.push(pn);
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phisSeen.insert(pn);
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}
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}
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else
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depsOfRoot.insert(cast<Instruction>(*UI));
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}
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// Walk paths of the CFG starting at the call instruction and insert
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// one sync before the first dependence on each path, if any.
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if (! depsOfRoot.empty())
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{
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stmtsVisited.clear(); // start a new DFS for this CallInst
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assert(CI.getNext() && "Call instruction cannot be a terminator!");
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DFSVisitInstr(CI.getNext(), &CI, depsOfRoot);
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}
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// Now, eliminate all users of the SSA value of the CallInst, i.e.,
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// if the call instruction returns a value, delete the return value
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// register and replace it by a stack slot.
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if (CI.getType() != Type::VoidTy)
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DemoteRegToStack(CI);
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}
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//----------------------------------------------------------------------------
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// class FindParallelCalls
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//
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// Find all CallInst instructions that have at least one other CallInst
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// that is independent. These are the instructions that can produce
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// useful parallelism.
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//----------------------------------------------------------------------------
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class FindParallelCalls: public InstVisitor<FindParallelCalls>,
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public NonCopyable
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{
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typedef hash_set<CallInst*> DependentsSet;
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typedef DependentsSet::iterator Dependents_iterator;
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typedef DependentsSet::const_iterator Dependents_const_iterator;
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PgmDependenceGraph& depGraph; // dependence graph for the function
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hash_set<Instruction*> stmtsVisited; // flags for DFS walk of depGraph
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hash_map<CallInst*, bool > completed; // flags marking if a CI is done
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hash_map<CallInst*, DependentsSet> dependents; // dependent CIs for each CI
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void VisitOutEdges(Instruction* I,
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CallInst* root,
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DependentsSet& depsOfRoot);
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public:
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std::vector<CallInst*> parallelCalls;
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public:
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/*ctor*/ FindParallelCalls (Function& F, PgmDependenceGraph& DG);
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void visitCallInst (CallInst& CI);
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};
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FindParallelCalls::FindParallelCalls(Function& F,
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PgmDependenceGraph& DG)
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: depGraph(DG)
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{
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// Find all CallInsts reachable from each CallInst using a recursive DFS
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visit(F);
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// Now we've found all CallInsts reachable from each CallInst.
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// Find those CallInsts that are parallel with at least one other CallInst
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// by counting total inEdges and outEdges.
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//
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unsigned long totalNumCalls = completed.size();
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if (totalNumCalls == 1)
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{ // Check first for the special case of a single call instruction not
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// in any loop. It is not parallel, even if it has no dependences
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// (this is why it is a special case).
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//
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// FIXME:
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// THIS CASE IS NOT HANDLED RIGHT NOW, I.E., THERE IS NO
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// PARALLELISM FOR CALLS IN DIFFERENT ITERATIONS OF A LOOP.
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//
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return;
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}
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hash_map<CallInst*, unsigned long> numDeps;
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for (hash_map<CallInst*, DependentsSet>::iterator II = dependents.begin(),
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IE = dependents.end(); II != IE; ++II)
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{
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CallInst* fromCI = II->first;
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numDeps[fromCI] += II->second.size();
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for (Dependents_iterator DI = II->second.begin(), DE = II->second.end();
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DI != DE; ++DI)
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numDeps[*DI]++; // *DI can be reached from II->first
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}
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for (hash_map<CallInst*, DependentsSet>::iterator
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II = dependents.begin(), IE = dependents.end(); II != IE; ++II)
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// FIXME: Remove "- 1" when considering parallelism in loops
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if (numDeps[II->first] < totalNumCalls - 1)
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parallelCalls.push_back(II->first);
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}
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void FindParallelCalls::VisitOutEdges(Instruction* I,
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CallInst* root,
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DependentsSet& depsOfRoot)
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{
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assert(stmtsVisited.find(I) == stmtsVisited.end() && "Stmt visited twice?");
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stmtsVisited.insert(I);
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if (CallInst* CI = dyn_cast<CallInst>(I))
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// FIXME: Ignoring parallelism in a loop. Here we're actually *ignoring*
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// a self-dependence in order to get the count comparison right above.
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// When we include loop parallelism, self-dependences should be included.
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//
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if (CI != root)
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{ // CallInst root has a path to CallInst I and any calls reachable from I
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depsOfRoot.insert(CI);
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if (completed[CI])
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{ // We have already visited I so we know all nodes it can reach!
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DependentsSet& depsOfI = dependents[CI];
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depsOfRoot.insert(depsOfI.begin(), depsOfI.end());
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return;
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}
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}
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// If we reach here, we need to visit all children of I
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for (PgmDependenceGraph::iterator DI = depGraph.outDepBegin(*I);
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! DI.fini(); ++DI)
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{
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Instruction* sink = &DI->getSink()->getInstr();
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if (stmtsVisited.find(sink) == stmtsVisited.end())
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VisitOutEdges(sink, root, depsOfRoot);
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}
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}
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void FindParallelCalls::visitCallInst(CallInst& CI)
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{
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if (completed[&CI])
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return;
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stmtsVisited.clear(); // clear flags to do a fresh DFS
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// Visit all children of CI using a recursive walk through dep graph
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DependentsSet& depsOfRoot = dependents[&CI];
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for (PgmDependenceGraph::iterator DI = depGraph.outDepBegin(CI);
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! DI.fini(); ++DI)
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{
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Instruction* sink = &DI->getSink()->getInstr();
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if (stmtsVisited.find(sink) == stmtsVisited.end())
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VisitOutEdges(sink, &CI, depsOfRoot);
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}
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completed[&CI] = true;
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}
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//----------------------------------------------------------------------------
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// class Parallelize
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//
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// (1) Find candidate parallel functions: any function F s.t.
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// there is a call C1 to the function F that is followed or preceded
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// by at least one other call C2 that is independent of this one
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// (i.e., there is no dependence path from C1 to C2 or C2 to C1)
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// (2) Label such a function F as a cilk function.
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// (3) Convert every call to F to a spawn
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// (4) For every function X, insert sync statements so that
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// every spawn is postdominated by a sync before any statements
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// with a data dependence to/from the call site for the spawn
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//
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//----------------------------------------------------------------------------
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namespace {
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class Parallelize: public Pass
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{
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public:
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/// Driver functions to transform a program
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///
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bool run(Module& M);
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/// getAnalysisUsage - Modifies extensively so preserve nothing.
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/// Uses the DependenceGraph and the Top-down DS Graph (only to find
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/// all functions called via an indirect call).
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///
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.addRequired<TDDataStructures>();
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AU.addRequired<MemoryDepAnalysis>(); // force this not to be released
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AU.addRequired<PgmDependenceGraph>(); // because it is needed by this
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}
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};
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RegisterOpt<Parallelize> X("parallel", "Parallelize program using Cilk");
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}
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static Function* FindMain(Module& M)
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{
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for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
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if (FI->getName() == std::string("main"))
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return FI;
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return NULL;
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}
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bool Parallelize::run(Module& M)
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{
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hash_set<Function*> parallelFunctions;
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hash_set<Function*> safeParallelFunctions;
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hash_set<const GlobalValue*> indirectlyCalled;
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// If there is no main (i.e., for an incomplete program), we can do nothing.
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// If there is a main, mark main as a parallel function.
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//
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Function* mainFunc = FindMain(M);
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if (!mainFunc)
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return false;
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// (1) Find candidate parallel functions and mark them as Cilk functions
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//
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for (Module::iterator FI = M.begin(), FE = M.end(); FI != FE; ++FI)
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if (! FI->isExternal())
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{
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Function* F = FI;
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DSGraph& tdg = getAnalysis<TDDataStructures>().getDSGraph(*F);
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// All the hard analysis work gets done here!
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//
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FindParallelCalls finder(*F,
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getAnalysis<PgmDependenceGraph>().getGraph(*F));
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/* getAnalysis<MemoryDepAnalysis>().getGraph(*F)); */
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// Now we know which call instructions are useful to parallelize.
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// Remember those callee functions.
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//
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for (std::vector<CallInst*>::iterator
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CII = finder.parallelCalls.begin(),
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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;
|
|
}
|