1 //===- SSAUpdaterBulk.cpp - Unstructured SSA Update Tool ------------------===//
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 // This file implements the SSAUpdaterBulk class.
10 //
11 //===----------------------------------------------------------------------===//
12
13 #include "llvm/Transforms/Utils/SSAUpdaterBulk.h"
14 #include "llvm/Analysis/IteratedDominanceFrontier.h"
15 #include "llvm/IR/BasicBlock.h"
16 #include "llvm/IR/Dominators.h"
17 #include "llvm/IR/IRBuilder.h"
18 #include "llvm/IR/Instructions.h"
19 #include "llvm/IR/Use.h"
20 #include "llvm/IR/Value.h"
21
22 using namespace llvm;
23
24 #define DEBUG_TYPE "ssaupdaterbulk"
25
26 /// Helper function for finding a block which should have a value for the given
27 /// user. For PHI-nodes this block is the corresponding predecessor, for other
28 /// instructions it's their parent block.
getUserBB(Use * U)29 static BasicBlock *getUserBB(Use *U) {
30 auto *User = cast<Instruction>(U->getUser());
31
32 if (auto *UserPN = dyn_cast<PHINode>(User))
33 return UserPN->getIncomingBlock(*U);
34 else
35 return User->getParent();
36 }
37
38 /// Add a new variable to the SSA rewriter. This needs to be called before
39 /// AddAvailableValue or AddUse calls.
AddVariable(StringRef Name,Type * Ty)40 unsigned SSAUpdaterBulk::AddVariable(StringRef Name, Type *Ty) {
41 unsigned Var = Rewrites.size();
42 LLVM_DEBUG(dbgs() << "SSAUpdater: Var=" << Var << ": initialized with Ty = "
43 << *Ty << ", Name = " << Name << "\n");
44 RewriteInfo RI(Name, Ty);
45 Rewrites.push_back(RI);
46 return Var;
47 }
48
49 /// Indicate that a rewritten value is available in the specified block with the
50 /// specified value.
AddAvailableValue(unsigned Var,BasicBlock * BB,Value * V)51 void SSAUpdaterBulk::AddAvailableValue(unsigned Var, BasicBlock *BB, Value *V) {
52 assert(Var < Rewrites.size() && "Variable not found!");
53 LLVM_DEBUG(dbgs() << "SSAUpdater: Var=" << Var
54 << ": added new available value " << *V << " in "
55 << BB->getName() << "\n");
56 Rewrites[Var].Defines.emplace_back(BB, V);
57 }
58
59 /// Record a use of the symbolic value. This use will be updated with a
60 /// rewritten value when RewriteAllUses is called.
AddUse(unsigned Var,Use * U)61 void SSAUpdaterBulk::AddUse(unsigned Var, Use *U) {
62 assert(Var < Rewrites.size() && "Variable not found!");
63 LLVM_DEBUG(dbgs() << "SSAUpdater: Var=" << Var << ": added a use" << *U->get()
64 << " in " << getUserBB(U)->getName() << "\n");
65 Rewrites[Var].Uses.push_back(U);
66 }
67
68 /// Given sets of UsingBlocks and DefBlocks, compute the set of LiveInBlocks.
69 /// This is basically a subgraph limited by DefBlocks and UsingBlocks.
70 static void
ComputeLiveInBlocks(const SmallPtrSetImpl<BasicBlock * > & UsingBlocks,const SmallPtrSetImpl<BasicBlock * > & DefBlocks,SmallPtrSetImpl<BasicBlock * > & LiveInBlocks,PredIteratorCache & PredCache)71 ComputeLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &UsingBlocks,
72 const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
73 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks,
74 PredIteratorCache &PredCache) {
75 // To determine liveness, we must iterate through the predecessors of blocks
76 // where the def is live. Blocks are added to the worklist if we need to
77 // check their predecessors. Start with all the using blocks.
78 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(UsingBlocks.begin(),
79 UsingBlocks.end());
80
81 // Now that we have a set of blocks where the phi is live-in, recursively add
82 // their predecessors until we find the full region the value is live.
83 while (!LiveInBlockWorklist.empty()) {
84 BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
85
86 // The block really is live in here, insert it into the set. If already in
87 // the set, then it has already been processed.
88 if (!LiveInBlocks.insert(BB).second)
89 continue;
90
91 // Since the value is live into BB, it is either defined in a predecessor or
92 // live into it to. Add the preds to the worklist unless they are a
93 // defining block.
94 for (BasicBlock *P : PredCache.get(BB)) {
95 // The value is not live into a predecessor if it defines the value.
96 if (DefBlocks.count(P))
97 continue;
98
99 // Otherwise it is, add to the worklist.
100 LiveInBlockWorklist.push_back(P);
101 }
102 }
103 }
104
105 struct BBValueInfo {
106 Value *LiveInValue = nullptr;
107 Value *LiveOutValue = nullptr;
108 };
109
110 /// Perform all the necessary updates, including new PHI-nodes insertion and the
111 /// requested uses update.
RewriteAllUses(DominatorTree * DT,SmallVectorImpl<PHINode * > * InsertedPHIs)112 void SSAUpdaterBulk::RewriteAllUses(DominatorTree *DT,
113 SmallVectorImpl<PHINode *> *InsertedPHIs) {
114 DenseMap<BasicBlock *, BBValueInfo> BBInfos;
115 for (auto &R : Rewrites) {
116 BBInfos.clear();
117
118 // Compute locations for new phi-nodes.
119 // For that we need to initialize DefBlocks from definitions in R.Defines,
120 // UsingBlocks from uses in R.Uses, then compute LiveInBlocks, and then use
121 // this set for computing iterated dominance frontier (IDF).
122 // The IDF blocks are the blocks where we need to insert new phi-nodes.
123 ForwardIDFCalculator IDF(*DT);
124 LLVM_DEBUG(dbgs() << "SSAUpdater: rewriting " << R.Uses.size()
125 << " use(s)\n");
126
127 SmallPtrSet<BasicBlock *, 2> DefBlocks(llvm::from_range,
128 llvm::make_first_range(R.Defines));
129 IDF.setDefiningBlocks(DefBlocks);
130
131 SmallPtrSet<BasicBlock *, 2> UsingBlocks;
132 for (Use *U : R.Uses)
133 UsingBlocks.insert(getUserBB(U));
134
135 SmallVector<BasicBlock *, 32> IDFBlocks;
136 SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
137 ComputeLiveInBlocks(UsingBlocks, DefBlocks, LiveInBlocks, PredCache);
138 IDF.setLiveInBlocks(LiveInBlocks);
139 IDF.calculate(IDFBlocks);
140
141 // Reserve sufficient buckets to prevent map growth. [1]
142 BBInfos.reserve(LiveInBlocks.size() + DefBlocks.size());
143
144 for (auto [BB, V] : R.Defines)
145 BBInfos[BB].LiveOutValue = V;
146
147 // We've computed IDF, now insert new phi-nodes there.
148 for (auto *FrontierBB : IDFBlocks) {
149 IRBuilder<> B(FrontierBB, FrontierBB->begin());
150 PHINode *PN = B.CreatePHI(R.Ty, 0, R.Name);
151 BBInfos[FrontierBB].LiveInValue = PN;
152 if (InsertedPHIs)
153 InsertedPHIs->push_back(PN);
154 }
155
156 // IsLiveOut indicates whether we are computing live-out values (true) or
157 // live-in values (false).
158 auto ComputeValue = [&](BasicBlock *BB, bool IsLiveOut) -> Value * {
159 auto *BBInfo = &BBInfos[BB];
160
161 if (IsLiveOut && BBInfo->LiveOutValue)
162 return BBInfo->LiveOutValue;
163
164 if (BBInfo->LiveInValue)
165 return BBInfo->LiveInValue;
166
167 SmallVector<BBValueInfo *, 4> Stack = {BBInfo};
168 Value *V = nullptr;
169
170 while (DT->isReachableFromEntry(BB) && !PredCache.get(BB).empty() &&
171 (BB = DT->getNode(BB)->getIDom()->getBlock())) {
172 BBInfo = &BBInfos[BB];
173
174 if (BBInfo->LiveOutValue) {
175 V = BBInfo->LiveOutValue;
176 break;
177 }
178
179 if (BBInfo->LiveInValue) {
180 V = BBInfo->LiveInValue;
181 break;
182 }
183
184 Stack.emplace_back(BBInfo);
185 }
186
187 if (!V)
188 V = UndefValue::get(R.Ty);
189
190 for (auto *BBInfo : Stack)
191 // Loop above can insert new entries into the BBInfos map: assume the
192 // map shouldn't grow due to [1] and BBInfo references are valid.
193 BBInfo->LiveInValue = V;
194
195 return V;
196 };
197
198 // Fill in arguments of the inserted PHIs.
199 for (auto *BB : IDFBlocks) {
200 auto *PHI = cast<PHINode>(&BB->front());
201 for (BasicBlock *Pred : PredCache.get(BB))
202 PHI->addIncoming(ComputeValue(Pred, /*IsLiveOut=*/true), Pred);
203 }
204
205 // Rewrite actual uses with the inserted definitions.
206 SmallPtrSet<Use *, 4> ProcessedUses;
207 for (Use *U : R.Uses) {
208 if (!ProcessedUses.insert(U).second)
209 continue;
210
211 auto *User = cast<Instruction>(U->getUser());
212 BasicBlock *BB = getUserBB(U);
213 Value *V = ComputeValue(BB, /*IsLiveOut=*/BB != User->getParent());
214 Value *OldVal = U->get();
215 assert(OldVal && "Invalid use!");
216 // Notify that users of the existing value that it is being replaced.
217 if (OldVal != V && OldVal->hasValueHandle())
218 ValueHandleBase::ValueIsRAUWd(OldVal, V);
219 LLVM_DEBUG(dbgs() << "SSAUpdater: replacing " << *OldVal << " with " << *V
220 << "\n");
221 U->set(V);
222 }
223 }
224 }
225