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DivergenceAnalysis.cpp
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1 //===- DivergenceAnalysis.cpp --------- Divergence Analysis Implementation -==//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file implements divergence analysis which determines whether a branch
11 // in a GPU program is divergent.It can help branch optimizations such as jump
12 // threading and loop unswitching to make better decisions.
13 //
14 // GPU programs typically use the SIMD execution model, where multiple threads
15 // in the same execution group have to execute in lock-step. Therefore, if the
16 // code contains divergent branches (i.e., threads in a group do not agree on
17 // which path of the branch to take), the group of threads has to execute all
18 // the paths from that branch with different subsets of threads enabled until
19 // they converge at the immediately post-dominating BB of the paths.
20 //
21 // Due to this execution model, some optimizations such as jump
22 // threading and loop unswitching can be unfortunately harmful when performed on
23 // divergent branches. Therefore, an analysis that computes which branches in a
24 // GPU program are divergent can help the compiler to selectively run these
25 // optimizations.
26 //
27 // This file defines divergence analysis which computes a conservative but
28 // non-trivial approximation of all divergent branches in a GPU program. It
29 // partially implements the approach described in
30 //
31 // Divergence Analysis
32 // Sampaio, Souza, Collange, Pereira
33 // TOPLAS '13
34 //
35 // The divergence analysis identifies the sources of divergence (e.g., special
36 // variables that hold the thread ID), and recursively marks variables that are
37 // data or sync dependent on a source of divergence as divergent.
38 //
39 // While data dependency is a well-known concept, the notion of sync dependency
40 // is worth more explanation. Sync dependence characterizes the control flow
41 // aspect of the propagation of branch divergence. For example,
42 //
43 // %cond = icmp slt i32 %tid, 10
44 // br i1 %cond, label %then, label %else
45 // then:
46 // br label %merge
47 // else:
48 // br label %merge
49 // merge:
50 // %a = phi i32 [ 0, %then ], [ 1, %else ]
51 //
52 // Suppose %tid holds the thread ID. Although %a is not data dependent on %tid
53 // because %tid is not on its use-def chains, %a is sync dependent on %tid
54 // because the branch "br i1 %cond" depends on %tid and affects which value %a
55 // is assigned to.
56 //
57 // The current implementation has the following limitations:
58 // 1. intra-procedural. It conservatively considers the arguments of a
59 // non-kernel-entry function and the return value of a function call as
60 // divergent.
61 // 2. memory as black box. It conservatively considers values loaded from
62 // generic or local address as divergent. This can be improved by leveraging
63 // pointer analysis.
64 //
65 //===----------------------------------------------------------------------===//
66 
68 #include "llvm/Analysis/Passes.h"
71 #include "llvm/IR/Dominators.h"
72 #include "llvm/IR/InstIterator.h"
73 #include "llvm/IR/Instructions.h"
74 #include "llvm/IR/Value.h"
75 #include "llvm/Support/Debug.h"
77 #include <vector>
78 using namespace llvm;
79 
80 namespace {
81 
82 class DivergencePropagator {
83 public:
84  DivergencePropagator(Function &F, TargetTransformInfo &TTI, DominatorTree &DT,
86  : F(F), TTI(TTI), DT(DT), PDT(PDT), DV(DV) {}
87  void populateWithSourcesOfDivergence();
88  void propagate();
89 
90 private:
91  // A helper function that explores data dependents of V.
92  void exploreDataDependency(Value *V);
93  // A helper function that explores sync dependents of TI.
94  void exploreSyncDependency(TerminatorInst *TI);
95  // Computes the influence region from Start to End. This region includes all
96  // basic blocks on any simple path from Start to End.
97  void computeInfluenceRegion(BasicBlock *Start, BasicBlock *End,
98  DenseSet<BasicBlock *> &InfluenceRegion);
99  // Finds all users of I that are outside the influence region, and add these
100  // users to Worklist.
101  void findUsersOutsideInfluenceRegion(
102  Instruction &I, const DenseSet<BasicBlock *> &InfluenceRegion);
103 
104  Function &F;
105  TargetTransformInfo &TTI;
106  DominatorTree &DT;
107  PostDominatorTree &PDT;
108  std::vector<Value *> Worklist; // Stack for DFS.
109  DenseSet<const Value *> &DV; // Stores all divergent values.
110 };
111 
112 void DivergencePropagator::populateWithSourcesOfDivergence() {
113  Worklist.clear();
114  DV.clear();
115  for (auto &I : instructions(F)) {
116  if (TTI.isSourceOfDivergence(&I)) {
117  Worklist.push_back(&I);
118  DV.insert(&I);
119  }
120  }
121  for (auto &Arg : F.args()) {
122  if (TTI.isSourceOfDivergence(&Arg)) {
123  Worklist.push_back(&Arg);
124  DV.insert(&Arg);
125  }
126  }
127 }
128 
129 void DivergencePropagator::exploreSyncDependency(TerminatorInst *TI) {
130  // Propagation rule 1: if branch TI is divergent, all PHINodes in TI's
131  // immediate post dominator are divergent. This rule handles if-then-else
132  // patterns. For example,
133  //
134  // if (tid < 5)
135  // a1 = 1;
136  // else
137  // a2 = 2;
138  // a = phi(a1, a2); // sync dependent on (tid < 5)
139  BasicBlock *ThisBB = TI->getParent();
140 
141  // Unreachable blocks may not be in the dominator tree.
142  if (!DT.isReachableFromEntry(ThisBB))
143  return;
144 
145  // If the function has no exit blocks or doesn't reach any exit blocks, the
146  // post dominator may be null.
147  DomTreeNode *ThisNode = PDT.getNode(ThisBB);
148  if (!ThisNode)
149  return;
150 
151  BasicBlock *IPostDom = ThisNode->getIDom()->getBlock();
152  if (IPostDom == nullptr)
153  return;
154 
155  for (auto I = IPostDom->begin(); isa<PHINode>(I); ++I) {
156  // A PHINode is uniform if it returns the same value no matter which path is
157  // taken.
158  if (!cast<PHINode>(I)->hasConstantOrUndefValue() && DV.insert(&*I).second)
159  Worklist.push_back(&*I);
160  }
161 
162  // Propagation rule 2: if a value defined in a loop is used outside, the user
163  // is sync dependent on the condition of the loop exits that dominate the
164  // user. For example,
165  //
166  // int i = 0;
167  // do {
168  // i++;
169  // if (foo(i)) ... // uniform
170  // } while (i < tid);
171  // if (bar(i)) ... // divergent
172  //
173  // A program may contain unstructured loops. Therefore, we cannot leverage
174  // LoopInfo, which only recognizes natural loops.
175  //
176  // The algorithm used here handles both natural and unstructured loops. Given
177  // a branch TI, we first compute its influence region, the union of all simple
178  // paths from TI to its immediate post dominator (IPostDom). Then, we search
179  // for all the values defined in the influence region but used outside. All
180  // these users are sync dependent on TI.
181  DenseSet<BasicBlock *> InfluenceRegion;
182  computeInfluenceRegion(ThisBB, IPostDom, InfluenceRegion);
183  // An insight that can speed up the search process is that all the in-region
184  // values that are used outside must dominate TI. Therefore, instead of
185  // searching every basic blocks in the influence region, we search all the
186  // dominators of TI until it is outside the influence region.
187  BasicBlock *InfluencedBB = ThisBB;
188  while (InfluenceRegion.count(InfluencedBB)) {
189  for (auto &I : *InfluencedBB)
190  findUsersOutsideInfluenceRegion(I, InfluenceRegion);
191  DomTreeNode *IDomNode = DT.getNode(InfluencedBB)->getIDom();
192  if (IDomNode == nullptr)
193  break;
194  InfluencedBB = IDomNode->getBlock();
195  }
196 }
197 
198 void DivergencePropagator::findUsersOutsideInfluenceRegion(
199  Instruction &I, const DenseSet<BasicBlock *> &InfluenceRegion) {
200  for (User *U : I.users()) {
201  Instruction *UserInst = cast<Instruction>(U);
202  if (!InfluenceRegion.count(UserInst->getParent())) {
203  if (DV.insert(UserInst).second)
204  Worklist.push_back(UserInst);
205  }
206  }
207 }
208 
209 // A helper function for computeInfluenceRegion that adds successors of "ThisBB"
210 // to the influence region.
211 static void
212 addSuccessorsToInfluenceRegion(BasicBlock *ThisBB, BasicBlock *End,
213  DenseSet<BasicBlock *> &InfluenceRegion,
214  std::vector<BasicBlock *> &InfluenceStack) {
215  for (BasicBlock *Succ : successors(ThisBB)) {
216  if (Succ != End && InfluenceRegion.insert(Succ).second)
217  InfluenceStack.push_back(Succ);
218  }
219 }
220 
221 void DivergencePropagator::computeInfluenceRegion(
222  BasicBlock *Start, BasicBlock *End,
223  DenseSet<BasicBlock *> &InfluenceRegion) {
224  assert(PDT.properlyDominates(End, Start) &&
225  "End does not properly dominate Start");
226 
227  // The influence region starts from the end of "Start" to the beginning of
228  // "End". Therefore, "Start" should not be in the region unless "Start" is in
229  // a loop that doesn't contain "End".
230  std::vector<BasicBlock *> InfluenceStack;
231  addSuccessorsToInfluenceRegion(Start, End, InfluenceRegion, InfluenceStack);
232  while (!InfluenceStack.empty()) {
233  BasicBlock *BB = InfluenceStack.back();
234  InfluenceStack.pop_back();
235  addSuccessorsToInfluenceRegion(BB, End, InfluenceRegion, InfluenceStack);
236  }
237 }
238 
239 void DivergencePropagator::exploreDataDependency(Value *V) {
240  // Follow def-use chains of V.
241  for (User *U : V->users()) {
242  Instruction *UserInst = cast<Instruction>(U);
243  if (!TTI.isAlwaysUniform(U) && DV.insert(UserInst).second)
244  Worklist.push_back(UserInst);
245  }
246 }
247 
249  // Traverse the dependency graph using DFS.
250  while (!Worklist.empty()) {
251  Value *V = Worklist.back();
252  Worklist.pop_back();
253  if (TerminatorInst *TI = dyn_cast<TerminatorInst>(V)) {
254  // Terminators with less than two successors won't introduce sync
255  // dependency. Ignore them.
256  if (TI->getNumSuccessors() > 1)
257  exploreSyncDependency(TI);
258  }
259  exploreDataDependency(V);
260  }
261 }
262 
263 } /// end namespace anonymous
264 
265 // Register this pass.
266 char DivergenceAnalysis::ID = 0;
267 INITIALIZE_PASS_BEGIN(DivergenceAnalysis, "divergence", "Divergence Analysis",
268  false, true)
272  false, true)
273 
275  return new DivergenceAnalysis();
276 }
277 
281  AU.setPreservesAll();
282 }
283 
285  auto *TTIWP = getAnalysisIfAvailable<TargetTransformInfoWrapperPass>();
286  if (TTIWP == nullptr)
287  return false;
288 
289  TargetTransformInfo &TTI = TTIWP->getTTI(F);
290  // Fast path: if the target does not have branch divergence, we do not mark
291  // any branch as divergent.
292  if (!TTI.hasBranchDivergence())
293  return false;
294 
295  DivergentValues.clear();
296  auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
297  DivergencePropagator DP(F, TTI,
298  getAnalysis<DominatorTreeWrapperPass>().getDomTree(),
299  PDT, DivergentValues);
300  DP.populateWithSourcesOfDivergence();
301  DP.propagate();
302  return false;
303 }
304 
305 void DivergenceAnalysis::print(raw_ostream &OS, const Module *) const {
306  if (DivergentValues.empty())
307  return;
308  const Value *FirstDivergentValue = *DivergentValues.begin();
309  const Function *F;
310  if (const Argument *Arg = dyn_cast<Argument>(FirstDivergentValue)) {
311  F = Arg->getParent();
312  } else if (const Instruction *I =
313  dyn_cast<Instruction>(FirstDivergentValue)) {
314  F = I->getParent()->getParent();
315  } else {
316  llvm_unreachable("Only arguments and instructions can be divergent");
317  }
318 
319  // Dumps all divergent values in F, arguments and then instructions.
320  for (auto &Arg : F->args()) {
321  if (DivergentValues.count(&Arg))
322  OS << "DIVERGENT: " << Arg << "\n";
323  }
324  // Iterate instructions using instructions() to ensure a deterministic order.
325  for (auto &I : instructions(F)) {
326  if (DivergentValues.count(&I))
327  OS << "DIVERGENT:" << I << "\n";
328  }
329 }
void getAnalysisUsage(AnalysisUsage &AU) const override
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
This class represents an incoming formal argument to a Function.
Definition: Argument.h:30
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
A Module instance is used to store all the information related to an LLVM module. ...
Definition: Module.h:63
bool runOnFunction(Function &F) override
runOnFunction - Virtual method overriden by subclasses to do the per-function processing of the pass...
F(f)
iterator begin()
Instruction iterator methods.
Definition: BasicBlock.h:264
AnalysisUsage & addRequired()
#define INITIALIZE_PASS_DEPENDENCY(depName)
Definition: PassSupport.h:51
bool hasBranchDivergence() const
Return true if branch divergence exists.
FunctionPass * createDivergenceAnalysisPass()
Concrete subclass of DominatorTreeBase that is used to compute a normal dominator tree...
Definition: Dominators.h:142
NodeT * getBlock() const
Subclasses of this class are all able to terminate a basic block.
Definition: InstrTypes.h:55
LLVM Basic Block Representation.
Definition: BasicBlock.h:59
void print(raw_ostream &OS, const Module *) const override
print - Print out the internal state of the pass.
DomTreeNodeBase * getIDom() const
Represent the analysis usage information of a pass.
const Instruction & back() const
Definition: BasicBlock.h:278
static const unsigned End
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:285
INITIALIZE_PASS_END(RegBankSelect, DEBUG_TYPE, "Assign register bank of generic virtual registers", false, false) RegBankSelect
#define llvm_unreachable(msg)
Marks that the current location is not supposed to be reachable.
This pass provides access to the codegen interfaces that are needed for IR-level transformations.
Divergence Analysis
static char ID
end namespace anonymous
void setPreservesAll()
Set by analyses that do not transform their input at all.
iterator_range< user_iterator > users()
Definition: Value.h:405
PostDominatorTree Class - Concrete subclass of DominatorTree that is used to compute the post-dominat...
static void propagate(InstantiatedValue From, InstantiatedValue To, MatchState State, ReachabilitySet &ReachSet, std::vector< WorkListItem > &WorkList)
amdgpu Simplify well known AMD library false Value Value * Arg
Basic Alias true
const Function * getParent() const
Return the enclosing method, or null if none.
Definition: BasicBlock.h:108
#define I(x, y, z)
Definition: MD5.cpp:58
size_type count(const_arg_type_t< ValueT > V) const
Return 1 if the specified key is in the set, 0 otherwise.
Definition: DenseSet.h:91
assert(ImpDefSCC.getReg()==AMDGPU::SCC &&ImpDefSCC.isDef())
unsigned getNumSuccessors() const
Return the number of successors that this terminator has.
LLVM Value Representation.
Definition: Value.h:73
succ_range successors(BasicBlock *BB)
Definition: CFG.h:143
This class implements an extremely fast bulk output stream that can only output to a stream...
Definition: raw_ostream.h:44
inst_range instructions(Function *F)
Definition: InstIterator.h:134
Legacy analysis pass which computes a DominatorTree.
Definition: Dominators.h:254
This pass exposes codegen information to IR-level passes.
INITIALIZE_PASS_BEGIN(DivergenceAnalysis, "divergence", "Divergence Analysis", false, true) INITIALIZE_PASS_END(DivergenceAnalysis
iterator_range< arg_iterator > args()
Definition: Function.h:670
const BasicBlock * getParent() const
Definition: Instruction.h:67