SimplifyCFGPass.cpp revision 363496 
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/IR/Attributes.h"
31#include "llvm/IR/CFG.h"
32#include "llvm/IR/Constants.h"
33#include "llvm/IR/DataLayout.h"
34#include "llvm/IR/Instructions.h"
35#include "llvm/IR/IntrinsicInst.h"
36#include "llvm/IR/Module.h"
37#include "llvm/InitializePasses.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>
44using namespace llvm;
45
46#define DEBUG_TYPE "simplifycfg"
47
48static cl::opt<unsigned> UserBonusInstThreshold(
49    "bonus-inst-threshold", cl::Hidden, cl::init(1),
50    cl::desc("Control the number of bonus instructions (default = 1)"));
51
52static cl::opt<bool> UserKeepLoops(
53    "keep-loops", cl::Hidden, cl::init(true),
54    cl::desc("Preserve canonical loop structure (default = true)"));
55
56static cl::opt<bool> UserSwitchToLookup(
57    "switch-to-lookup", cl::Hidden, cl::init(false),
58    cl::desc("Convert switches to lookup tables (default = false)"));
59
60static cl::opt<bool> UserForwardSwitchCond(
61    "forward-switch-cond", cl::Hidden, cl::init(false),
62    cl::desc("Forward switch condition to phi ops (default = false)"));
63
64static cl::opt<bool> UserSinkCommonInsts(
65    "sink-common-insts", cl::Hidden, cl::init(false),
66    cl::desc("Sink common instructions (default = false)"));
67
68
69STATISTIC(NumSimpl, "Number of blocks simplified");
70
71/// If we have more than one empty (other than phi node) return blocks,
72/// merge them together to promote recursive block merging.
73static bool mergeEmptyReturnBlocks(Function &F) {
74  bool Changed = false;
75
76  BasicBlock *RetBlock = nullptr;
77
78  // Scan all the blocks in the function, looking for empty return blocks.
79  for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
80    BasicBlock &BB = *BBI++;
81
82    // Only look at return blocks.
83    ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
84    if (!Ret) continue;
85
86    // Only look at the block if it is empty or the only other thing in it is a
87    // single PHI node that is the operand to the return.
88    if (Ret != &BB.front()) {
89      // Check for something else in the block.
90      BasicBlock::iterator I(Ret);
91      --I;
92      // Skip over debug info.
93      while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
94        --I;
95      if (!isa<DbgInfoIntrinsic>(I) &&
96          (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 ||
97           Ret->getOperand(0) != &*I))
98        continue;
99    }
100
101    // If this is the first returning block, remember it and keep going.
102    if (!RetBlock) {
103      RetBlock = &BB;
104      continue;
105    }
106
107    // Skip merging if this would result in a CallBr instruction with a
108    // duplicate destination. FIXME: See note in CodeGenPrepare.cpp.
109    bool SkipCallBr = false;
110    for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB);
111         PI != E && !SkipCallBr; ++PI) {
112      if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator()))
113        for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i)
114          if (RetBlock == CBI->getSuccessor(i)) {
115            SkipCallBr = true;
116            break;
117          }
118    }
119    if (SkipCallBr)
120      continue;
121
122    // Otherwise, we found a duplicate return block.  Merge the two.
123    Changed = true;
124
125    // Case when there is no input to the return or when the returned values
126    // agree is trivial.  Note that they can't agree if there are phis in the
127    // blocks.
128    if (Ret->getNumOperands() == 0 ||
129        Ret->getOperand(0) ==
130          cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
131      BB.replaceAllUsesWith(RetBlock);
132      BB.eraseFromParent();
133      continue;
134    }
135
136    // If the canonical return block has no PHI node, create one now.
137    PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
138    if (!RetBlockPHI) {
139      Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
140      pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
141      RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
142                                    std::distance(PB, PE), "merge",
143                                    &RetBlock->front());
144
145      for (pred_iterator PI = PB; PI != PE; ++PI)
146        RetBlockPHI->addIncoming(InVal, *PI);
147      RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
148    }
149
150    // Turn BB into a block that just unconditionally branches to the return
151    // block.  This handles the case when the two return blocks have a common
152    // predecessor but that return different things.
153    RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
154    BB.getTerminator()->eraseFromParent();
155    BranchInst::Create(RetBlock, &BB);
156  }
157
158  return Changed;
159}
160
161/// Call SimplifyCFG on all the blocks in the function,
162/// iterating until no more changes are made.
163static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
164                                   const SimplifyCFGOptions &Options) {
165  bool Changed = false;
166  bool LocalChange = true;
167
168  SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
169  FindFunctionBackedges(F, Edges);
170  SmallPtrSet<BasicBlock *, 16> LoopHeaders;
171  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
172    LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second));
173
174  while (LocalChange) {
175    LocalChange = false;
176
177    // Loop over all of the basic blocks and remove them if they are unneeded.
178    for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
179      if (simplifyCFG(&*BBIt++, TTI, Options, &LoopHeaders)) {
180        LocalChange = true;
181        ++NumSimpl;
182      }
183    }
184    Changed |= LocalChange;
185  }
186  return Changed;
187}
188
189static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
190                                const SimplifyCFGOptions &Options) {
191  bool EverChanged = removeUnreachableBlocks(F);
192  EverChanged |= mergeEmptyReturnBlocks(F);
193  EverChanged |= iterativelySimplifyCFG(F, TTI, Options);
194
195  // If neither pass changed anything, we're done.
196  if (!EverChanged) return false;
197
198  // iterativelySimplifyCFG can (rarely) make some loops dead.  If this happens,
199  // removeUnreachableBlocks is needed to nuke them, which means we should
200  // iterate between the two optimizations.  We structure the code like this to
201  // avoid rerunning iterativelySimplifyCFG if the second pass of
202  // removeUnreachableBlocks doesn't do anything.
203  if (!removeUnreachableBlocks(F))
204    return true;
205
206  do {
207    EverChanged = iterativelySimplifyCFG(F, TTI, Options);
208    EverChanged |= removeUnreachableBlocks(F);
209  } while (EverChanged);
210
211  return true;
212}
213
214// Command-line settings override compile-time settings.
215SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) {
216  Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
217                                   ? UserBonusInstThreshold
218                                   : Opts.BonusInstThreshold;
219  Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
220                                       ? UserForwardSwitchCond
221                                       : Opts.ForwardSwitchCondToPhi;
222  Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
223                                           ? UserSwitchToLookup
224                                           : Opts.ConvertSwitchToLookupTable;
225  Options.NeedCanonicalLoop = UserKeepLoops.getNumOccurrences()
226                                  ? UserKeepLoops
227                                  : Opts.NeedCanonicalLoop;
228  Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
229                                ? UserSinkCommonInsts
230                                : Opts.SinkCommonInsts;
231}
232
233PreservedAnalyses SimplifyCFGPass::run(Function &F,
234                                       FunctionAnalysisManager &AM) {
235  auto &TTI = AM.getResult<TargetIRAnalysis>(F);
236  Options.AC = &AM.getResult<AssumptionAnalysis>(F);
237  if (!simplifyFunctionCFG(F, TTI, Options))
238    return PreservedAnalyses::all();
239  PreservedAnalyses PA;
240  PA.preserve<GlobalsAA>();
241  return PA;
242}
243
244namespace {
245struct CFGSimplifyPass : public FunctionPass {
246  static char ID;
247  SimplifyCFGOptions Options;
248  std::function<bool(const Function &)> PredicateFtor;
249
250  CFGSimplifyPass(unsigned Threshold = 1, bool ForwardSwitchCond = false,
251                  bool ConvertSwitch = false, bool KeepLoops = true,
252                  bool SinkCommon = false,
253                  std::function<bool(const Function &)> Ftor = nullptr)
254      : FunctionPass(ID), PredicateFtor(std::move(Ftor)) {
255
256    initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
257
258    // Check for command-line overrides of options for debug/customization.
259    Options.BonusInstThreshold = UserBonusInstThreshold.getNumOccurrences()
260                                    ? UserBonusInstThreshold
261                                    : Threshold;
262
263    Options.ForwardSwitchCondToPhi = UserForwardSwitchCond.getNumOccurrences()
264                                         ? UserForwardSwitchCond
265                                         : ForwardSwitchCond;
266
267    Options.ConvertSwitchToLookupTable = UserSwitchToLookup.getNumOccurrences()
268                                             ? UserSwitchToLookup
269                                             : ConvertSwitch;
270
271    Options.NeedCanonicalLoop =
272        UserKeepLoops.getNumOccurrences() ? UserKeepLoops : KeepLoops;
273
274    Options.SinkCommonInsts = UserSinkCommonInsts.getNumOccurrences()
275                                  ? UserSinkCommonInsts
276                                  : SinkCommon;
277  }
278
279  bool runOnFunction(Function &F) override {
280    if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
281      return false;
282
283    Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
284    auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
285    return simplifyFunctionCFG(F, TTI, Options);
286  }
287  void getAnalysisUsage(AnalysisUsage &AU) const override {
288    AU.addRequired<AssumptionCacheTracker>();
289    AU.addRequired<TargetTransformInfoWrapperPass>();
290    AU.addPreserved<GlobalsAAWrapperPass>();
291  }
292};
293}
294
295char CFGSimplifyPass::ID = 0;
296INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
297                      false)
298INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
299INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
300INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
301                    false)
302
303// Public interface to the CFGSimplification pass
304FunctionPass *
305llvm::createCFGSimplificationPass(unsigned Threshold, bool ForwardSwitchCond,
306                                  bool ConvertSwitch, bool KeepLoops,
307                                  bool SinkCommon,
308                                  std::function<bool(const Function &)> Ftor) {
309  return new CFGSimplifyPass(Threshold, ForwardSwitchCond, ConvertSwitch,
310                             KeepLoops, SinkCommon, std::move(Ftor));
311}
312