1 //===- DeadCodeElimination.cpp - Eliminate dead iteration  ----------------===//
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 // The polyhedral dead code elimination pass analyses a SCoP to eliminate
11 // statement instances that can be proven dead.
12 // As a consequence, the code generated for this SCoP may execute a statement
13 // less often. This means, a statement may be executed only in certain loop
14 // iterations or it may not even be part of the generated code at all.
15 //
16 // This code:
17 //
18 //    for (i = 0; i < N; i++)
19 //        arr[i] = 0;
20 //    for (i = 0; i < N; i++)
21 //        arr[i] = 10;
22 //    for (i = 0; i < N; i++)
23 //        arr[i] = i;
24 //
25 // is e.g. simplified to:
26 //
27 //    for (i = 0; i < N; i++)
28 //        arr[i] = i;
29 //
30 // The idea and the algorithm used was first implemented by Sven Verdoolaege in
31 // the 'ppcg' tool.
32 //
33 //===----------------------------------------------------------------------===//
34 
35 #include "polly/DependenceInfo.h"
36 #include "polly/LinkAllPasses.h"
37 #include "polly/Options.h"
38 #include "polly/ScopInfo.h"
39 #include "llvm/Support/CommandLine.h"
40 #include "isl/flow.h"
41 #include "isl/isl-noexceptions.h"
42 #include "isl/map.h"
43 #include "isl/set.h"
44 #include "isl/union_map.h"
45 #include "isl/union_set.h"
46 
47 using namespace llvm;
48 using namespace polly;
49 
50 namespace {
51 
52 cl::opt<int> DCEPreciseSteps(
53     "polly-dce-precise-steps",
54     cl::desc("The number of precise steps between two approximating "
55              "iterations. (A value of -1 schedules another approximation stage "
56              "before the actual dead code elimination."),
57     cl::ZeroOrMore, cl::init(-1), cl::cat(PollyCategory));
58 
59 class DeadCodeElim : public ScopPass {
60 public:
61   static char ID;
62   explicit DeadCodeElim() : ScopPass(ID) {}
63 
64   /// Remove dead iterations from the schedule of @p S.
65   bool runOnScop(Scop &S) override;
66 
67   /// Register all analyses and transformation required.
68   void getAnalysisUsage(AnalysisUsage &AU) const override;
69 
70 private:
71   /// Return the set of live iterations.
72   ///
73   /// The set of live iterations are all iterations that write to memory and for
74   /// which we can not prove that there will be a later write that _must_
75   /// overwrite the same memory location and is consequently the only one that
76   /// is visible after the execution of the SCoP.
77   ///
78   isl::union_set getLiveOut(Scop &S);
79   bool eliminateDeadCode(Scop &S, int PreciseSteps);
80 };
81 } // namespace
82 
83 char DeadCodeElim::ID = 0;
84 
85 // To compute the live outs, we compute for the data-locations that are
86 // must-written to the last statement that touches these locations. On top of
87 // this we add all statements that perform may-write accesses.
88 //
89 // We could be more precise by removing may-write accesses for which we know
90 // that they are overwritten by a must-write after. However, at the moment the
91 // only may-writes we introduce access the full (unbounded) array, such that
92 // bounded write accesses can not overwrite all of the data-locations. As
93 // this means may-writes are in the current situation always live, there is
94 // no point in trying to remove them from the live-out set.
95 isl::union_set DeadCodeElim::getLiveOut(Scop &S) {
96   isl::union_map Schedule = S.getSchedule();
97   isl::union_map MustWrites = S.getMustWrites();
98   isl::union_map WriteIterations = MustWrites.reverse();
99   isl::union_map WriteTimes = WriteIterations.apply_range(Schedule);
100 
101   isl::union_map LastWriteTimes = WriteTimes.lexmax();
102   isl::union_map LastWriteIterations =
103       LastWriteTimes.apply_range(Schedule.reverse());
104 
105   isl::union_set Live = LastWriteIterations.range();
106   isl::union_map MayWrites = S.getMayWrites();
107   Live = Live.unite(MayWrites.domain());
108   return Live.coalesce();
109 }
110 
111 /// Performs polyhedral dead iteration elimination by:
112 /// o Assuming that the last write to each location is live.
113 /// o Following each RAW dependency from a live iteration backwards and adding
114 ///   that iteration to the live set.
115 ///
116 /// To ensure the set of live iterations does not get too complex we always
117 /// combine a certain number of precise steps with one approximating step that
118 /// simplifies the life set with an affine hull.
119 bool DeadCodeElim::eliminateDeadCode(Scop &S, int PreciseSteps) {
120   DependenceInfo &DI = getAnalysis<DependenceInfo>();
121   const Dependences &D = DI.getDependences(Dependences::AL_Statement);
122 
123   if (!D.hasValidDependences())
124     return false;
125 
126   isl::union_set Live = getLiveOut(S);
127   isl::union_map Dep = isl::manage(
128       D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED));
129   Dep = Dep.reverse();
130 
131   if (PreciseSteps == -1)
132     Live = Live.affine_hull();
133 
134   isl::union_set OriginalDomain = S.getDomains();
135   int Steps = 0;
136   while (true) {
137     Steps++;
138 
139     isl::union_set Extra = Live.apply(Dep);
140 
141     if (Extra.is_subset(Live))
142       break;
143 
144     Live = Live.unite(Extra);
145 
146     if (Steps > PreciseSteps) {
147       Steps = 0;
148       Live = Live.affine_hull();
149     }
150 
151     Live = Live.intersect(OriginalDomain);
152   }
153 
154   Live = Live.coalesce();
155 
156   bool Changed = S.restrictDomains(Live);
157 
158   // FIXME: We can probably avoid the recomputation of all dependences by
159   // updating them explicitly.
160   if (Changed)
161     DI.recomputeDependences(Dependences::AL_Statement);
162   return Changed;
163 }
164 
165 bool DeadCodeElim::runOnScop(Scop &S) {
166   return eliminateDeadCode(S, DCEPreciseSteps);
167 }
168 
169 void DeadCodeElim::getAnalysisUsage(AnalysisUsage &AU) const {
170   ScopPass::getAnalysisUsage(AU);
171   AU.addRequired<DependenceInfo>();
172 }
173 
174 Pass *polly::createDeadCodeElimPass() { return new DeadCodeElim(); }
175 
176 INITIALIZE_PASS_BEGIN(DeadCodeElim, "polly-dce",
177                       "Polly - Remove dead iterations", false, false)
178 INITIALIZE_PASS_DEPENDENCY(DependenceInfo)
179 INITIALIZE_PASS_DEPENDENCY(ScopInfoRegionPass)
180 INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
181                     false, false)
182