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This file implements a pass that automatically parallelizes a program,

Browse Source 
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
This commit is contained in:
Vikram S. Adve 2002-12-10 00:43:34 +00:00
parent c8ea11cf8e
commit 849346ee23
1 changed files with 548 additions and 0 deletions
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//===- 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;
}