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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                             Pass *P = nullptr);
     33   /// \brief If \param BB is the merge block of an if-region, attempt to merge
     34   /// the if-region with an adjacent if-region upstream if two if-regions
     35   /// contain identical instructions.
     36   bool MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder, Pass *P = nullptr);
     37   /// \brief Compare a pair of blocks: \p Block1 and \p Block2, which
     38   /// are from two if-regions whose entry blocks are \p Head1 and \p
     39   /// Head2.  \returns true if \p Block1 and \p Block2 contain identical
     40   /// instructions, and have no memory reference alias with \p Head2.
     41   /// This is used as a legality check for merging if-regions.
     42   bool CompareIfRegionBlock(BasicBlock *Head1, BasicBlock *Head2,
     43                             BasicBlock *Block1, BasicBlock *Block2);
     44 
     45 public:
     46   FlattenCFGOpt(AliasAnalysis *AA) : AA(AA) {}
     47   bool run(BasicBlock *BB);
     48 };
     49 }
     50 
     51 /// If \param [in] BB has more than one predecessor that is a conditional
     52 /// branch, attempt to use parallel and/or for the branch condition. \returns
     53 /// true on success.
     54 ///
     55 /// Before:
     56 ///   ......
     57 ///   %cmp10 = fcmp une float %tmp1, %tmp2
     58 ///   br i1 %cmp1, label %if.then, label %lor.rhs
     59 ///
     60 /// lor.rhs:
     61 ///   ......
     62 ///   %cmp11 = fcmp une float %tmp3, %tmp4
     63 ///   br i1 %cmp11, label %if.then, label %ifend
     64 ///
     65 /// if.end:  // the merge block
     66 ///   ......
     67 ///
     68 /// if.then: // has two predecessors, both of them contains conditional branch.
     69 ///   ......
     70 ///   br label %if.end;
     71 ///
     72 /// After:
     73 ///  ......
     74 ///  %cmp10 = fcmp une float %tmp1, %tmp2
     75 ///  ......
     76 ///  %cmp11 = fcmp une float %tmp3, %tmp4
     77 ///  %cmp12 = or i1 %cmp10, %cmp11    // parallel-or mode.
     78 ///  br i1 %cmp12, label %if.then, label %ifend
     79 ///
     80 ///  if.end:
     81 ///    ......
     82 ///
     83 ///  if.then:
     84 ///    ......
     85 ///    br label %if.end;
     86 ///
     87 ///  Current implementation handles two cases.
     88 ///  Case 1: \param BB is on the else-path.
     89 ///
     90 ///          BB1
     91 ///        /     |
     92 ///       BB2    |
     93 ///      /   \   |
     94 ///     BB3   \  |     where, BB1, BB2 contain conditional branches.
     95 ///      \    |  /     BB3 contains unconditional branch.
     96 ///       \   | /      BB4 corresponds to \param BB which is also the merge.
     97 ///  BB => BB4
     98 ///
     99 ///
    100 ///  Corresponding source code:
    101 ///
    102 ///  if (a == b && c == d)
    103 ///    statement; // BB3
    104 ///
    105 ///  Case 2: \param BB BB is on the then-path.
    106 ///
    107 ///             BB1
    108 ///          /      |
    109 ///         |      BB2
    110 ///         \    /    |  where BB1, BB2 contain conditional branches.
    111 ///  BB =>   BB3      |  BB3 contains unconditiona branch and corresponds
    112 ///           \     /    to \param BB.  BB4 is the merge.
    113 ///             BB4
    114 ///
    115 ///  Corresponding source code:
    116 ///
    117 ///  if (a == b || c == d)
    118 ///    statement;  // BB3
    119 ///
    120 ///  In both cases,  \param BB is the common successor of conditional branches.
    121 ///  In Case 1, \param BB (BB4) has an unconditional branch (BB3) as
    122 ///  its predecessor.  In Case 2, \param BB (BB3) only has conditional branches
    123 ///  as its predecessors.
    124 ///
    125 bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder,
    126                                          Pass *P) {
    127   PHINode *PHI = dyn_cast<PHINode>(BB->begin());
    128   if (PHI)
    129     return false; // For simplicity, avoid cases containing PHI nodes.
    130 
    131   BasicBlock *LastCondBlock = nullptr;
    132   BasicBlock *FirstCondBlock = nullptr;
    133   BasicBlock *UnCondBlock = nullptr;
    134   int Idx = -1;
    135 
    136   // Check predecessors of \param BB.
    137   SmallPtrSet<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
    138   for (SmallPtrSetIterator<BasicBlock *> PI = Preds.begin(), PE = Preds.end();
    139        PI != PE; ++PI) {
    140     BasicBlock *Pred = *PI;
    141     BranchInst *PBI = dyn_cast<BranchInst>(Pred->getTerminator());
    142 
    143     // All predecessors should terminate with a branch.
    144     if (!PBI)
    145       return false;
    146 
    147     BasicBlock *PP = Pred->getSinglePredecessor();
    148 
    149     if (PBI->isUnconditional()) {
    150       // Case 1: Pred (BB3) is an unconditional block, it should
    151       // have a single predecessor (BB2) that is also a predecessor
    152       // of \param BB (BB4) and should not have address-taken.
    153       // There should exist only one such unconditional
    154       // branch among the predecessors.
    155       if (UnCondBlock || !PP || (Preds.count(PP) == 0) ||
    156           Pred->hasAddressTaken())
    157         return false;
    158 
    159       UnCondBlock = Pred;
    160       continue;
    161     }
    162 
    163     // Only conditional branches are allowed beyond this point.
    164     assert(PBI->isConditional());
    165 
    166     // Condition's unique use should be the branch instruction.
    167     Value *PC = PBI->getCondition();
    168     if (!PC || !PC->hasOneUse())
    169       return false;
    170 
    171     if (PP && Preds.count(PP)) {
    172       // These are internal condition blocks to be merged from, e.g.,
    173       // BB2 in both cases.
    174       // Should not be address-taken.
    175       if (Pred->hasAddressTaken())
    176         return false;
    177 
    178       // Instructions in the internal condition blocks should be safe
    179       // to hoist up.
    180       for (BasicBlock::iterator BI = Pred->begin(), BE = PBI; BI != BE;) {
    181         Instruction *CI = BI++;
    182         if (isa<PHINode>(CI) || !isSafeToSpeculativelyExecute(CI))
    183           return false;
    184       }
    185     } else {
    186       // This is the condition block to be merged into, e.g. BB1 in
    187       // both cases.
    188       if (FirstCondBlock)
    189         return false;
    190       FirstCondBlock = Pred;
    191     }
    192 
    193     // Find whether BB is uniformly on the true (or false) path
    194     // for all of its predecessors.
    195     BasicBlock *PS1 = PBI->getSuccessor(0);
    196     BasicBlock *PS2 = PBI->getSuccessor(1);
    197     BasicBlock *PS = (PS1 == BB) ? PS2 : PS1;
    198     int CIdx = (PS1 == BB) ? 0 : 1;
    199 
    200     if (Idx == -1)
    201       Idx = CIdx;
    202     else if (CIdx != Idx)
    203       return false;
    204 
    205     // PS is the successor which is not BB. Check successors to identify
    206     // the last conditional branch.
    207     if (Preds.count(PS) == 0) {
    208       // Case 2.
    209       LastCondBlock = Pred;
    210     } else {
    211       // Case 1
    212       BranchInst *BPS = dyn_cast<BranchInst>(PS->getTerminator());
    213       if (BPS && BPS->isUnconditional()) {
    214         // Case 1: PS(BB3) should be an unconditional branch.
    215         LastCondBlock = Pred;
    216       }
    217     }
    218   }
    219 
    220   if (!FirstCondBlock || !LastCondBlock || (FirstCondBlock == LastCondBlock))
    221     return false;
    222 
    223   TerminatorInst *TBB = LastCondBlock->getTerminator();
    224   BasicBlock *PS1 = TBB->getSuccessor(0);
    225   BasicBlock *PS2 = TBB->getSuccessor(1);
    226   BranchInst *PBI1 = dyn_cast<BranchInst>(PS1->getTerminator());
    227   BranchInst *PBI2 = dyn_cast<BranchInst>(PS2->getTerminator());
    228 
    229   // If PS1 does not jump into PS2, but PS2 jumps into PS1,
    230   // attempt branch inversion.
    231   if (!PBI1 || !PBI1->isUnconditional() ||
    232       (PS1->getTerminator()->getSuccessor(0) != PS2)) {
    233     // Check whether PS2 jumps into PS1.
    234     if (!PBI2 || !PBI2->isUnconditional() ||
    235         (PS2->getTerminator()->getSuccessor(0) != PS1))
    236       return false;
    237 
    238     // Do branch inversion.
    239     BasicBlock *CurrBlock = LastCondBlock;
    240     bool EverChanged = false;
    241     while (1) {
    242       BranchInst *BI = dyn_cast<BranchInst>(CurrBlock->getTerminator());
    243       CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
    244       CmpInst::Predicate Predicate = CI->getPredicate();
    245       // Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
    246       if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
    247         CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
    248         BI->swapSuccessors();
    249         EverChanged = true;
    250       }
    251       if (CurrBlock == FirstCondBlock)
    252         break;
    253       CurrBlock = CurrBlock->getSinglePredecessor();
    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->begin();
    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();
    330   BasicBlock::iterator iter2 = Block2->begin();
    331   BasicBlock::iterator end2 = Block2->getTerminator();
    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                                   Pass *P) {
    390   BasicBlock *IfTrue2, *IfFalse2;
    391   Value *IfCond2 = GetIfCondition(BB, IfTrue2, IfFalse2);
    392   Instruction *CInst2 = dyn_cast_or_null<Instruction>(IfCond2);
    393   if (!CInst2)
    394     return false;
    395 
    396   BasicBlock *SecondEntryBlock = CInst2->getParent();
    397   if (SecondEntryBlock->hasAddressTaken())
    398     return false;
    399 
    400   BasicBlock *IfTrue1, *IfFalse1;
    401   Value *IfCond1 = GetIfCondition(SecondEntryBlock, IfTrue1, IfFalse1);
    402   Instruction *CInst1 = dyn_cast_or_null<Instruction>(IfCond1);
    403   if (!CInst1)
    404     return false;
    405 
    406   BasicBlock *FirstEntryBlock = CInst1->getParent();
    407 
    408   // Either then-path or else-path should be empty.
    409   if ((IfTrue1 != FirstEntryBlock) && (IfFalse1 != FirstEntryBlock))
    410     return false;
    411   if ((IfTrue2 != SecondEntryBlock) && (IfFalse2 != SecondEntryBlock))
    412     return false;
    413 
    414   TerminatorInst *PTI2 = SecondEntryBlock->getTerminator();
    415   Instruction *PBI2 = SecondEntryBlock->begin();
    416 
    417   if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfTrue1,
    418                             IfTrue2))
    419     return false;
    420 
    421   if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfFalse1,
    422                             IfFalse2))
    423     return false;
    424 
    425   // Check whether \param SecondEntryBlock has side-effect and is safe to
    426   // speculate.
    427   for (BasicBlock::iterator BI = PBI2, BE = PTI2; BI != BE; ++BI) {
    428     Instruction *CI = BI;
    429     if (isa<PHINode>(CI) || CI->mayHaveSideEffects() ||
    430         !isSafeToSpeculativelyExecute(CI))
    431       return false;
    432   }
    433 
    434   // Merge \param SecondEntryBlock into \param FirstEntryBlock.
    435   FirstEntryBlock->getInstList().pop_back();
    436   FirstEntryBlock->getInstList()
    437       .splice(FirstEntryBlock->end(), SecondEntryBlock->getInstList());
    438   BranchInst *PBI = dyn_cast<BranchInst>(FirstEntryBlock->getTerminator());
    439   Value *CC = PBI->getCondition();
    440   BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
    441   BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
    442   Builder.SetInsertPoint(PBI);
    443   Value *NC = Builder.CreateOr(CInst1, CC);
    444   PBI->replaceUsesOfWith(CC, NC);
    445   Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
    446 
    447   // Remove IfTrue1
    448   if (IfTrue1 != FirstEntryBlock) {
    449     IfTrue1->dropAllReferences();
    450     IfTrue1->eraseFromParent();
    451   }
    452 
    453   // Remove IfFalse1
    454   if (IfFalse1 != FirstEntryBlock) {
    455     IfFalse1->dropAllReferences();
    456     IfFalse1->eraseFromParent();
    457   }
    458 
    459   // Remove \param SecondEntryBlock
    460   SecondEntryBlock->dropAllReferences();
    461   SecondEntryBlock->eraseFromParent();
    462   DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
    463   return true;
    464 }
    465 
    466 bool FlattenCFGOpt::run(BasicBlock *BB) {
    467   bool Changed = false;
    468   assert(BB && BB->getParent() && "Block not embedded in function!");
    469   assert(BB->getTerminator() && "Degenerate basic block encountered!");
    470 
    471   IRBuilder<> Builder(BB);
    472 
    473   if (FlattenParallelAndOr(BB, Builder))
    474     return true;
    475 
    476   if (MergeIfRegion(BB, Builder))
    477     return true;
    478 
    479   return Changed;
    480 }
    481 
    482 /// FlattenCFG - This function is used to flatten a CFG.  For
    483 /// example, it uses parallel-and and parallel-or mode to collapse
    484 //  if-conditions and merge if-regions with identical statements.
    485 ///
    486 bool llvm::FlattenCFG(BasicBlock *BB, AliasAnalysis *AA) {
    487   return FlattenCFGOpt(AA).run(BB);
    488 }
    489