1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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 // This file implements dead code elimination and basic block merging, along
11 // with a collection of other peephole control flow optimizations. For example:
12 //
13 // * Removes basic blocks with no predecessors.
14 // * Merges a basic block into its predecessor if there is only one and the
15 // predecessor only has one successor.
16 // * Eliminates PHI nodes for basic blocks with a single predecessor.
17 // * Eliminates a basic block that only contains an unconditional branch.
18 // * Changes invoke instructions to nounwind functions to be calls.
19 // * Change things like "if (x) if (y)" into "if (x&y)".
20 // * etc..
21 //
22 //===----------------------------------------------------------------------===//
23
24 #include "llvm/ADT/SmallPtrSet.h"
25 #include "llvm/ADT/SmallVector.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/Analysis/AssumptionCache.h"
28 #include "llvm/Analysis/CFG.h"
29 #include "llvm/Analysis/GlobalsModRef.h"
30 #include "llvm/Analysis/TargetTransformInfo.h"
31 #include "llvm/IR/Attributes.h"
32 #include "llvm/IR/CFG.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/DataLayout.h"
35 #include "llvm/IR/Instructions.h"
36 #include "llvm/IR/IntrinsicInst.h"
37 #include "llvm/IR/Module.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Support/CommandLine.h"
40 #include "llvm/Transforms/Scalar.h"
41 #include "llvm/Transforms/Scalar/SimplifyCFG.h"
42 #include "llvm/Transforms/Utils/Local.h"
43 #include <utility>
44 using namespace llvm;
45
46 #define DEBUG_TYPE "simplifycfg"
47
48 static cl::opt<unsigned>
49 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
50 cl::desc("Control the number of bonus instructions (default = 1)"));
51
52 STATISTIC(NumSimpl, "Number of blocks simplified");
53
54 /// If we have more than one empty (other than phi node) return blocks,
55 /// merge them together to promote recursive block merging.
mergeEmptyReturnBlocks(Function & F)56 static bool mergeEmptyReturnBlocks(Function &F) {
57 bool Changed = false;
58
59 BasicBlock *RetBlock = nullptr;
60
61 // Scan all the blocks in the function, looking for empty return blocks.
62 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
63 BasicBlock &BB = *BBI++;
64
65 // Only look at return blocks.
66 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
67 if (!Ret) continue;
68
69 // Only look at the block if it is empty or the only other thing in it is a
70 // single PHI node that is the operand to the return.
71 if (Ret != &BB.front()) {
72 // Check for something else in the block.
73 BasicBlock::iterator I(Ret);
74 --I;
75 // Skip over debug info.
76 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
77 --I;
78 if (!isa<DbgInfoIntrinsic>(I) &&
79 (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
80 Ret->getOperand(0) != &*I))
81 continue;
82 }
83
84 // If this is the first returning block, remember it and keep going.
85 if (!RetBlock) {
86 RetBlock = &BB;
87 continue;
88 }
89
90 // Otherwise, we found a duplicate return block. Merge the two.
91 Changed = true;
92
93 // Case when there is no input to the return or when the returned values
94 // agree is trivial. Note that they can't agree if there are phis in the
95 // blocks.
96 if (Ret->getNumOperands() == 0 ||
97 Ret->getOperand(0) ==
98 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
99 BB.replaceAllUsesWith(RetBlock);
100 BB.eraseFromParent();
101 continue;
102 }
103
104 // If the canonical return block has no PHI node, create one now.
105 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
106 if (!RetBlockPHI) {
107 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
108 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
109 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
110 std::distance(PB, PE), "merge",
111 &RetBlock->front());
112
113 for (pred_iterator PI = PB; PI != PE; ++PI)
114 RetBlockPHI->addIncoming(InVal, *PI);
115 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
116 }
117
118 // Turn BB into a block that just unconditionally branches to the return
119 // block. This handles the case when the two return blocks have a common
120 // predecessor but that return different things.
121 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
122 BB.getTerminator()->eraseFromParent();
123 BranchInst::Create(RetBlock, &BB);
124 }
125
126 return Changed;
127 }
128
129 /// Call SimplifyCFG on all the blocks in the function,
130 /// iterating until no more changes are made.
iterativelySimplifyCFG(Function & F,const TargetTransformInfo & TTI,AssumptionCache * AC,unsigned BonusInstThreshold)131 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
132 AssumptionCache *AC,
133 unsigned BonusInstThreshold) {
134 bool Changed = false;
135 bool LocalChange = true;
136
137 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
138 FindFunctionBackedges(F, Edges);
139 SmallPtrSet<BasicBlock *, 16> LoopHeaders;
140 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
141 LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
142
143 while (LocalChange) {
144 LocalChange = false;
145
146 // Loop over all of the basic blocks and remove them if they are unneeded.
147 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
148 if (SimplifyCFG(&*BBIt++, TTI, BonusInstThreshold, AC, &LoopHeaders)) {
149 LocalChange = true;
150 ++NumSimpl;
151 }
152 }
153 Changed |= LocalChange;
154 }
155 return Changed;
156 }
157
simplifyFunctionCFG(Function & F,const TargetTransformInfo & TTI,AssumptionCache * AC,int BonusInstThreshold)158 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
159 AssumptionCache *AC, int BonusInstThreshold) {
160 bool EverChanged = removeUnreachableBlocks(F);
161 EverChanged |= mergeEmptyReturnBlocks(F);
162 EverChanged |= iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
163
164 // If neither pass changed anything, we're done.
165 if (!EverChanged) return false;
166
167 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
168 // removeUnreachableBlocks is needed to nuke them, which means we should
169 // iterate between the two optimizations. We structure the code like this to
170 // avoid rerunning iterativelySimplifyCFG if the second pass of
171 // removeUnreachableBlocks doesn't do anything.
172 if (!removeUnreachableBlocks(F))
173 return true;
174
175 do {
176 EverChanged = iterativelySimplifyCFG(F, TTI, AC, BonusInstThreshold);
177 EverChanged |= removeUnreachableBlocks(F);
178 } while (EverChanged);
179
180 return true;
181 }
182
SimplifyCFGPass()183 SimplifyCFGPass::SimplifyCFGPass()
184 : BonusInstThreshold(UserBonusInstThreshold) {}
185
SimplifyCFGPass(int BonusInstThreshold)186 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
187 : BonusInstThreshold(BonusInstThreshold) {}
188
run(Function & F,AnalysisManager<Function> & AM)189 PreservedAnalyses SimplifyCFGPass::run(Function &F,
190 AnalysisManager<Function> &AM) {
191 auto &TTI = AM.getResult<TargetIRAnalysis>(F);
192 auto &AC = AM.getResult<AssumptionAnalysis>(F);
193
194 if (!simplifyFunctionCFG(F, TTI, &AC, BonusInstThreshold))
195 return PreservedAnalyses::all();
196 PreservedAnalyses PA;
197 PA.preserve<GlobalsAA>();
198 return PA;
199 }
200
201 namespace {
202 struct CFGSimplifyPass : public FunctionPass {
203 static char ID; // Pass identification, replacement for typeid
204 unsigned BonusInstThreshold;
205 std::function<bool(const Function &)> PredicateFtor;
206
CFGSimplifyPass__anonbefdac880111::CFGSimplifyPass207 CFGSimplifyPass(int T = -1,
208 std::function<bool(const Function &)> Ftor = nullptr)
209 : FunctionPass(ID), PredicateFtor(std::move(Ftor)) {
210 BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
211 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
212 }
runOnFunction__anonbefdac880111::CFGSimplifyPass213 bool runOnFunction(Function &F) override {
214 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
215 return false;
216
217 AssumptionCache *AC =
218 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
219 const TargetTransformInfo &TTI =
220 getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
221 return simplifyFunctionCFG(F, TTI, AC, BonusInstThreshold);
222 }
223
getAnalysisUsage__anonbefdac880111::CFGSimplifyPass224 void getAnalysisUsage(AnalysisUsage &AU) const override {
225 AU.addRequired<AssumptionCacheTracker>();
226 AU.addRequired<TargetTransformInfoWrapperPass>();
227 AU.addPreserved<GlobalsAAWrapperPass>();
228 }
229 };
230 }
231
232 char CFGSimplifyPass::ID = 0;
233 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
234 false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)235 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
236 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
237 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
238 false)
239
240 // Public interface to the CFGSimplification pass
241 FunctionPass *
242 llvm::createCFGSimplificationPass(int Threshold,
243 std::function<bool(const Function &)> Ftor) {
244 return new CFGSimplifyPass(Threshold, std::move(Ftor));
245 }
246