LCOV - code coverage report
Current view: top level - lib/Target/AArch64 - AArch64ConditionOptimizer.cpp (source / functions) Hit Total Coverage
Test: llvm-toolchain.info Lines: 118 131 90.1 %
Date: 2017-09-14 15:23:50 Functions: 12 13 92.3 %
Legend: Lines: hit not hit

          Line data    Source code
       1             : //=- AArch64ConditionOptimizer.cpp - Remove useless comparisons for AArch64 -=//
       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 pass tries to make consecutive compares of values use same operands to
      11             : // allow CSE pass to remove duplicated instructions.  For this it analyzes
      12             : // branches and adjusts comparisons with immediate values by converting:
      13             : //  * GE -> GT
      14             : //  * GT -> GE
      15             : //  * LT -> LE
      16             : //  * LE -> LT
      17             : // and adjusting immediate values appropriately.  It basically corrects two
      18             : // immediate values towards each other to make them equal.
      19             : //
      20             : // Consider the following example in C:
      21             : //
      22             : //   if ((a < 5 && ...) || (a > 5 && ...)) {
      23             : //        ~~~~~             ~~~~~
      24             : //          ^                 ^
      25             : //          x                 y
      26             : //
      27             : // Here both "x" and "y" expressions compare "a" with "5".  When "x" evaluates
      28             : // to "false", "y" can just check flags set by the first comparison.  As a
      29             : // result of the canonicalization employed by
      30             : // SelectionDAGBuilder::visitSwitchCase, DAGCombine, and other target-specific
      31             : // code, assembly ends up in the form that is not CSE friendly:
      32             : //
      33             : //     ...
      34             : //     cmp      w8, #4
      35             : //     b.gt     .LBB0_3
      36             : //     ...
      37             : //   .LBB0_3:
      38             : //     cmp      w8, #6
      39             : //     b.lt     .LBB0_6
      40             : //     ...
      41             : //
      42             : // Same assembly after the pass:
      43             : //
      44             : //     ...
      45             : //     cmp      w8, #5
      46             : //     b.ge     .LBB0_3
      47             : //     ...
      48             : //   .LBB0_3:
      49             : //     cmp      w8, #5     // <-- CSE pass removes this instruction
      50             : //     b.le     .LBB0_6
      51             : //     ...
      52             : //
      53             : // Currently only SUBS and ADDS followed by b.?? are supported.
      54             : //
      55             : // TODO: maybe handle TBNZ/TBZ the same way as CMP when used instead for "a < 0"
      56             : // TODO: handle other conditional instructions (e.g. CSET)
      57             : // TODO: allow second branching to be anything if it doesn't require adjusting
      58             : //
      59             : //===----------------------------------------------------------------------===//
      60             : 
      61             : #include "AArch64.h"
      62             : #include "MCTargetDesc/AArch64AddressingModes.h"
      63             : #include "Utils/AArch64BaseInfo.h"
      64             : #include "llvm/ADT/ArrayRef.h"
      65             : #include "llvm/ADT/DepthFirstIterator.h"
      66             : #include "llvm/ADT/SmallVector.h"
      67             : #include "llvm/ADT/Statistic.h"
      68             : #include "llvm/CodeGen/MachineBasicBlock.h"
      69             : #include "llvm/CodeGen/MachineDominators.h"
      70             : #include "llvm/CodeGen/MachineFunction.h"
      71             : #include "llvm/CodeGen/MachineFunctionPass.h"
      72             : #include "llvm/CodeGen/MachineInstr.h"
      73             : #include "llvm/CodeGen/MachineInstrBuilder.h"
      74             : #include "llvm/CodeGen/MachineOperand.h"
      75             : #include "llvm/CodeGen/MachineRegisterInfo.h"
      76             : #include "llvm/Pass.h"
      77             : #include "llvm/Support/Debug.h"
      78             : #include "llvm/Support/ErrorHandling.h"
      79             : #include "llvm/Support/raw_ostream.h"
      80             : #include "llvm/Target/TargetInstrInfo.h"
      81             : #include "llvm/Target/TargetSubtargetInfo.h"
      82             : #include <cassert>
      83             : #include <cstdlib>
      84             : #include <tuple>
      85             : 
      86             : using namespace llvm;
      87             : 
      88             : #define DEBUG_TYPE "aarch64-condopt"
      89             : 
      90             : STATISTIC(NumConditionsAdjusted, "Number of conditions adjusted");
      91             : 
      92             : namespace {
      93             : 
      94         907 : class AArch64ConditionOptimizer : public MachineFunctionPass {
      95             :   const TargetInstrInfo *TII;
      96             :   MachineDominatorTree *DomTree;
      97             :   const MachineRegisterInfo *MRI;
      98             : 
      99             : public:
     100             :   // Stores immediate, compare instruction opcode and branch condition (in this
     101             :   // order) of adjusted comparison.
     102             :   using CmpInfo = std::tuple<int, unsigned, AArch64CC::CondCode>;
     103             : 
     104             :   static char ID;
     105             : 
     106         914 :   AArch64ConditionOptimizer() : MachineFunctionPass(ID) {
     107         914 :     initializeAArch64ConditionOptimizerPass(*PassRegistry::getPassRegistry());
     108         914 :   }
     109             : 
     110             :   void getAnalysisUsage(AnalysisUsage &AU) const override;
     111             :   MachineInstr *findSuitableCompare(MachineBasicBlock *MBB);
     112             :   CmpInfo adjustCmp(MachineInstr *CmpMI, AArch64CC::CondCode Cmp);
     113             :   void modifyCmp(MachineInstr *CmpMI, const CmpInfo &Info);
     114             :   bool adjustTo(MachineInstr *CmpMI, AArch64CC::CondCode Cmp, MachineInstr *To,
     115             :                 int ToImm);
     116             :   bool runOnMachineFunction(MachineFunction &MF) override;
     117             : 
     118         913 :   StringRef getPassName() const override {
     119         913 :     return "AArch64 Condition Optimizer";
     120             :   }
     121             : };
     122             : 
     123             : } // end anonymous namespace
     124             : 
     125             : char AArch64ConditionOptimizer::ID = 0;
     126             : 
     127       53045 : INITIALIZE_PASS_BEGIN(AArch64ConditionOptimizer, "aarch64-condopt",
     128             :                       "AArch64 CondOpt Pass", false, false)
     129       53045 : INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
     130      315295 : INITIALIZE_PASS_END(AArch64ConditionOptimizer, "aarch64-condopt",
     131             :                     "AArch64 CondOpt Pass", false, false)
     132             : 
     133         914 : FunctionPass *llvm::createAArch64ConditionOptimizerPass() {
     134         914 :   return new AArch64ConditionOptimizer();
     135             : }
     136             : 
     137         907 : void AArch64ConditionOptimizer::getAnalysisUsage(AnalysisUsage &AU) const {
     138         907 :   AU.addRequired<MachineDominatorTree>();
     139         907 :   AU.addPreserved<MachineDominatorTree>();
     140         907 :   MachineFunctionPass::getAnalysisUsage(AU);
     141         907 : }
     142             : 
     143             : // Finds compare instruction that corresponds to supported types of branching.
     144             : // Returns the instruction or nullptr on failures or detecting unsupported
     145             : // instructions.
     146         366 : MachineInstr *AArch64ConditionOptimizer::findSuitableCompare(
     147             :     MachineBasicBlock *MBB) {
     148         366 :   MachineBasicBlock::iterator I = MBB->getFirstTerminator();
     149         732 :   if (I == MBB->end())
     150             :     return nullptr;
     151             : 
     152         670 :   if (I->getOpcode() != AArch64::Bcc)
     153             :     return nullptr;
     154             : 
     155             :   // Since we may modify cmp of this MBB, make sure NZCV does not live out.
     156         698 :   for (auto SuccBB : MBB->successors())
     157         349 :     if (SuccBB->isLiveIn(AArch64::NZCV))
     158             :       return nullptr;
     159             : 
     160             :   // Now find the instruction controlling the terminator.
     161         377 :   for (MachineBasicBlock::iterator B = MBB->begin(); I != B;) {
     162         195 :     --I;
     163             :     assert(!I->isTerminator() && "Spurious terminator");
     164             :     // Check if there is any use of NZCV between CMP and Bcc.
     165         390 :     if (I->readsRegister(AArch64::NZCV))
     166             :       return nullptr;
     167         388 :     switch (I->getOpcode()) {
     168             :     // cmp is an alias for subs with a dead destination register.
     169         105 :     case AArch64::SUBSWri:
     170             :     case AArch64::SUBSXri:
     171             :     // cmn is an alias for adds with a dead destination register.
     172             :     case AArch64::ADDSWri:
     173             :     case AArch64::ADDSXri: {
     174         210 :       unsigned ShiftAmt = AArch64_AM::getShiftValue(I->getOperand(3).getImm());
     175         210 :       if (!I->getOperand(2).isImm()) {
     176             :         DEBUG(dbgs() << "Immediate of cmp is symbolic, " << *I << '\n');
     177             :         return nullptr;
     178         105 :       } else if (I->getOperand(2).getImm() << ShiftAmt >= 0xfff) {
     179             :         DEBUG(dbgs() << "Immediate of cmp may be out of range, " << *I << '\n');
     180             :         return nullptr;
     181         309 :       } else if (!MRI->use_empty(I->getOperand(0).getReg())) {
     182             :         DEBUG(dbgs() << "Destination of cmp is not dead, " << *I << '\n');
     183             :         return nullptr;
     184             :       }
     185         103 :       return &*I;
     186             :     }
     187             :     // Prevent false positive case like:
     188             :     // cmp      w19, #0
     189             :     // cinc     w0, w19, gt
     190             :     // ...
     191             :     // fcmp     d8, #0.0
     192             :     // b.gt     .LBB0_5
     193             :     case AArch64::FCMPDri:
     194             :     case AArch64::FCMPSri:
     195             :     case AArch64::FCMPESri:
     196             :     case AArch64::FCMPEDri:
     197             : 
     198             :     case AArch64::SUBSWrr:
     199             :     case AArch64::SUBSXrr:
     200             :     case AArch64::ADDSWrr:
     201             :     case AArch64::ADDSXrr:
     202             :     case AArch64::FCMPSrr:
     203             :     case AArch64::FCMPDrr:
     204             :     case AArch64::FCMPESrr:
     205             :     case AArch64::FCMPEDrr:
     206             :       // Skip comparison instructions without immediate operands.
     207             :       return nullptr;
     208             :     }
     209             :   }
     210             :   DEBUG(dbgs() << "Flags not defined in BB#" << MBB->getNumber() << '\n');
     211             :   return nullptr;
     212             : }
     213             : 
     214             : // Changes opcode adds <-> subs considering register operand width.
     215             : static int getComplementOpc(int Opc) {
     216           0 :   switch (Opc) {
     217             :   case AArch64::ADDSWri: return AArch64::SUBSWri;
     218           0 :   case AArch64::ADDSXri: return AArch64::SUBSXri;
     219           0 :   case AArch64::SUBSWri: return AArch64::ADDSWri;
     220           0 :   case AArch64::SUBSXri: return AArch64::ADDSXri;
     221           0 :   default:
     222           0 :     llvm_unreachable("Unexpected opcode");
     223             :   }
     224             : }
     225             : 
     226             : // Changes form of comparison inclusive <-> exclusive.
     227             : static AArch64CC::CondCode getAdjustedCmp(AArch64CC::CondCode Cmp) {
     228          13 :   switch (Cmp) {
     229             :   case AArch64CC::GT: return AArch64CC::GE;
     230           0 :   case AArch64CC::GE: return AArch64CC::GT;
     231           6 :   case AArch64CC::LT: return AArch64CC::LE;
     232           0 :   case AArch64CC::LE: return AArch64CC::LT;
     233           0 :   default:
     234           0 :     llvm_unreachable("Unexpected condition code");
     235             :   }
     236             : }
     237             : 
     238             : // Transforms GT -> GE, GE -> GT, LT -> LE, LE -> LT by updating comparison
     239             : // operator and condition code.
     240          13 : AArch64ConditionOptimizer::CmpInfo AArch64ConditionOptimizer::adjustCmp(
     241             :     MachineInstr *CmpMI, AArch64CC::CondCode Cmp) {
     242          26 :   unsigned Opc = CmpMI->getOpcode();
     243             : 
     244             :   // CMN (compare with negative immediate) is an alias to ADDS (as
     245             :   // "operand - negative" == "operand + positive")
     246          13 :   bool Negative = (Opc == AArch64::ADDSWri || Opc == AArch64::ADDSXri);
     247             : 
     248          13 :   int Correction = (Cmp == AArch64CC::GT) ? 1 : -1;
     249             :   // Negate Correction value for comparison with negative immediate (CMN).
     250          13 :   if (Negative) {
     251           5 :     Correction = -Correction;
     252             :   }
     253             : 
     254          13 :   const int OldImm = (int)CmpMI->getOperand(2).getImm();
     255          13 :   const int NewImm = std::abs(OldImm + Correction);
     256             : 
     257             :   // Handle +0 -> -1 and -0 -> +1 (CMN with 0 immediate) transitions by
     258             :   // adjusting compare instruction opcode.
     259          14 :   if (OldImm == 0 && ((Negative && Correction == 1) ||
     260           1 :                       (!Negative && Correction == -1))) {
     261           0 :     Opc = getComplementOpc(Opc);
     262             :   }
     263             : 
     264          26 :   return CmpInfo(NewImm, Opc, getAdjustedCmp(Cmp));
     265             : }
     266             : 
     267             : // Applies changes to comparison instruction suggested by adjustCmp().
     268          10 : void AArch64ConditionOptimizer::modifyCmp(MachineInstr *CmpMI,
     269             :     const CmpInfo &Info) {
     270             :   int Imm;
     271             :   unsigned Opc;
     272             :   AArch64CC::CondCode Cmp;
     273          20 :   std::tie(Imm, Opc, Cmp) = Info;
     274             : 
     275          10 :   MachineBasicBlock *const MBB = CmpMI->getParent();
     276             : 
     277             :   // Change immediate in comparison instruction (ADDS or SUBS).
     278          40 :   BuildMI(*MBB, CmpMI, CmpMI->getDebugLoc(), TII->get(Opc))
     279          20 :       .add(CmpMI->getOperand(0))
     280          30 :       .add(CmpMI->getOperand(1))
     281          20 :       .addImm(Imm)
     282          30 :       .add(CmpMI->getOperand(3));
     283          10 :   CmpMI->eraseFromParent();
     284             : 
     285             :   // The fact that this comparison was picked ensures that it's related to the
     286             :   // first terminator instruction.
     287          20 :   MachineInstr &BrMI = *MBB->getFirstTerminator();
     288             : 
     289             :   // Change condition in branch instruction.
     290          30 :   BuildMI(*MBB, BrMI, BrMI.getDebugLoc(), TII->get(AArch64::Bcc))
     291          20 :       .addImm(Cmp)
     292          30 :       .add(BrMI.getOperand(1));
     293          10 :   BrMI.eraseFromParent();
     294             : 
     295          10 :   MBB->updateTerminator();
     296             : 
     297          10 :   ++NumConditionsAdjusted;
     298          10 : }
     299             : 
     300             : // Parse a condition code returned by AnalyzeBranch, and compute the CondCode
     301             : // corresponding to TBB.
     302             : // Returns true if parsing was successful, otherwise false is returned.
     303             : static bool parseCond(ArrayRef<MachineOperand> Cond, AArch64CC::CondCode &CC) {
     304             :   // A normal br.cond simply has the condition code.
     305          50 :   if (Cond[0].getImm() != -1) {
     306             :     assert(Cond.size() == 1 && "Unknown Cond array format");
     307          50 :     CC = (AArch64CC::CondCode)(int)Cond[0].getImm();
     308             :     return true;
     309             :   }
     310             :   return false;
     311             : }
     312             : 
     313             : // Adjusts one cmp instruction to another one if result of adjustment will allow
     314             : // CSE.  Returns true if compare instruction was changed, otherwise false is
     315             : // returned.
     316           3 : bool AArch64ConditionOptimizer::adjustTo(MachineInstr *CmpMI,
     317             :   AArch64CC::CondCode Cmp, MachineInstr *To, int ToImm)
     318             : {
     319           3 :   CmpInfo Info = adjustCmp(CmpMI, Cmp);
     320           9 :   if (std::get<0>(Info) == ToImm && std::get<1>(Info) == To->getOpcode()) {
     321           2 :     modifyCmp(CmpMI, Info);
     322           2 :     return true;
     323             :   }
     324             :   return false;
     325             : }
     326             : 
     327       11003 : bool AArch64ConditionOptimizer::runOnMachineFunction(MachineFunction &MF) {
     328             :   DEBUG(dbgs() << "********** AArch64 Conditional Compares **********\n"
     329             :                << "********** Function: " << MF.getName() << '\n');
     330       11003 :   if (skipFunction(*MF.getFunction()))
     331             :     return false;
     332             : 
     333       11002 :   TII = MF.getSubtarget().getInstrInfo();
     334       11002 :   DomTree = &getAnalysis<MachineDominatorTree>();
     335       11002 :   MRI = &MF.getRegInfo();
     336             : 
     337       11002 :   bool Changed = false;
     338             : 
     339             :   // Visit blocks in dominator tree pre-order. The pre-order enables multiple
     340             :   // cmp-conversions from the same head block.
     341             :   // Note that updateDomTree() modifies the children of the DomTree node
     342             :   // currently being visited. The df_iterator supports that; it doesn't look at
     343             :   // child_begin() / child_end() until after a node has been visited.
     344       81238 :   for (MachineDomTreeNode *I : depth_first(DomTree)) {
     345       13114 :     MachineBasicBlock *HBB = I->getBlock();
     346             : 
     347       13139 :     SmallVector<MachineOperand, 4> HeadCond;
     348       13114 :     MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
     349       13114 :     if (TII->analyzeBranch(*HBB, TBB, FBB, HeadCond)) {
     350       24363 :       continue;
     351             :     }
     352             : 
     353             :     // Equivalence check is to skip loops.
     354        1840 :     if (!TBB || TBB == HBB) {
     355         862 :       continue;
     356             :     }
     357             : 
     358        1003 :     SmallVector<MachineOperand, 4> TrueCond;
     359         978 :     MachineBasicBlock *TBB_TBB = nullptr, *TBB_FBB = nullptr;
     360         978 :     if (TII->analyzeBranch(*TBB, TBB_TBB, TBB_FBB, TrueCond)) {
     361         690 :       continue;
     362             :     }
     363             : 
     364         288 :     MachineInstr *HeadCmpMI = findSuitableCompare(HBB);
     365         288 :     if (!HeadCmpMI) {
     366         210 :       continue;
     367             :     }
     368             : 
     369          78 :     MachineInstr *TrueCmpMI = findSuitableCompare(TBB);
     370          78 :     if (!TrueCmpMI) {
     371          53 :       continue;
     372             :     }
     373             : 
     374             :     AArch64CC::CondCode HeadCmp;
     375          75 :     if (HeadCond.empty() || !parseCond(HeadCond, HeadCmp)) {
     376           0 :       continue;
     377             :     }
     378             : 
     379             :     AArch64CC::CondCode TrueCmp;
     380          75 :     if (TrueCond.empty() || !parseCond(TrueCond, TrueCmp)) {
     381           0 :       continue;
     382             :     }
     383             : 
     384          25 :     const int HeadImm = (int)HeadCmpMI->getOperand(2).getImm();
     385          25 :     const int TrueImm = (int)TrueCmpMI->getOperand(2).getImm();
     386             : 
     387             :     DEBUG(dbgs() << "Head branch:\n");
     388             :     DEBUG(dbgs() << "\tcondition: "
     389             :           << AArch64CC::getCondCodeName(HeadCmp) << '\n');
     390             :     DEBUG(dbgs() << "\timmediate: " << HeadImm << '\n');
     391             : 
     392             :     DEBUG(dbgs() << "True branch:\n");
     393             :     DEBUG(dbgs() << "\tcondition: "
     394             :           << AArch64CC::getCondCodeName(TrueCmp) << '\n');
     395             :     DEBUG(dbgs() << "\timmediate: " << TrueImm << '\n');
     396             : 
     397          32 :     if (((HeadCmp == AArch64CC::GT && TrueCmp == AArch64CC::LT) ||
     398           8 :          (HeadCmp == AArch64CC::LT && TrueCmp == AArch64CC::GT)) &&
     399           5 :         std::abs(TrueImm - HeadImm) == 2) {
     400             :       // This branch transforms machine instructions that correspond to
     401             :       //
     402             :       // 1) (a > {TrueImm} && ...) || (a < {HeadImm} && ...)
     403             :       // 2) (a < {TrueImm} && ...) || (a > {HeadImm} && ...)
     404             :       //
     405             :       // into
     406             :       //
     407             :       // 1) (a >= {NewImm} && ...) || (a <= {NewImm} && ...)
     408             :       // 2) (a <= {NewImm} && ...) || (a >= {NewImm} && ...)
     409             : 
     410           5 :       CmpInfo HeadCmpInfo = adjustCmp(HeadCmpMI, HeadCmp);
     411           5 :       CmpInfo TrueCmpInfo = adjustCmp(TrueCmpMI, TrueCmp);
     412          14 :       if (std::get<0>(HeadCmpInfo) == std::get<0>(TrueCmpInfo) &&
     413           8 :           std::get<1>(HeadCmpInfo) == std::get<1>(TrueCmpInfo)) {
     414           4 :         modifyCmp(HeadCmpMI, HeadCmpInfo);
     415           4 :         modifyCmp(TrueCmpMI, TrueCmpInfo);
     416           4 :         Changed = true;
     417           5 :       }
     418          20 :     } else if (((HeadCmp == AArch64CC::GT && TrueCmp == AArch64CC::GT) ||
     419           4 :                 (HeadCmp == AArch64CC::LT && TrueCmp == AArch64CC::LT)) &&
     420           3 :                 std::abs(TrueImm - HeadImm) == 1) {
     421             :       // This branch transforms machine instructions that correspond to
     422             :       //
     423             :       // 1) (a > {TrueImm} && ...) || (a > {HeadImm} && ...)
     424             :       // 2) (a < {TrueImm} && ...) || (a < {HeadImm} && ...)
     425             :       //
     426             :       // into
     427             :       //
     428             :       // 1) (a <= {NewImm} && ...) || (a >  {NewImm} && ...)
     429             :       // 2) (a <  {NewImm} && ...) || (a >= {NewImm} && ...)
     430             : 
     431             :       // GT -> GE transformation increases immediate value, so picking the
     432             :       // smaller one; LT -> LE decreases immediate value so invert the choice.
     433           3 :       bool adjustHeadCond = (HeadImm < TrueImm);
     434           3 :       if (HeadCmp == AArch64CC::LT) {
     435           1 :           adjustHeadCond = !adjustHeadCond;
     436             :       }
     437             : 
     438           3 :       if (adjustHeadCond) {
     439           2 :         Changed |= adjustTo(HeadCmpMI, HeadCmp, TrueCmpMI, TrueImm);
     440             :       } else {
     441           1 :         Changed |= adjustTo(TrueCmpMI, TrueCmp, HeadCmpMI, HeadImm);
     442             :       }
     443             :     }
     444             :     // Other transformation cases almost never occur due to generation of < or >
     445             :     // comparisons instead of <= and >=.
     446             :   }
     447             : 
     448       11002 :   return Changed;
     449             : }

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