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      1 //===- FlatternCFG.cpp - Code to perform CFG flattening ---------------===//
      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 // Reduce conditional branches in CFG.
     11 //
     12 //===----------------------------------------------------------------------===//
     13 
     14 #include "llvm/Transforms/Utils/Local.h"
     15 #include "llvm/ADT/SmallPtrSet.h"
     16 #include "llvm/Analysis/AliasAnalysis.h"
     17 #include "llvm/Analysis/ValueTracking.h"
     18 #include "llvm/IR/IRBuilder.h"
     19 #include "llvm/Support/Debug.h"
     20 #include "llvm/Support/raw_ostream.h"
     21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
     22 using namespace llvm;
     23 
     24 #define DEBUG_TYPE "flattencfg"
     25 
     26 namespace {
     27 class FlattenCFGOpt {
     28   AliasAnalysis *AA;
     29   /// \brief Use parallel-and or parallel-or to generate conditions for
     30   /// conditional branches.
     31   bool FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder);
     32   /// \brief If \param BB is the merge block of an if-region, attempt to merge
     33   /// the if-region with an adjacent if-region upstream if two if-regions
     34   /// contain identical instructions.
     35   bool MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder);
     36   /// \brief Compare a pair of blocks: \p Block1 and \p Block2, which
     37   /// are from two if-regions whose entry blocks are \p Head1 and \p
     38   /// Head2.  \returns true if \p Block1 and \p Block2 contain identical
     39   /// instructions, and have no memory reference alias with \p Head2.
     40   /// This is used as a legality check for merging if-regions.
     41   bool CompareIfRegionBlock(BasicBlock *Head1, BasicBlock *Head2,
     42                             BasicBlock *Block1, BasicBlock *Block2);
     43 
     44 public:
     45   FlattenCFGOpt(AliasAnalysis *AA) : AA(AA) {}
     46   bool run(BasicBlock *BB);
     47 };
     48 }
     49 
     50 /// If \param [in] BB has more than one predecessor that is a conditional
     51 /// branch, attempt to use parallel and/or for the branch condition. \returns
     52 /// true on success.
     53 ///
     54 /// Before:
     55 ///   ......
     56 ///   %cmp10 = fcmp une float %tmp1, %tmp2
     57 ///   br i1 %cmp1, label %if.then, label %lor.rhs
     58 ///
     59 /// lor.rhs:
     60 ///   ......
     61 ///   %cmp11 = fcmp une float %tmp3, %tmp4
     62 ///   br i1 %cmp11, label %if.then, label %ifend
     63 ///
     64 /// if.end:  // the merge block
     65 ///   ......
     66 ///
     67 /// if.then: // has two predecessors, both of them contains conditional branch.
     68 ///   ......
     69 ///   br label %if.end;
     70 ///
     71 /// After:
     72 ///  ......
     73 ///  %cmp10 = fcmp une float %tmp1, %tmp2
     74 ///  ......
     75 ///  %cmp11 = fcmp une float %tmp3, %tmp4
     76 ///  %cmp12 = or i1 %cmp10, %cmp11    // parallel-or mode.
     77 ///  br i1 %cmp12, label %if.then, label %ifend
     78 ///
     79 ///  if.end:
     80 ///    ......
     81 ///
     82 ///  if.then:
     83 ///    ......
     84 ///    br label %if.end;
     85 ///
     86 ///  Current implementation handles two cases.
     87 ///  Case 1: \param BB is on the else-path.
     88 ///
     89 ///          BB1
     90 ///        /     |
     91 ///       BB2    |
     92 ///      /   \   |
     93 ///     BB3   \  |     where, BB1, BB2 contain conditional branches.
     94 ///      \    |  /     BB3 contains unconditional branch.
     95 ///       \   | /      BB4 corresponds to \param BB which is also the merge.
     96 ///  BB => BB4
     97 ///
     98 ///
     99 ///  Corresponding source code:
    100 ///
    101 ///  if (a == b && c == d)
    102 ///    statement; // BB3
    103 ///
    104 ///  Case 2: \param BB BB is on the then-path.
    105 ///
    106 ///             BB1
    107 ///          /      |
    108 ///         |      BB2
    109 ///         \    /    |  where BB1, BB2 contain conditional branches.
    110 ///  BB =>   BB3      |  BB3 contains unconditiona branch and corresponds
    111 ///           \     /    to \param BB.  BB4 is the merge.
    112 ///             BB4
    113 ///
    114 ///  Corresponding source code:
    115 ///
    116 ///  if (a == b || c == d)
    117 ///    statement;  // BB3
    118 ///
    119 ///  In both cases,  \param BB is the common successor of conditional branches.
    120 ///  In Case 1, \param BB (BB4) has an unconditional branch (BB3) as
    121 ///  its predecessor.  In Case 2, \param BB (BB3) only has conditional branches
    122 ///  as its predecessors.
    123 ///
    124 bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder) {
    125   PHINode *PHI = dyn_cast<PHINode>(BB->begin());
    126   if (PHI)
    127     return false; // For simplicity, avoid cases containing PHI nodes.
    128 
    129   BasicBlock *LastCondBlock = nullptr;
    130   BasicBlock *FirstCondBlock = nullptr;
    131   BasicBlock *UnCondBlock = nullptr;
    132   int Idx = -1;
    133 
    134   // Check predecessors of \param BB.
    135   SmallPtrSet<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
    136   for (SmallPtrSetIterator<BasicBlock *> PI = Preds.begin(), PE = Preds.end();
    137        PI != PE; ++PI) {
    138     BasicBlock *Pred = *PI;
    139     BranchInst *PBI = dyn_cast<BranchInst>(Pred->getTerminator());
    140 
    141     // All predecessors should terminate with a branch.
    142     if (!PBI)
    143       return false;
    144 
    145     BasicBlock *PP = Pred->getSinglePredecessor();
    146 
    147     if (PBI->isUnconditional()) {
    148       // Case 1: Pred (BB3) is an unconditional block, it should
    149       // have a single predecessor (BB2) that is also a predecessor
    150       // of \param BB (BB4) and should not have address-taken.
    151       // There should exist only one such unconditional
    152       // branch among the predecessors.
    153       if (UnCondBlock || !PP || (Preds.count(PP) == 0) ||
    154           Pred->hasAddressTaken())
    155         return false;
    156 
    157       UnCondBlock = Pred;
    158       continue;
    159     }
    160 
    161     // Only conditional branches are allowed beyond this point.
    162     assert(PBI->isConditional());
    163 
    164     // Condition's unique use should be the branch instruction.
    165     Value *PC = PBI->getCondition();
    166     if (!PC || !PC->hasOneUse())
    167       return false;
    168 
    169     if (PP && Preds.count(PP)) {
    170       // These are internal condition blocks to be merged from, e.g.,
    171       // BB2 in both cases.
    172       // Should not be address-taken.
    173       if (Pred->hasAddressTaken())
    174         return false;
    175 
    176       // Instructions in the internal condition blocks should be safe
    177       // to hoist up.
    178       for (BasicBlock::iterator BI = Pred->begin(), BE = PBI->getIterator();
    179            BI != BE;) {
    180         Instruction *CI = &*BI++;
    181         if (isa<PHINode>(CI) || !isSafeToSpeculativelyExecute(CI))
    182           return false;
    183       }
    184     } else {
    185       // This is the condition block to be merged into, e.g. BB1 in
    186       // both cases.
    187       if (FirstCondBlock)
    188         return false;
    189       FirstCondBlock = Pred;
    190     }
    191 
    192     // Find whether BB is uniformly on the true (or false) path
    193     // for all of its predecessors.
    194     BasicBlock *PS1 = PBI->getSuccessor(0);
    195     BasicBlock *PS2 = PBI->getSuccessor(1);
    196     BasicBlock *PS = (PS1 == BB) ? PS2 : PS1;
    197     int CIdx = (PS1 == BB) ? 0 : 1;
    198 
    199     if (Idx == -1)
    200       Idx = CIdx;
    201     else if (CIdx != Idx)
    202       return false;
    203 
    204     // PS is the successor which is not BB. Check successors to identify
    205     // the last conditional branch.
    206     if (Preds.count(PS) == 0) {
    207       // Case 2.
    208       LastCondBlock = Pred;
    209     } else {
    210       // Case 1
    211       BranchInst *BPS = dyn_cast<BranchInst>(PS->getTerminator());
    212       if (BPS && BPS->isUnconditional()) {
    213         // Case 1: PS(BB3) should be an unconditional branch.
    214         LastCondBlock = Pred;
    215       }
    216     }
    217   }
    218 
    219   if (!FirstCondBlock || !LastCondBlock || (FirstCondBlock == LastCondBlock))
    220     return false;
    221 
    222   TerminatorInst *TBB = LastCondBlock->getTerminator();
    223   BasicBlock *PS1 = TBB->getSuccessor(0);
    224   BasicBlock *PS2 = TBB->getSuccessor(1);
    225   BranchInst *PBI1 = dyn_cast<BranchInst>(PS1->getTerminator());
    226   BranchInst *PBI2 = dyn_cast<BranchInst>(PS2->getTerminator());
    227 
    228   // If PS1 does not jump into PS2, but PS2 jumps into PS1,
    229   // attempt branch inversion.
    230   if (!PBI1 || !PBI1->isUnconditional() ||
    231       (PS1->getTerminator()->getSuccessor(0) != PS2)) {
    232     // Check whether PS2 jumps into PS1.
    233     if (!PBI2 || !PBI2->isUnconditional() ||
    234         (PS2->getTerminator()->getSuccessor(0) != PS1))
    235       return false;
    236 
    237     // Do branch inversion.
    238     BasicBlock *CurrBlock = LastCondBlock;
    239     bool EverChanged = false;
    240     for (;CurrBlock != FirstCondBlock;
    241           CurrBlock = CurrBlock->getSinglePredecessor()) {
    242       BranchInst *BI = dyn_cast<BranchInst>(CurrBlock->getTerminator());
    243       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
    244       if (!CI)
    245         continue;
    246 
    247       CmpInst::Predicate Predicate = CI->getPredicate();
    248       // Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
    249       if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
    250         CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
    251         BI->swapSuccessors();
    252         EverChanged = true;
    253       }
    254     }
    255     return EverChanged;
    256   }
    257 
    258   // PS1 must have a conditional branch.
    259   if (!PBI1 || !PBI1->isUnconditional())
    260     return false;
    261 
    262   // PS2 should not contain PHI node.
    263   PHI = dyn_cast<PHINode>(PS2->begin());
    264   if (PHI)
    265     return false;
    266 
    267   // Do the transformation.
    268   BasicBlock *CB;
    269   BranchInst *PBI = dyn_cast<BranchInst>(FirstCondBlock->getTerminator());
    270   bool Iteration = true;
    271   IRBuilder<>::InsertPointGuard Guard(Builder);
    272   Value *PC = PBI->getCondition();
    273 
    274   do {
    275     CB = PBI->getSuccessor(1 - Idx);
    276     // Delete the conditional branch.
    277     FirstCondBlock->getInstList().pop_back();
    278     FirstCondBlock->getInstList()
    279         .splice(FirstCondBlock->end(), CB->getInstList());
    280     PBI = cast<BranchInst>(FirstCondBlock->getTerminator());
    281     Value *CC = PBI->getCondition();
    282     // Merge conditions.
    283     Builder.SetInsertPoint(PBI);
    284     Value *NC;
    285     if (Idx == 0)
    286       // Case 2, use parallel or.
    287       NC = Builder.CreateOr(PC, CC);
    288     else
    289       // Case 1, use parallel and.
    290       NC = Builder.CreateAnd(PC, CC);
    291 
    292     PBI->replaceUsesOfWith(CC, NC);
    293     PC = NC;
    294     if (CB == LastCondBlock)
    295       Iteration = false;
    296     // Remove internal conditional branches.
    297     CB->dropAllReferences();
    298     // make CB unreachable and let downstream to delete the block.
    299     new UnreachableInst(CB->getContext(), CB);
    300   } while (Iteration);
    301 
    302   DEBUG(dbgs() << "Use parallel and/or in:\n" << *FirstCondBlock);
    303   return true;
    304 }
    305 
    306 /// Compare blocks from two if-regions, where \param Head1 is the entry of the
    307 /// 1st if-region. \param Head2 is the entry of the 2nd if-region. \param
    308 /// Block1 is a block in the 1st if-region to compare. \param Block2 is a block
    309 //  in the 2nd if-region to compare.  \returns true if \param Block1 and \param
    310 /// Block2 have identical instructions and do not have memory reference alias
    311 /// with \param Head2.
    312 ///
    313 bool FlattenCFGOpt::CompareIfRegionBlock(BasicBlock *Head1, BasicBlock *Head2,
    314                                          BasicBlock *Block1,
    315                                          BasicBlock *Block2) {
    316   TerminatorInst *PTI2 = Head2->getTerminator();
    317   Instruction *PBI2 = &Head2->front();
    318 
    319   bool eq1 = (Block1 == Head1);
    320   bool eq2 = (Block2 == Head2);
    321   if (eq1 || eq2) {
    322     // An empty then-path or else-path.
    323     return (eq1 == eq2);
    324   }
    325 
    326   // Check whether instructions in Block1 and Block2 are identical
    327   // and do not alias with instructions in Head2.
    328   BasicBlock::iterator iter1 = Block1->begin();
    329   BasicBlock::iterator end1 = Block1->getTerminator()->getIterator();
    330   BasicBlock::iterator iter2 = Block2->begin();
    331   BasicBlock::iterator end2 = Block2->getTerminator()->getIterator();
    332 
    333   while (1) {
    334     if (iter1 == end1) {
    335       if (iter2 != end2)
    336         return false;
    337       break;
    338     }
    339 
    340     if (!iter1->isIdenticalTo(&*iter2))
    341       return false;
    342 
    343     // Illegal to remove instructions with side effects except
    344     // non-volatile stores.
    345     if (iter1->mayHaveSideEffects()) {
    346       Instruction *CurI = &*iter1;
    347       StoreInst *SI = dyn_cast<StoreInst>(CurI);
    348       if (!SI || SI->isVolatile())
    349         return false;
    350     }
    351 
    352     // For simplicity and speed, data dependency check can be
    353     // avoided if read from memory doesn't exist.
    354     if (iter1->mayReadFromMemory())
    355       return false;
    356 
    357     if (iter1->mayWriteToMemory()) {
    358       for (BasicBlock::iterator BI(PBI2), BE(PTI2); BI != BE; ++BI) {
    359         if (BI->mayReadFromMemory() || BI->mayWriteToMemory()) {
    360           // Check alias with Head2.
    361           if (!AA || AA->alias(&*iter1, &*BI))
    362             return false;
    363         }
    364       }
    365     }
    366     ++iter1;
    367     ++iter2;
    368   }
    369 
    370   return true;
    371 }
    372 
    373 /// Check whether \param BB is the merge block of a if-region.  If yes, check
    374 /// whether there exists an adjacent if-region upstream, the two if-regions
    375 /// contain identical instructions and can be legally merged.  \returns true if
    376 /// the two if-regions are merged.
    377 ///
    378 /// From:
    379 /// if (a)
    380 ///   statement;
    381 /// if (b)
    382 ///   statement;
    383 ///
    384 /// To:
    385 /// if (a || b)
    386 ///   statement;
    387 ///
    388 bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
    389   BasicBlock *IfTrue2, *IfFalse2;
    390   Value *IfCond2 = GetIfCondition(BB, IfTrue2, IfFalse2);
    391   Instruction *CInst2 = dyn_cast_or_null<Instruction>(IfCond2);
    392   if (!CInst2)
    393     return false;
    394 
    395   BasicBlock *SecondEntryBlock = CInst2->getParent();
    396   if (SecondEntryBlock->hasAddressTaken())
    397     return false;
    398 
    399   BasicBlock *IfTrue1, *IfFalse1;
    400   Value *IfCond1 = GetIfCondition(SecondEntryBlock, IfTrue1, IfFalse1);
    401   Instruction *CInst1 = dyn_cast_or_null<Instruction>(IfCond1);
    402   if (!CInst1)
    403     return false;
    404 
    405   BasicBlock *FirstEntryBlock = CInst1->getParent();
    406 
    407   // Either then-path or else-path should be empty.
    408   if ((IfTrue1 != FirstEntryBlock) && (IfFalse1 != FirstEntryBlock))
    409     return false;
    410   if ((IfTrue2 != SecondEntryBlock) && (IfFalse2 != SecondEntryBlock))
    411     return false;
    412 
    413   TerminatorInst *PTI2 = SecondEntryBlock->getTerminator();
    414   Instruction *PBI2 = &SecondEntryBlock->front();
    415 
    416   if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfTrue1,
    417                             IfTrue2))
    418     return false;
    419 
    420   if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfFalse1,
    421                             IfFalse2))
    422     return false;
    423 
    424   // Check whether \param SecondEntryBlock has side-effect and is safe to
    425   // speculate.
    426   for (BasicBlock::iterator BI(PBI2), BE(PTI2); BI != BE; ++BI) {
    427     Instruction *CI = &*BI;
    428     if (isa<PHINode>(CI) || CI->mayHaveSideEffects() ||
    429         !isSafeToSpeculativelyExecute(CI))
    430       return false;
    431   }
    432 
    433   // Merge \param SecondEntryBlock into \param FirstEntryBlock.
    434   FirstEntryBlock->getInstList().pop_back();
    435   FirstEntryBlock->getInstList()
    436       .splice(FirstEntryBlock->end(), SecondEntryBlock->getInstList());
    437   BranchInst *PBI = dyn_cast<BranchInst>(FirstEntryBlock->getTerminator());
    438   Value *CC = PBI->getCondition();
    439   BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
    440   BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
    441   Builder.SetInsertPoint(PBI);
    442   Value *NC = Builder.CreateOr(CInst1, CC);
    443   PBI->replaceUsesOfWith(CC, NC);
    444   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
    445 
    446   // Remove IfTrue1
    447   if (IfTrue1 != FirstEntryBlock) {
    448     IfTrue1->dropAllReferences();
    449     IfTrue1->eraseFromParent();
    450   }
    451 
    452   // Remove IfFalse1
    453   if (IfFalse1 != FirstEntryBlock) {
    454     IfFalse1->dropAllReferences();
    455     IfFalse1->eraseFromParent();
    456   }
    457 
    458   // Remove \param SecondEntryBlock
    459   SecondEntryBlock->dropAllReferences();
    460   SecondEntryBlock->eraseFromParent();
    461   DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
    462   return true;
    463 }
    464 
    465 bool FlattenCFGOpt::run(BasicBlock *BB) {
    466   bool Changed = false;
    467   assert(BB && BB->getParent() && "Block not embedded in function!");
    468   assert(BB->getTerminator() && "Degenerate basic block encountered!");
    469 
    470   IRBuilder<> Builder(BB);
    471 
    472   if (FlattenParallelAndOr(BB, Builder))
    473     return true;
    474 
    475   if (MergeIfRegion(BB, Builder))
    476     return true;
    477 
    478   return Changed;
    479 }
    480 
    481 /// FlattenCFG - This function is used to flatten a CFG.  For
    482 /// example, it uses parallel-and and parallel-or mode to collapse
    483 //  if-conditions and merge if-regions with identical statements.
    484 ///
    485 bool llvm::FlattenCFG(BasicBlock *BB, AliasAnalysis *AA) {
    486   return FlattenCFGOpt(AA).run(BB);
    487 }
    488