1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
|
//===- DeadCodeElimination.cpp - Eliminate dead iteration ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// The polyhedral dead code elimination pass analyses a SCoP to eliminate
// statement instances that can be proven dead.
// As a consequence, the code generated for this SCoP may execute a statement
// less often. This means, a statement may be executed only in certain loop
// iterations or it may not even be part of the generated code at all.
//
// This code:
//
// for (i = 0; i < N; i++)
// arr[i] = 0;
// for (i = 0; i < N; i++)
// arr[i] = 10;
// for (i = 0; i < N; i++)
// arr[i] = i;
//
// is e.g. simplified to:
//
// for (i = 0; i < N; i++)
// arr[i] = i;
//
// The idea and the algorithm used was first implemented by Sven Verdoolaege in
// the 'ppcg' tool.
//
//===----------------------------------------------------------------------===//
#include "polly/DependenceInfo.h"
#include "polly/LinkAllPasses.h"
#include "polly/ScopInfo.h"
#include "llvm/Support/CommandLine.h"
#include "isl/flow.h"
#include "isl/set.h"
#include "isl/map.h"
#include "isl/union_map.h"
using namespace llvm;
using namespace polly;
namespace {
cl::opt<int> DCEPreciseSteps(
"polly-dce-precise-steps",
cl::desc("The number of precise steps between two approximating "
"iterations. (A value of -1 schedules another approximation stage "
"before the actual dead code elimination."),
cl::ZeroOrMore, cl::init(-1));
class DeadCodeElim : public ScopPass {
public:
static char ID;
explicit DeadCodeElim() : ScopPass(ID) {}
bool runOnScop(Scop &S) override;
void printScop(raw_ostream &OS, Scop &S) const override;
void getAnalysisUsage(AnalysisUsage &AU) const override;
private:
/// @brief Return the set of live iterations.
///
/// The set of live iterations are all iterations that write to memory and for
/// which we can not prove that there will be a later write that _must_
/// overwrite the same memory location and is consequently the only one that
/// is visible after the execution of the SCoP.
///
isl_union_set *getLiveOut(Scop &S);
bool eliminateDeadCode(Scop &S, int PreciseSteps);
};
}
char DeadCodeElim::ID = 0;
// To compute the live outs, we compute for the data-locations that are
// must-written to the last statement that touches these locations. On top of
// this we add all statements that perform may-write accesses.
//
// We could be more precise by removing may-write accesses for which we know
// that they are overwritten by a must-write after. However, at the moment the
// only may-writes we introduce access the full (unbounded) array, such that
// bounded write accesses can not overwrite all of the data-locations. As
// this means may-writes are in the current situation always live, there is
// no point in trying to remove them from the live-out set.
isl_union_set *DeadCodeElim::getLiveOut(Scop &S) {
isl_union_map *Schedule = S.getSchedule();
isl_union_map *WriteIterations = isl_union_map_reverse(S.getMustWrites());
isl_union_map *WriteTimes =
isl_union_map_apply_range(WriteIterations, isl_union_map_copy(Schedule));
isl_union_map *LastWriteTimes = isl_union_map_lexmax(WriteTimes);
isl_union_map *LastWriteIterations = isl_union_map_apply_range(
LastWriteTimes, isl_union_map_reverse(Schedule));
isl_union_set *Live = isl_union_map_range(LastWriteIterations);
Live = isl_union_set_union(Live, isl_union_map_domain(S.getMayWrites()));
return isl_union_set_coalesce(Live);
}
/// Performs polyhedral dead iteration elimination by:
/// o Assuming that the last write to each location is live.
/// o Following each RAW dependency from a live iteration backwards and adding
/// that iteration to the live set.
///
/// To ensure the set of live iterations does not get too complex we always
/// combine a certain number of precise steps with one approximating step that
/// simplifies the life set with an affine hull.
bool DeadCodeElim::eliminateDeadCode(Scop &S, int PreciseSteps) {
DependenceInfo &DI = getAnalysis<DependenceInfo>();
const Dependences &D = DI.getDependences();
if (!D.hasValidDependences())
return false;
isl_union_set *Live = getLiveOut(S);
isl_union_map *Dep =
D.getDependences(Dependences::TYPE_RAW | Dependences::TYPE_RED);
Dep = isl_union_map_reverse(Dep);
if (PreciseSteps == -1)
Live = isl_union_set_affine_hull(Live);
isl_union_set *OriginalDomain = S.getDomains();
int Steps = 0;
while (true) {
isl_union_set *Extra;
Steps++;
Extra =
isl_union_set_apply(isl_union_set_copy(Live), isl_union_map_copy(Dep));
if (isl_union_set_is_subset(Extra, Live)) {
isl_union_set_free(Extra);
break;
}
Live = isl_union_set_union(Live, Extra);
if (Steps > PreciseSteps) {
Steps = 0;
Live = isl_union_set_affine_hull(Live);
}
Live = isl_union_set_intersect(Live, isl_union_set_copy(OriginalDomain));
}
isl_union_map_free(Dep);
isl_union_set_free(OriginalDomain);
bool Changed = S.restrictDomains(isl_union_set_coalesce(Live));
// FIXME: We can probably avoid the recomputation of all dependences by
// updating them explicitly.
if (Changed)
DI.recomputeDependences();
return Changed;
}
bool DeadCodeElim::runOnScop(Scop &S) {
return eliminateDeadCode(S, DCEPreciseSteps);
}
void DeadCodeElim::printScop(raw_ostream &, Scop &) const {}
void DeadCodeElim::getAnalysisUsage(AnalysisUsage &AU) const {
ScopPass::getAnalysisUsage(AU);
AU.addRequired<DependenceInfo>();
}
Pass *polly::createDeadCodeElimPass() { return new DeadCodeElim(); }
INITIALIZE_PASS_BEGIN(DeadCodeElim, "polly-dce",
"Polly - Remove dead iterations", false, false)
INITIALIZE_PASS_DEPENDENCY(DependenceInfo)
INITIALIZE_PASS_DEPENDENCY(ScopInfo)
INITIALIZE_PASS_END(DeadCodeElim, "polly-dce", "Polly - Remove dead iterations",
false, false)
|
