llvm/lib/Support/DAGDeltaAlgorithm.cpp

   1 //===--- DAGDeltaAlgorithm.cpp - A DAG Minimization Algorithm --*- C++ -*--===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //===----------------------------------------------------------------------===//
   7 //
   8 // The algorithm we use attempts to exploit the dependency information by
   9 // minimizing top-down. We start by constructing an initial root set R, and
  10 // then iteratively:
  11 //
  12 //   1. Minimize the set R using the test predicate:
  13 //       P'(S) = P(S union pred*(S))
  14 //
  15 //   2. Extend R to R' = R union pred(R).
  16 //
  17 // until a fixed point is reached.
  18 //
  19 // The idea is that we want to quickly prune entire portions of the graph, so we
  20 // try to find high-level nodes that can be eliminated with all of their
  21 // dependents.
  22 //
  23 // FIXME: The current algorithm doesn't actually provide a strong guarantee
  24 // about the minimality of the result. The problem is that after adding nodes to
  25 // the required set, we no longer consider them for elimination. For strictly
  26 // well formed predicates, this doesn't happen, but it commonly occurs in
  27 // practice when there are unmodelled dependencies. I believe we can resolve
  28 // this by allowing the required set to be minimized as well, but need more test
  29 // cases first.
  30 //
  31 //===----------------------------------------------------------------------===//
  32
  33 #include "llvm/ADT/DAGDeltaAlgorithm.h"
  34 #include "llvm/ADT/DeltaAlgorithm.h"
  35 #include "llvm/Support/Debug.h"
  36 #include "llvm/Support/Format.h"
  37 #include "llvm/Support/raw_ostream.h"
  38 #include <cassert>
  39 #include <map>
  40 using namespace llvm;
  41
  42 #define DEBUG_TYPE "dag-delta"
  43
  44 namespace {
  45
  46 class DAGDeltaAlgorithmImpl {
  47   friend class DeltaActiveSetHelper;
  48
  49 public:
  50   typedef DAGDeltaAlgorithm::change_ty change_ty;
  51   typedef DAGDeltaAlgorithm::changeset_ty changeset_ty;
  52   typedef DAGDeltaAlgorithm::changesetlist_ty changesetlist_ty;
  53   typedef DAGDeltaAlgorithm::edge_ty edge_ty;
  54
  55 private:
  56   typedef std::vector<change_ty>::iterator pred_iterator_ty;
  57   typedef std::vector<change_ty>::iterator succ_iterator_ty;
  58   typedef std::set<change_ty>::iterator pred_closure_iterator_ty;
  59   typedef std::set<change_ty>::iterator succ_closure_iterator_ty;
  60
  61   DAGDeltaAlgorithm &DDA;
  62
  63   std::vector<change_ty> Roots;
  64
  65   /// Cache of failed test results. Successful test results are never cached
  66   /// since we always reduce following a success. We maintain an independent
  67   /// cache from that used by the individual delta passes because we may get
  68   /// hits across multiple individual delta invocations.
  69   mutable std::set<changeset_ty> FailedTestsCache;
  70
  71   // FIXME: Gross.
  72   std::map<change_ty, std::vector<change_ty> > Predecessors;
  73   std::map<change_ty, std::vector<change_ty> > Successors;
  74
  75   std::map<change_ty, std::set<change_ty> > PredClosure;
  76   std::map<change_ty, std::set<change_ty> > SuccClosure;
  77
  78 private:
  79   pred_iterator_ty pred_begin(change_ty Node) {
  80     assert(Predecessors.count(Node) && "Invalid node!");
  81     return Predecessors[Node].begin();
  82   }
  83   pred_iterator_ty pred_end(change_ty Node) {
  84     assert(Predecessors.count(Node) && "Invalid node!");
  85     return Predecessors[Node].end();
  86   }
  87
  88   pred_closure_iterator_ty pred_closure_begin(change_ty Node) {
  89     assert(PredClosure.count(Node) && "Invalid node!");
  90     return PredClosure[Node].begin();
  91   }
  92   pred_closure_iterator_ty pred_closure_end(change_ty Node) {
  93     assert(PredClosure.count(Node) && "Invalid node!");
  94     return PredClosure[Node].end();
  95   }
  96
  97   succ_iterator_ty succ_begin(change_ty Node) {
  98     assert(Successors.count(Node) && "Invalid node!");
  99     return Successors[Node].begin();
 100   }
 101   succ_iterator_ty succ_end(change_ty Node) {
 102     assert(Successors.count(Node) && "Invalid node!");
 103     return Successors[Node].end();
 104   }
 105
 106   succ_closure_iterator_ty succ_closure_begin(change_ty Node) {
 107     assert(SuccClosure.count(Node) && "Invalid node!");
 108     return SuccClosure[Node].begin();
 109   }
 110   succ_closure_iterator_ty succ_closure_end(change_ty Node) {
 111     assert(SuccClosure.count(Node) && "Invalid node!");
 112     return SuccClosure[Node].end();
 113   }
 114
 115   void UpdatedSearchState(const changeset_ty &Changes,
 116                           const changesetlist_ty &Sets,
 117                           const changeset_ty &Required) {
 118     DDA.UpdatedSearchState(Changes, Sets, Required);
 119   }
 120
 121   /// ExecuteOneTest - Execute a single test predicate on the change set \p S.
 122   bool ExecuteOneTest(const changeset_ty &S) {
 123     // Check dependencies invariant.
 124     LLVM_DEBUG({
 125       for (changeset_ty::const_iterator it = S.begin(), ie = S.end(); it != ie;
 126            ++it)
 127         for (succ_iterator_ty it2 = succ_begin(*it), ie2 = succ_end(*it);
 128              it2 != ie2; ++it2)
 129           assert(S.count(*it2) && "Attempt to run invalid changeset!");
 130     });
 131
 132     return DDA.ExecuteOneTest(S);
 133   }
 134
 135 public:
 136   DAGDeltaAlgorithmImpl(DAGDeltaAlgorithm &DDA, const changeset_ty &Changes,
 137                         const std::vector<edge_ty> &Dependencies);
 138
 139   changeset_ty Run();
 140
 141   /// GetTestResult - Get the test result for the active set \p Changes with
 142   /// \p Required changes from the cache, executing the test if necessary.
 143   ///
 144   /// \param Changes - The set of active changes being minimized, which should
 145   /// have their pred closure included in the test.
 146   /// \param Required - The set of changes which have previously been
 147   /// established to be required.
 148   /// \return - The test result.
 149   bool GetTestResult(const changeset_ty &Changes, const changeset_ty &Required);
 150 };
 151
 152 /// Helper object for minimizing an active set of changes.
 153 class DeltaActiveSetHelper : public DeltaAlgorithm {
 154   DAGDeltaAlgorithmImpl &DDAI;
 155
 156   const changeset_ty &Required;
 157
 158 protected:
 159   /// UpdatedSearchState - Callback used when the search state changes.
 160   void UpdatedSearchState(const changeset_ty &Changes,
 161                                   const changesetlist_ty &Sets) override {
 162     DDAI.UpdatedSearchState(Changes, Sets, Required);
 163   }
 164
 165   bool ExecuteOneTest(const changeset_ty &S) override {
 166     return DDAI.GetTestResult(S, Required);
 167   }
 168
 169 public:
 170   DeltaActiveSetHelper(DAGDeltaAlgorithmImpl &DDAI,
 171                        const changeset_ty &Required)
 172       : DDAI(DDAI), Required(Required) {}
 173 };
 174
 175 } // namespace
 176
 177 DAGDeltaAlgorithmImpl::DAGDeltaAlgorithmImpl(
 178     DAGDeltaAlgorithm &DDA, const changeset_ty &Changes,
 179     const std::vector<edge_ty> &Dependencies)
 180     : DDA(DDA) {
 181   for (change_ty Change : Changes) {
 182     Predecessors.insert(std::make_pair(Change, std::vector<change_ty>()));
 183     Successors.insert(std::make_pair(Change, std::vector<change_ty>()));
 184   }
 185   for (const edge_ty &Dep : Dependencies) {
 186     Predecessors[Dep.second].push_back(Dep.first);
 187     Successors[Dep.first].push_back(Dep.second);
 188   }
 189
 190   // Compute the roots.
 191   for (change_ty Change : Changes)
 192     if (succ_begin(Change) == succ_end(Change))
 193       Roots.push_back(Change);
 194
 195   // Pre-compute the closure of the successor relation.
 196   std::vector<change_ty> Worklist(Roots.begin(), Roots.end());
 197   while (!Worklist.empty()) {
 198     change_ty Change = Worklist.back();
 199     Worklist.pop_back();
 200
 201     std::set<change_ty> &ChangeSuccs = SuccClosure[Change];
 202     for (pred_iterator_ty it = pred_begin(Change),
 203            ie = pred_end(Change); it != ie; ++it) {
 204       SuccClosure[*it].insert(Change);
 205       SuccClosure[*it].insert(ChangeSuccs.begin(), ChangeSuccs.end());
 206       Worklist.push_back(*it);
 207     }
 208   }
 209
 210   // Invert to form the predecessor closure map.
 211   for (change_ty Change : Changes)
 212     PredClosure.insert(std::make_pair(Change, std::set<change_ty>()));
 213   for (change_ty Change : Changes)
 214     for (succ_closure_iterator_ty it2 = succ_closure_begin(Change),
 215                                   ie2 = succ_closure_end(Change);
 216          it2 != ie2; ++it2)
 217       PredClosure[*it2].insert(Change);
 218
 219   // Dump useful debug info.
 220   LLVM_DEBUG({
 221     llvm::errs() << "-- DAGDeltaAlgorithmImpl --\n";
 222     llvm::errs() << "Changes: [";
 223     for (changeset_ty::const_iterator it = Changes.begin(), ie = Changes.end();
 224          it != ie; ++it) {
 225       if (it != Changes.begin())
 226         llvm::errs() << ", ";
 227       llvm::errs() << *it;
 228
 229       if (succ_begin(*it) != succ_end(*it)) {
 230         llvm::errs() << "(";
 231         for (succ_iterator_ty it2 = succ_begin(*it), ie2 = succ_end(*it);
 232              it2 != ie2; ++it2) {
 233           if (it2 != succ_begin(*it))
 234             llvm::errs() << ", ";
 235           llvm::errs() << "->" << *it2;
 236         }
 237         llvm::errs() << ")";
 238       }
 239     }
 240     llvm::errs() << "]\n";
 241
 242     llvm::errs() << "Roots: [";
 243     for (std::vector<change_ty>::const_iterator it = Roots.begin(),
 244                                                 ie = Roots.end();
 245          it != ie; ++it) {
 246       if (it != Roots.begin())
 247         llvm::errs() << ", ";
 248       llvm::errs() << *it;
 249     }
 250     llvm::errs() << "]\n";
 251
 252     llvm::errs() << "Predecessor Closure:\n";
 253     for (change_ty Change : Changes) {
 254       llvm::errs() << format("  %-4d: [", Change);
 255       for (pred_closure_iterator_ty it2 = pred_closure_begin(Change),
 256                                     ie2 = pred_closure_end(Change);
 257            it2 != ie2; ++it2) {
 258         if (it2 != pred_closure_begin(Change))
 259           llvm::errs() << ", ";
 260         llvm::errs() << *it2;
 261       }
 262       llvm::errs() << "]\n";
 263     }
 264
 265     llvm::errs() << "Successor Closure:\n";
 266     for (change_ty Change : Changes) {
 267       llvm::errs() << format("  %-4d: [", Change);
 268       for (succ_closure_iterator_ty it2 = succ_closure_begin(Change),
 269                                     ie2 = succ_closure_end(Change);
 270            it2 != ie2; ++it2) {
 271         if (it2 != succ_closure_begin(Change))
 272           llvm::errs() << ", ";
 273         llvm::errs() << *it2;
 274       }
 275       llvm::errs() << "]\n";
 276     }
 277
 278     llvm::errs() << "\n\n";
 279   });
 280 }
 281
 282 bool DAGDeltaAlgorithmImpl::GetTestResult(const changeset_ty &Changes,
 283                                           const changeset_ty &Required) {
 284   changeset_ty Extended(Required);
 285   Extended.insert(Changes.begin(), Changes.end());
 286   for (change_ty Change : Changes)
 287     Extended.insert(pred_closure_begin(Change), pred_closure_end(Change));
 288
 289   if (FailedTestsCache.count(Extended))
 290     return false;
 291
 292   bool Result = ExecuteOneTest(Extended);
 293   if (!Result)
 294     FailedTestsCache.insert(Extended);
 295
 296   return Result;
 297 }
 298
 299 DAGDeltaAlgorithm::changeset_ty
 300 DAGDeltaAlgorithmImpl::Run() {
 301   // The current set of changes we are minimizing, starting at the roots.
 302   changeset_ty CurrentSet(Roots.begin(), Roots.end());
 303
 304   // The set of required changes.
 305   changeset_ty Required;
 306
 307   // Iterate until the active set of changes is empty. Convergence is guaranteed
 308   // assuming input was a DAG.
 309   //
 310   // Invariant:  CurrentSet intersect Required == {}
 311   // Invariant:  Required == (Required union succ*(Required))
 312   while (!CurrentSet.empty()) {
 313     LLVM_DEBUG({
 314       llvm::errs() << "DAG_DD - " << CurrentSet.size() << " active changes, "
 315                    << Required.size() << " required changes\n";
 316     });
 317
 318     // Minimize the current set of changes.
 319     DeltaActiveSetHelper Helper(*this, Required);
 320     changeset_ty CurrentMinSet = Helper.Run(CurrentSet);
 321
 322     // Update the set of required changes. Since
 323     //   CurrentMinSet subset CurrentSet
 324     // and after the last iteration,
 325     //   succ(CurrentSet) subset Required
 326     // then
 327     //   succ(CurrentMinSet) subset Required
 328     // and our invariant on Required is maintained.
 329     Required.insert(CurrentMinSet.begin(), CurrentMinSet.end());
 330
 331     // Replace the current set with the predecssors of the minimized set of
 332     // active changes.
 333     CurrentSet.clear();
 334     for (change_ty CT : CurrentMinSet)
 335       CurrentSet.insert(pred_begin(CT), pred_end(CT));
 336
 337     // FIXME: We could enforce CurrentSet intersect Required == {} here if we
 338     // wanted to protect against cyclic graphs.
 339   }
 340
 341   return Required;
 342 }
 343
 344 void DAGDeltaAlgorithm::anchor() {
 345 }
 346
 347 DAGDeltaAlgorithm::changeset_ty
 348 DAGDeltaAlgorithm::Run(const changeset_ty &Changes,
 349                        const std::vector<edge_ty> &Dependencies) {
 350   return DAGDeltaAlgorithmImpl(*this, Changes, Dependencies).Run();
 351 }