1 //===----------------------------------------------------------------------===//
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
9 #include "ReduceOperandsSkip.h"
10 #include "llvm/ADT/Sequence.h"
11 #include "llvm/ADT/SetVector.h"
12 #include "llvm/IR/Constants.h"
13 #include "llvm/IR/Dominators.h"
14 #include "llvm/IR/InstIterator.h"
15 #include "llvm/IR/Instructions.h"
16 #include "llvm/IR/Operator.h"
17
18 using namespace llvm;
19
20 /// Collect all values that are directly or indirectly referenced by @p Root,
21 /// including Root itself. This is a BF search such that the more steps needed
22 /// to get to the reference, the more behind it is found in @p Collection. Each
23 /// step could be its own reduction, therefore we consider later values "more
24 /// reduced".
collectReferencedValues(Value * Root)25 static SetVector<Value *> collectReferencedValues(Value *Root) {
26 SetVector<Value *> Refs;
27 std::deque<Value *> Worklist;
28 Worklist.push_back(Root);
29
30 while (!Worklist.empty()) {
31 Value *Val = Worklist.front();
32 Worklist.pop_front();
33 if (!Refs.insert(Val))
34 continue;
35
36 if (auto *O = dyn_cast<Operator>(Val)) {
37 for (Use &Op : O->operands())
38 Worklist.push_back(Op.get());
39 }
40 }
41
42 return Refs;
43 }
44
shouldReduceOperand(Use & Op)45 static bool shouldReduceOperand(Use &Op) {
46 Type *Ty = Op->getType();
47 if (Ty->isLabelTy() || Ty->isMetadataTy())
48 return false;
49 // TODO: be more precise about which GEP operands we can reduce (e.g. array
50 // indexes)
51 if (isa<GEPOperator>(Op.getUser()))
52 return false;
53 if (auto *CB = dyn_cast<CallBase>(Op.getUser())) {
54 if (&CB->getCalledOperandUse() == &Op)
55 return false;
56 }
57 return true;
58 }
59
60 /// Return a reduction priority for @p V. A higher values means "more reduced".
classifyReductivePower(Value * V)61 static int classifyReductivePower(Value *V) {
62 if (auto *C = dyn_cast<ConstantData>(V)) {
63 if (isa<UndefValue>(V))
64 return -2;
65 if (C->isNullValue())
66 return 7;
67 if (C->isOneValue())
68 return 6;
69 return 5;
70 }
71
72 if (isa<Argument>(V))
73 return 3;
74
75 if (isa<GlobalValue>(V))
76 return 2;
77
78 if (isa<Constant>(V))
79 return 1;
80
81 if (isa<Instruction>(V))
82 return -1;
83
84 return 0;
85 }
86
87 /// Calls @p Callback for every reduction opportunity in @p F. Used by
88 /// countOperands() and extractOperandsFromModule() to ensure consistency
89 /// between the two.
90 static void
opportunities(Function & F,function_ref<void (Use &,ArrayRef<Value * >)> Callback)91 opportunities(Function &F,
92 function_ref<void(Use &, ArrayRef<Value *>)> Callback) {
93 if (F.isDeclaration())
94 return;
95
96 // Need DominatorTree to find out whether an SSA value can be referenced.
97 DominatorTree DT(F);
98
99 // Return whether @p LHS is "more reduced" that @p RHS. That is, whether
100 // @p RHS should be preferred over @p LHS in a reduced output. This is a
101 // partial order, a Value may not be preferable over another.
102 auto IsMoreReduced = [&DT](Value *LHS, Value *RHS) -> bool {
103 // A value is not more reduced than itself.
104 if (LHS == RHS)
105 return false;
106
107 int ReductivePowerDiff =
108 classifyReductivePower(RHS) - classifyReductivePower(LHS);
109 if (ReductivePowerDiff != 0)
110 return ReductivePowerDiff < 0;
111
112 // LHS is more reduced if it is defined further up the dominance tree. In a
113 // chain of definitions,
114 //
115 // %a = ..
116 // %b = op %a
117 // %c = op %b
118 //
119 // every use of %b can be replaced by %a, but not by a use of %c. That is, a
120 // use %c can be replaced in steps first by %b, then by %a, making %a the
121 // "more reduced" choice that skips over more instructions.
122 auto *LHSInst = dyn_cast<Instruction>(LHS);
123 auto *RHSInst = dyn_cast<Instruction>(RHS);
124 if (LHSInst && RHSInst) {
125 if (DT.dominates(LHSInst, RHSInst))
126 return true;
127 }
128
129 // Compress the number of used arguments by prefering the first ones. Unused
130 // trailing argument can be removed by the arguments pass.
131 auto *LHSArg = dyn_cast<Argument>(LHS);
132 auto *RHSArg = dyn_cast<Argument>(RHS);
133 if (LHSArg && RHSArg) {
134 if (LHSArg->getArgNo() < RHSArg->getArgNo())
135 return true;
136 }
137
138 return false;
139 };
140
141 for (Instruction &I : instructions(&F)) {
142 for (Use &Op : I.operands()) {
143 if (!shouldReduceOperand(Op))
144 continue;
145 Value *OpVal = Op.get();
146
147 // Collect refenced values as potential replacement candidates.
148 SetVector<Value *> ReferencedVals = collectReferencedValues(OpVal);
149
150 // Regardless whether referenced, add the function arguments as
151 // replacement possibility with the goal of reducing the number of (used)
152 // function arguments, possibly created by the the operands-to-args.
153 for (Argument &Arg : F.args())
154 ReferencedVals.insert(&Arg);
155
156 // After all candidates have been added, it doesn't need to be a set
157 // anymore.
158 std::vector<Value *> Candidates = ReferencedVals.takeVector();
159
160 // Remove ineligible candidates.
161 llvm::erase_if(Candidates, [&, OpVal](Value *V) {
162 // Candidate value must have the same type.
163 if (OpVal->getType() != V->getType())
164 return true;
165
166 // Only consider candidates that are "more reduced" than the original
167 // value. This explicitly also rules out candidates with the same
168 // reduction power. This is to ensure that repeated invocations of this
169 // pass eventually reach a fixpoint without switch back and forth
170 // between two opportunities with the same reductive power.
171 return !IsMoreReduced(V, OpVal);
172 });
173
174 if (Candidates.empty())
175 continue;
176
177 // collectReferencedValues pushed the more reductive values to the end of
178 // the collection, but we need them at the front.
179 std::reverse(Candidates.begin(), Candidates.end());
180
181 // Independency of collectReferencedValues's idea of reductive power,
182 // ensure the the partial order of IsMoreReduced is enforced.
183 llvm::stable_sort(Candidates, IsMoreReduced);
184
185 Callback(Op, Candidates);
186 }
187 }
188 }
189
extractOperandsFromModule(Oracle & O,ReducerWorkItem & WorkItem)190 static void extractOperandsFromModule(Oracle &O, ReducerWorkItem &WorkItem) {
191 Module &Program = WorkItem.getModule();
192
193 for (Function &F : Program.functions()) {
194 SmallVector<std::pair<Use *, Value *>> Replacements;
195 opportunities(F, [&](Use &Op, ArrayRef<Value *> Candidates) {
196 // Only apply the candidate the Oracle selected to keep that is the most
197 // reduced. Candidates with less reductive power can be interpreted as an
198 // intermediate step that is immediately replaced with the more reduced
199 // one. The number of shouldKeep() calls must be independent of the result
200 // of previous shouldKeep() calls to keep the total number of calls
201 // in-sync with what countOperands() has computed.
202 bool AlreadyReplaced = false;
203 for (Value *C : Candidates) {
204 bool Keep = O.shouldKeep();
205 if (AlreadyReplaced || Keep)
206 continue;
207
208 // Replacing the operand value immediately would influence the candidate
209 // set for the following operands. Delay it until after all candidates
210 // have been determined.
211 Replacements.push_back({&Op, C});
212
213 AlreadyReplaced = true;
214 }
215 });
216
217 for (std::pair<Use *, Value *> P : Replacements) {
218 if (PHINode *Phi = dyn_cast<PHINode>(P.first->getUser()))
219 Phi->setIncomingValueForBlock(Phi->getIncomingBlock(*P.first), P.second);
220 else
221 P.first->set(P.second);
222 }
223 }
224 }
225
reduceOperandsSkipDeltaPass(TestRunner & Test)226 void llvm::reduceOperandsSkipDeltaPass(TestRunner &Test) {
227 runDeltaPass(Test, extractOperandsFromModule,
228 "Reducing operands by skipping over instructions");
229 }
230