1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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 dead code elimination and basic block merging, along
10 // with a collection of other peephole control flow optimizations.  For example:
11 //
12 //   * Removes basic blocks with no predecessors.
13 //   * Merges a basic block into its predecessor if there is only one and the
14 //     predecessor only has one successor.
15 //   * Eliminates PHI nodes for basic blocks with a single predecessor.
16 //   * Eliminates a basic block that only contains an unconditional branch.
17 //   * Changes invoke instructions to nounwind functions to be calls.
18 //   * Change things like "if (x) if (y)" into "if (x&y)".
19 //   * etc..
20 //
21 //===----------------------------------------------------------------------===//
22 
23 #include "llvm/ADT/SmallPtrSet.h"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/Analysis/AssumptionCache.h"
27 #include "llvm/Analysis/CFG.h"
28 #include "llvm/Analysis/GlobalsModRef.h"
29 #include "llvm/Analysis/TargetTransformInfo.h"
30 #include "llvm/Transforms/Utils/Local.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 <utility>
43 using namespace llvm;
44 
45 #define DEBUG_TYPE "simplifycfg"
46 
47 static cl::opt<unsigned> UserBonusInstThreshold(
48     "bonus-inst-threshold", cl::Hidden, cl::init(1),
49     cl::desc("Control the number of bonus instructions (default = 1)"));
50 
51 static cl::opt<bool> UserKeepLoops(
52     "keep-loops", cl::Hidden, cl::init(true),
53     cl::desc("Preserve canonical loop structure (default = true)"));
54 
55 static cl::opt<bool> UserSwitchToLookup(
56     "switch-to-lookup", cl::Hidden, cl::init(false),
57     cl::desc("Convert switches to lookup tables (default = false)"));
58 
59 static cl::opt<bool> UserForwardSwitchCond(
60     "forward-switch-cond", cl::Hidden, cl::init(false),
61     cl::desc("Forward switch condition to phi ops (default = false)"));
62 
63 static cl::opt<bool> UserSinkCommonInsts(
64     "sink-common-insts", cl::Hidden, cl::init(false),
65     cl::desc("Sink common instructions (default = false)"));
66 
67 
68 STATISTIC(NumSimpl, "Number of blocks simplified");
69 
70 /// If we have more than one empty (other than phi node) return blocks,
71 /// merge them together to promote recursive block merging.
72 static bool mergeEmptyReturnBlocks(Function &F) {
73   bool Changed = false;
74 
75   BasicBlock *RetBlock = nullptr;
76 
77   // Scan all the blocks in the function, looking for empty return blocks.
78   for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
79     BasicBlock &BB = *BBI++;
80 
81     // Only look at return blocks.
82     ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
83     if (!Ret) continue;
84 
85     // Only look at the block if it is empty or the only other thing in it is a
86     // single PHI node that is the operand to the return.
87     if (Ret != &BB.front()) {
88       // Check for something else in the block.
89       BasicBlock::iterator I(Ret);
90       --I;
91       // Skip over debug info.
92       while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
93         --I;
94       if (!isa<DbgInfoIntrinsic>(I) &&
95           (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
96            Ret->getOperand(0) != &*I))
97         continue;
98     }
99 
100     // If this is the first returning block, remember it and keep going.
101     if (!RetBlock) {
102       RetBlock = &BB;
103       continue;
104     }
105 
106     // Otherwise, we found a duplicate return block.  Merge the two.
107     Changed = true;
108 
109     // Case when there is no input to the return or when the returned values
110     // agree is trivial.  Note that they can't agree if there are phis in the
111     // blocks.
112     if (Ret->getNumOperands() == 0 ||
113         Ret->getOperand(0) ==
114           cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
115       BB.replaceAllUsesWith(RetBlock);
116       BB.eraseFromParent();
117       continue;
118     }
119 
120     // If the canonical return block has no PHI node, create one now.
121     PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
122     if (!RetBlockPHI) {
123       Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
124       pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
125       RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
126                                     std::distance(PB, PE), "merge",
127                                     &RetBlock->front());
128 
129       for (pred_iterator PI = PB; PI != PE; ++PI)
130         RetBlockPHI->addIncoming(InVal, *PI);
131       RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
132     }
133 
134     // Turn BB into a block that just unconditionally branches to the return
135     // block.  This handles the case when the two return blocks have a common
136     // predecessor but that return different things.
137     RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
138     BB.getTerminator()->eraseFromParent();
139     BranchInst::Create(RetBlock, &BB);
140   }
141 
142   return Changed;
143 }
144 
145 /// Call SimplifyCFG on all the blocks in the function,
146 /// iterating until no more changes are made.
147 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
148                                    const SimplifyCFGOptions &Options) {
149   bool Changed = false;
150   bool LocalChange = true;
151 
152   SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
153   FindFunctionBackedges(F, Edges);
154   SmallPtrSet<BasicBlock *, 16> LoopHeaders;
155   for (unsigned i = 0, e = Edges.size(); i != e; ++i)
156     LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
157 
158   while (LocalChange) {
159     LocalChange = false;
160 
161     // Loop over all of the basic blocks and remove them if they are unneeded.
162     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
163       if (simplifyCFG(&*BBIt++, TTI, Options, &LoopHeaders)) {
164         LocalChange = true;
165         ++NumSimpl;
166       }
167     }
168     Changed |= LocalChange;
169   }
170   return Changed;
171 }
172 
173 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
174                                 const SimplifyCFGOptions &Options) {
175   bool EverChanged = removeUnreachableBlocks(F);
176   EverChanged |= mergeEmptyReturnBlocks(F);
177   EverChanged |= iterativelySimplifyCFG(F, TTI, Options);
178 
179   // If neither pass changed anything, we're done.
180   if (!EverChanged) return false;
181 
182   // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
183   // removeUnreachableBlocks is needed to nuke them, which means we should
184   // iterate between the two optimizations.  We structure the code like this to
185   // avoid rerunning iterativelySimplifyCFG if the second pass of
186   // removeUnreachableBlocks doesn't do anything.
187   if (!removeUnreachableBlocks(F))
188     return true;
189 
190   do {
191     EverChanged = iterativelySimplifyCFG(F, TTI, Options);
192     EverChanged |= removeUnreachableBlocks(F);
193   } while (EverChanged);
194 
195   return true;
196 }
197 
198 // Command-line settings override compile-time settings.
199 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) {
200   Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
201                                    ? UserBonusInstThreshold
202                                    : Opts.BonusInstThreshold;
203   Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
204                                        ? UserForwardSwitchCond
205                                        : Opts.ForwardSwitchCondToPhi;
206   Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
207                                            ? UserSwitchToLookup
208                                            : Opts.ConvertSwitchToLookupTable;
209   Options.NeedCanonicalLoop = UserKeepLoops.getNumOccurrences()
210                                   ? UserKeepLoops
211                                   : Opts.NeedCanonicalLoop;
212   Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
213                                 ? UserSinkCommonInsts
214                                 : Opts.SinkCommonInsts;
215 }
216 
217 PreservedAnalyses SimplifyCFGPass::run(Function &F,
218                                        FunctionAnalysisManager &AM) {
219   auto &TTI = AM.getResult<TargetIRAnalysis>(F);
220   Options.AC = &AM.getResult<AssumptionAnalysis>(F);
221   if (!simplifyFunctionCFG(F, TTI, Options))
222     return PreservedAnalyses::all();
223   PreservedAnalyses PA;
224   PA.preserve<GlobalsAA>();
225   return PA;
226 }
227 
228 namespace {
229 struct CFGSimplifyPass : public FunctionPass {
230   static char ID;
231   SimplifyCFGOptions Options;
232   std::function<bool(const Function &)> PredicateFtor;
233 
234   CFGSimplifyPass(unsigned Threshold = 1, bool ForwardSwitchCond = false,
235                   bool ConvertSwitch = false, bool KeepLoops = true,
236                   bool SinkCommon = false,
237                   std::function<bool(const Function &)> Ftor = nullptr)
238       : FunctionPass(ID), PredicateFtor(std::move(Ftor)) {
239 
240     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
241 
242     // Check for command-line overrides of options for debug/customization.
243     Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
244                                     ? UserBonusInstThreshold
245                                     : Threshold;
246 
247     Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
248                                          ? UserForwardSwitchCond
249                                          : ForwardSwitchCond;
250 
251     Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
252                                              ? UserSwitchToLookup
253                                              : ConvertSwitch;
254 
255     Options.NeedCanonicalLoop =
256         UserKeepLoops.getNumOccurrences() ? UserKeepLoops : KeepLoops;
257 
258     Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
259                                   ? UserSinkCommonInsts
260                                   : SinkCommon;
261   }
262 
263   bool runOnFunction(Function &F) override {
264     if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
265       return false;
266 
267     Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
268     auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
269     return simplifyFunctionCFG(F, TTI, Options);
270   }
271   void getAnalysisUsage(AnalysisUsage &AU) const override {
272     AU.addRequired<AssumptionCacheTracker>();
273     AU.addRequired<TargetTransformInfoWrapperPass>();
274     AU.addPreserved<GlobalsAAWrapperPass>();
275   }
276 };
277 }
278 
279 char CFGSimplifyPass::ID = 0;
280 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
281                       false)
282 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
283 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
284 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
285                     false)
286 
287 // Public interface to the CFGSimplification pass
288 FunctionPass *
289 llvm::createCFGSimplificationPass(unsigned Threshold, bool ForwardSwitchCond,
290                                   bool ConvertSwitch, bool KeepLoops,
291                                   bool SinkCommon,
292                                   std::function<bool(const Function &)> Ftor) {
293   return new CFGSimplifyPass(Threshold, ForwardSwitchCond, ConvertSwitch,
294                              KeepLoops, SinkCommon, std::move(Ftor));
295 }
296