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[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 // Compute value at the given block BB. We either should already know it, or we
69 // should be able to recursively reach it going up dominator tree.
computeValueAt(BasicBlock * BB,RewriteInfo & R,DominatorTree * DT)70 Value *SSAUpdaterBulk::computeValueAt(BasicBlock *BB, RewriteInfo &R,
71 DominatorTree *DT) {
72 if (!R.Defines.count(BB)) {
73 if (DT->isReachableFromEntry(BB) && PredCache.get(BB).size()) {
74 BasicBlock *IDom = DT->getNode(BB)->getIDom()->getBlock();
75 Value *V = computeValueAt(IDom, R, DT);
76 R.Defines[BB] = V;
77 } else
78 R.Defines[BB] = UndefValue::get(R.Ty);
79 }
80 return R.Defines[BB];
81 }
82
83 /// Given sets of UsingBlocks and DefBlocks, compute the set of LiveInBlocks.
84 /// This is basically a subgraph limited by DefBlocks and UsingBlocks.
85 static void
ComputeLiveInBlocks(const SmallPtrSetImpl<BasicBlock * > & UsingBlocks,const SmallPtrSetImpl<BasicBlock * > & DefBlocks,SmallPtrSetImpl<BasicBlock * > & LiveInBlocks,PredIteratorCache & PredCache)86 ComputeLiveInBlocks(const SmallPtrSetImpl<BasicBlock *> &UsingBlocks,
87 const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
88 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks,
89 PredIteratorCache &PredCache) {
90 // To determine liveness, we must iterate through the predecessors of blocks
91 // where the def is live. Blocks are added to the worklist if we need to
92 // check their predecessors. Start with all the using blocks.
93 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(UsingBlocks.begin(),
94 UsingBlocks.end());
95
96 // Now that we have a set of blocks where the phi is live-in, recursively add
97 // their predecessors until we find the full region the value is live.
98 while (!LiveInBlockWorklist.empty()) {
99 BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
100
101 // The block really is live in here, insert it into the set. If already in
102 // the set, then it has already been processed.
103 if (!LiveInBlocks.insert(BB).second)
104 continue;
105
106 // Since the value is live into BB, it is either defined in a predecessor or
107 // live into it to. Add the preds to the worklist unless they are a
108 // defining block.
109 for (BasicBlock *P : PredCache.get(BB)) {
110 // The value is not live into a predecessor if it defines the value.
111 if (DefBlocks.count(P))
112 continue;
113
114 // Otherwise it is, add to the worklist.
115 LiveInBlockWorklist.push_back(P);
116 }
117 }
118 }
119
120 /// Perform all the necessary updates, including new PHI-nodes insertion and the
121 /// requested uses update.
RewriteAllUses(DominatorTree * DT,SmallVectorImpl<PHINode * > * InsertedPHIs)122 void SSAUpdaterBulk::RewriteAllUses(DominatorTree *DT,
123 SmallVectorImpl<PHINode *> *InsertedPHIs) {
124 for (auto &R : Rewrites) {
125 // Compute locations for new phi-nodes.
126 // For that we need to initialize DefBlocks from definitions in R.Defines,
127 // UsingBlocks from uses in R.Uses, then compute LiveInBlocks, and then use
128 // this set for computing iterated dominance frontier (IDF).
129 // The IDF blocks are the blocks where we need to insert new phi-nodes.
130 ForwardIDFCalculator IDF(*DT);
131 LLVM_DEBUG(dbgs() << "SSAUpdater: rewriting " << R.Uses.size()
132 << " use(s)\n");
133
134 SmallPtrSet<BasicBlock *, 2> DefBlocks;
135 for (auto &Def : R.Defines)
136 DefBlocks.insert(Def.first);
137 IDF.setDefiningBlocks(DefBlocks);
138
139 SmallPtrSet<BasicBlock *, 2> UsingBlocks;
140 for (Use *U : R.Uses)
141 UsingBlocks.insert(getUserBB(U));
142
143 SmallVector<BasicBlock *, 32> IDFBlocks;
144 SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
145 ComputeLiveInBlocks(UsingBlocks, DefBlocks, LiveInBlocks, PredCache);
146 IDF.resetLiveInBlocks();
147 IDF.setLiveInBlocks(LiveInBlocks);
148 IDF.calculate(IDFBlocks);
149
150 // We've computed IDF, now insert new phi-nodes there.
151 SmallVector<PHINode *, 4> InsertedPHIsForVar;
152 for (auto *FrontierBB : IDFBlocks) {
153 IRBuilder<> B(FrontierBB, FrontierBB->begin());
154 PHINode *PN = B.CreatePHI(R.Ty, 0, R.Name);
155 R.Defines[FrontierBB] = PN;
156 InsertedPHIsForVar.push_back(PN);
157 if (InsertedPHIs)
158 InsertedPHIs->push_back(PN);
159 }
160
161 // Fill in arguments of the inserted PHIs.
162 for (auto *PN : InsertedPHIsForVar) {
163 BasicBlock *PBB = PN->getParent();
164 for (BasicBlock *Pred : PredCache.get(PBB))
165 PN->addIncoming(computeValueAt(Pred, R, DT), Pred);
166 }
167
168 // Rewrite actual uses with the inserted definitions.
169 SmallPtrSet<Use *, 4> ProcessedUses;
170 for (Use *U : R.Uses) {
171 if (!ProcessedUses.insert(U).second)
172 continue;
173 Value *V = computeValueAt(getUserBB(U), R, DT);
174 Value *OldVal = U->get();
175 assert(OldVal && "Invalid use!");
176 // Notify that users of the existing value that it is being replaced.
177 if (OldVal != V && OldVal->hasValueHandle())
178 ValueHandleBase::ValueIsRAUWd(OldVal, V);
179 LLVM_DEBUG(dbgs() << "SSAUpdater: replacing " << *OldVal << " with " << *V
180 << "\n");
181 U->set(V);
182 }
183 }
184 }
185