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/ScopInfo.h"
38 #include "llvm/Support/CommandLine.h"
39 #include "isl/flow.h"
40 #include "isl/map.h"
41 #include "isl/set.h"
42 #include "isl/union_map.h"
43 #include "isl/union_set.h"
44 
45 using namespace llvm;
46 using namespace polly;
47 
48 namespace {
49 
50 cl::opt<int> DCEPreciseSteps(
51     "polly-dce-precise-steps",
52     cl::desc("The number of precise steps between two approximating "
53              "iterations. (A value of -1 schedules another approximation stage "
54              "before the actual dead code elimination."),
55     cl::ZeroOrMore, cl::init(-1));
56 
57 class DeadCodeElim : public ScopPass {
58 public:
59   static char ID;
60   explicit DeadCodeElim() : ScopPass(ID) {}
61 
62   bool runOnScop(Scop &S) override;
63 
64   void printScop(raw_ostream &OS, Scop &S) const override;
65   void getAnalysisUsage(AnalysisUsage &AU) const override;
66 
67 private:
68   /// @brief Return the set of live iterations.
69   ///
70   /// The set of live iterations are all iterations that write to memory and for
71   /// which we can not prove that there will be a later write that _must_
72   /// overwrite the same memory location and is consequently the only one that
73   /// is visible after the execution of the SCoP.
74   ///
75   isl_union_set *getLiveOut(Scop &S);
76   bool eliminateDeadCode(Scop &S, int PreciseSteps);
77 };
78 }
79 
80 char DeadCodeElim::ID = 0;
81 
82 // To compute the live outs, we compute for the data-locations that are
83 // must-written to the last statement that touches these locations. On top of
84 // this we add all statements that perform may-write accesses.
85 //
86 // We could be more precise by removing may-write accesses for which we know
87 // that they are overwritten by a must-write after. However, at the moment the
88 // only may-writes we introduce access the full (unbounded) array, such that
89 // bounded write accesses can not overwrite all of the data-locations. As
90 // this means may-writes are in the current situation always live, there is
91 // no point in trying to remove them from the live-out set.
92 isl_union_set *DeadCodeElim::getLiveOut(Scop &S) {
93   isl_union_map *Schedule = S.getSchedule();
94   isl_union_map *WriteIterations = isl_union_map_reverse(S.getMustWrites());
95   isl_union_map *WriteTimes =
96       isl_union_map_apply_range(WriteIterations, isl_union_map_copy(Schedule));
97 
98   isl_union_map *LastWriteTimes = isl_union_map_lexmax(WriteTimes);
99   isl_union_map *LastWriteIterations = isl_union_map_apply_range(
100       LastWriteTimes, isl_union_map_reverse(Schedule));
101 
102   isl_union_set *Live = isl_union_map_range(LastWriteIterations);
103   Live = isl_union_set_union(Live, isl_union_map_domain(S.getMayWrites()));
104   return isl_union_set_coalesce(Live);
105 }
106 
107 /// Performs polyhedral dead iteration elimination by:
108 /// o Assuming that the last write to each location is live.
109 /// o Following each RAW dependency from a live iteration backwards and adding
110 ///   that iteration to the live set.
111 ///
112 /// To ensure the set of live iterations does not get too complex we always
113 /// combine a certain number of precise steps with one approximating step that
114 /// simplifies the life set with an affine hull.
115 bool DeadCodeElim::eliminateDeadCode(Scop &S, int PreciseSteps) {
116   DependenceInfo &DI = getAnalysis<DependenceInfo>();
117   const Dependences &D = DI.getDependences();
118 
119   if (!D.hasValidDependences())
120     return false;
121 
122   isl_union_set *Live = getLiveOut(S);
123   isl_union_map *Dep =
124       D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED);
125   Dep = isl_union_map_reverse(Dep);
126 
127   if (PreciseSteps == -1)
128     Live = isl_union_set_affine_hull(Live);
129 
130   isl_union_set *OriginalDomain = S.getDomains();
131   int Steps = 0;
132   while (true) {
133     isl_union_set *Extra;
134     Steps++;
135 
136     Extra =
137         isl_union_set_apply(isl_union_set_copy(Live), isl_union_map_copy(Dep));
138 
139     if (isl_union_set_is_subset(Extra, Live)) {
140       isl_union_set_free(Extra);
141       break;
142     }
143 
144     Live = isl_union_set_union(Live, Extra);
145 
146     if (Steps > PreciseSteps) {
147       Steps = 0;
148       Live = isl_union_set_affine_hull(Live);
149     }
150 
151     Live = isl_union_set_intersect(Live, isl_union_set_copy(OriginalDomain));
152   }
153   isl_union_map_free(Dep);
154   isl_union_set_free(OriginalDomain);
155 
156   bool Changed = S.restrictDomains(isl_union_set_coalesce(Live));
157 
158   // FIXME: We can probably avoid the recomputation of all dependences by
159   // updating them explicitly.
160   if (Changed)
161     DI.recomputeDependences();
162   return Changed;
163 }
164 
165 bool DeadCodeElim::runOnScop(Scop &S) {
166   return eliminateDeadCode(S, DCEPreciseSteps);
167 }
168 
169 void DeadCodeElim::printScop(raw_ostream &, Scop &) const {}
170 
171 void DeadCodeElim::getAnalysisUsage(AnalysisUsage &AU) const {
172   ScopPass::getAnalysisUsage(AU);
173   AU.addRequired<DependenceInfo>();
174 }
175 
176 Pass *polly::createDeadCodeElimPass() { return new DeadCodeElim(); }
177 
178 INITIALIZE_PASS_BEGIN(DeadCodeElim, "polly-dce",
179                       "Polly - Remove dead iterations", false, false)
180 INITIALIZE_PASS_DEPENDENCY(DependenceInfo)
181 INITIALIZE_PASS_DEPENDENCY(ScopInfo)
182 INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
183                     false, false)
184