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      1 //===-- AArch64AddressTypePromotion.cpp --- Promote type for addr accesses -==//
      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 promote the computations use to obtained a sign extended
     11 // value used into memory accesses.
     12 // E.g.
     13 // a = add nsw i32 b, 3
     14 // d = sext i32 a to i64
     15 // e = getelementptr ..., i64 d
     16 //
     17 // =>
     18 // f = sext i32 b to i64
     19 // a = add nsw i64 f, 3
     20 // e = getelementptr ..., i64 a
     21 //
     22 // This is legal to do if the computations are marked with either nsw or nuw
     23 // markers.
     24 // Moreover, the current heuristic is simple: it does not create new sext
     25 // operations, i.e., it gives up when a sext would have forked (e.g., if
     26 // a = add i32 b, c, two sexts are required to promote the computation).
     27 //
     28 // FIXME: This pass may be useful for other targets too.
     29 // ===---------------------------------------------------------------------===//
     30 
     31 #include "AArch64.h"
     32 #include "llvm/ADT/DenseMap.h"
     33 #include "llvm/ADT/SmallPtrSet.h"
     34 #include "llvm/ADT/SmallVector.h"
     35 #include "llvm/IR/Constants.h"
     36 #include "llvm/IR/Dominators.h"
     37 #include "llvm/IR/Function.h"
     38 #include "llvm/IR/Instructions.h"
     39 #include "llvm/IR/Module.h"
     40 #include "llvm/IR/Operator.h"
     41 #include "llvm/Pass.h"
     42 #include "llvm/Support/CommandLine.h"
     43 #include "llvm/Support/Debug.h"
     44 #include "llvm/Support/raw_ostream.h"
     45 
     46 using namespace llvm;
     47 
     48 #define DEBUG_TYPE "aarch64-type-promotion"
     49 
     50 static cl::opt<bool>
     51 EnableAddressTypePromotion("aarch64-type-promotion", cl::Hidden,
     52                            cl::desc("Enable the type promotion pass"),
     53                            cl::init(true));
     54 static cl::opt<bool>
     55 EnableMerge("aarch64-type-promotion-merge", cl::Hidden,
     56             cl::desc("Enable merging of redundant sexts when one is dominating"
     57                      " the other."),
     58             cl::init(true));
     59 
     60 //===----------------------------------------------------------------------===//
     61 //                       AArch64AddressTypePromotion
     62 //===----------------------------------------------------------------------===//
     63 
     64 namespace llvm {
     65 void initializeAArch64AddressTypePromotionPass(PassRegistry &);
     66 }
     67 
     68 namespace {
     69 class AArch64AddressTypePromotion : public FunctionPass {
     70 
     71 public:
     72   static char ID;
     73   AArch64AddressTypePromotion()
     74       : FunctionPass(ID), Func(nullptr), ConsideredSExtType(nullptr) {
     75     initializeAArch64AddressTypePromotionPass(*PassRegistry::getPassRegistry());
     76   }
     77 
     78   const char *getPassName() const override {
     79     return "AArch64 Address Type Promotion";
     80   }
     81 
     82   /// Iterate over the functions and promote the computation of interesting
     83   // sext instructions.
     84   bool runOnFunction(Function &F) override;
     85 
     86 private:
     87   /// The current function.
     88   Function *Func;
     89   /// Filter out all sexts that does not have this type.
     90   /// Currently initialized with Int64Ty.
     91   Type *ConsideredSExtType;
     92 
     93   // This transformation requires dominator info.
     94   void getAnalysisUsage(AnalysisUsage &AU) const override {
     95     AU.setPreservesCFG();
     96     AU.addRequired<DominatorTreeWrapperPass>();
     97     AU.addPreserved<DominatorTreeWrapperPass>();
     98     FunctionPass::getAnalysisUsage(AU);
     99   }
    100 
    101   typedef SmallPtrSet<Instruction *, 32> SetOfInstructions;
    102   typedef SmallVector<Instruction *, 16> Instructions;
    103   typedef DenseMap<Value *, Instructions> ValueToInsts;
    104 
    105   /// Check if it is profitable to move a sext through this instruction.
    106   /// Currently, we consider it is profitable if:
    107   /// - Inst is used only once (no need to insert truncate).
    108   /// - Inst has only one operand that will require a sext operation (we do
    109   ///   do not create new sext operation).
    110   bool shouldGetThrough(const Instruction *Inst);
    111 
    112   /// Check if it is possible and legal to move a sext through this
    113   /// instruction.
    114   /// Current heuristic considers that we can get through:
    115   /// - Arithmetic operation marked with the nsw or nuw flag.
    116   /// - Other sext operation.
    117   /// - Truncate operation if it was just dropping sign extended bits.
    118   bool canGetThrough(const Instruction *Inst);
    119 
    120   /// Move sext operations through safe to sext instructions.
    121   bool propagateSignExtension(Instructions &SExtInsts);
    122 
    123   /// Is this sext should be considered for code motion.
    124   /// We look for sext with ConsideredSExtType and uses in at least one
    125   // GetElementPtrInst.
    126   bool shouldConsiderSExt(const Instruction *SExt) const;
    127 
    128   /// Collect all interesting sext operations, i.e., the ones with the right
    129   /// type and used in memory accesses.
    130   /// More precisely, a sext instruction is considered as interesting if it
    131   /// is used in a "complex" getelementptr or it exits at least another
    132   /// sext instruction that sign extended the same initial value.
    133   /// A getelementptr is considered as "complex" if it has more than 2
    134   // operands.
    135   void analyzeSExtension(Instructions &SExtInsts);
    136 
    137   /// Merge redundant sign extension operations in common dominator.
    138   void mergeSExts(ValueToInsts &ValToSExtendedUses,
    139                   SetOfInstructions &ToRemove);
    140 };
    141 } // end anonymous namespace.
    142 
    143 char AArch64AddressTypePromotion::ID = 0;
    144 
    145 INITIALIZE_PASS_BEGIN(AArch64AddressTypePromotion, "aarch64-type-promotion",
    146                       "AArch64 Type Promotion Pass", false, false)
    147 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
    148 INITIALIZE_PASS_END(AArch64AddressTypePromotion, "aarch64-type-promotion",
    149                     "AArch64 Type Promotion Pass", false, false)
    150 
    151 FunctionPass *llvm::createAArch64AddressTypePromotionPass() {
    152   return new AArch64AddressTypePromotion();
    153 }
    154 
    155 bool AArch64AddressTypePromotion::canGetThrough(const Instruction *Inst) {
    156   if (isa<SExtInst>(Inst))
    157     return true;
    158 
    159   const BinaryOperator *BinOp = dyn_cast<BinaryOperator>(Inst);
    160   if (BinOp && isa<OverflowingBinaryOperator>(BinOp) &&
    161       (BinOp->hasNoUnsignedWrap() || BinOp->hasNoSignedWrap()))
    162     return true;
    163 
    164   // sext(trunc(sext)) --> sext
    165   if (isa<TruncInst>(Inst) && isa<SExtInst>(Inst->getOperand(0))) {
    166     const Instruction *Opnd = cast<Instruction>(Inst->getOperand(0));
    167     // Check that the truncate just drop sign extended bits.
    168     if (Inst->getType()->getIntegerBitWidth() >=
    169             Opnd->getOperand(0)->getType()->getIntegerBitWidth() &&
    170         Inst->getOperand(0)->getType()->getIntegerBitWidth() <=
    171             ConsideredSExtType->getIntegerBitWidth())
    172       return true;
    173   }
    174 
    175   return false;
    176 }
    177 
    178 bool AArch64AddressTypePromotion::shouldGetThrough(const Instruction *Inst) {
    179   // If the type of the sext is the same as the considered one, this sext
    180   // will become useless.
    181   // Otherwise, we will have to do something to preserve the original value,
    182   // unless it is used once.
    183   if (isa<SExtInst>(Inst) &&
    184       (Inst->getType() == ConsideredSExtType || Inst->hasOneUse()))
    185     return true;
    186 
    187   // If the Inst is used more that once, we may need to insert truncate
    188   // operations and we don't do that at the moment.
    189   if (!Inst->hasOneUse())
    190     return false;
    191 
    192   // This truncate is used only once, thus if we can get thourgh, it will become
    193   // useless.
    194   if (isa<TruncInst>(Inst))
    195     return true;
    196 
    197   // If both operands are not constant, a new sext will be created here.
    198   // Current heuristic is: each step should be profitable.
    199   // Therefore we don't allow to increase the number of sext even if it may
    200   // be profitable later on.
    201   if (isa<BinaryOperator>(Inst) && isa<ConstantInt>(Inst->getOperand(1)))
    202     return true;
    203 
    204   return false;
    205 }
    206 
    207 static bool shouldSExtOperand(const Instruction *Inst, int OpIdx) {
    208   if (isa<SelectInst>(Inst) && OpIdx == 0)
    209     return false;
    210   return true;
    211 }
    212 
    213 bool
    214 AArch64AddressTypePromotion::shouldConsiderSExt(const Instruction *SExt) const {
    215   if (SExt->getType() != ConsideredSExtType)
    216     return false;
    217 
    218   for (const User *U : SExt->users()) {
    219     if (isa<GetElementPtrInst>(U))
    220       return true;
    221   }
    222 
    223   return false;
    224 }
    225 
    226 // Input:
    227 // - SExtInsts contains all the sext instructions that are used directly in
    228 //   GetElementPtrInst, i.e., access to memory.
    229 // Algorithm:
    230 // - For each sext operation in SExtInsts:
    231 //   Let var be the operand of sext.
    232 //   while it is profitable (see shouldGetThrough), legal, and safe
    233 //   (see canGetThrough) to move sext through var's definition:
    234 //   * promote the type of var's definition.
    235 //   * fold var into sext uses.
    236 //   * move sext above var's definition.
    237 //   * update sext operand to use the operand of var that should be sign
    238 //     extended (by construction there is only one).
    239 //
    240 //   E.g.,
    241 //   a = ... i32 c, 3
    242 //   b = sext i32 a to i64 <- is it legal/safe/profitable to get through 'a'
    243 //   ...
    244 //   = b
    245 // => Yes, update the code
    246 //   b = sext i32 c to i64
    247 //   a = ... i64 b, 3
    248 //   ...
    249 //   = a
    250 // Iterate on 'c'.
    251 bool
    252 AArch64AddressTypePromotion::propagateSignExtension(Instructions &SExtInsts) {
    253   DEBUG(dbgs() << "*** Propagate Sign Extension ***\n");
    254 
    255   bool LocalChange = false;
    256   SetOfInstructions ToRemove;
    257   ValueToInsts ValToSExtendedUses;
    258   while (!SExtInsts.empty()) {
    259     // Get through simple chain.
    260     Instruction *SExt = SExtInsts.pop_back_val();
    261 
    262     DEBUG(dbgs() << "Consider:\n" << *SExt << '\n');
    263 
    264     // If this SExt has already been merged continue.
    265     if (SExt->use_empty() && ToRemove.count(SExt)) {
    266       DEBUG(dbgs() << "No uses => marked as delete\n");
    267       continue;
    268     }
    269 
    270     // Now try to get through the chain of definitions.
    271     while (auto *Inst = dyn_cast<Instruction>(SExt->getOperand(0))) {
    272       DEBUG(dbgs() << "Try to get through:\n" << *Inst << '\n');
    273       if (!canGetThrough(Inst) || !shouldGetThrough(Inst)) {
    274         // We cannot get through something that is not an Instruction
    275         // or not safe to SExt.
    276         DEBUG(dbgs() << "Cannot get through\n");
    277         break;
    278       }
    279 
    280       LocalChange = true;
    281       // If this is a sign extend, it becomes useless.
    282       if (isa<SExtInst>(Inst) || isa<TruncInst>(Inst)) {
    283         DEBUG(dbgs() << "SExt or trunc, mark it as to remove\n");
    284         // We cannot use replaceAllUsesWith here because we may trigger some
    285         // assertion on the type as all involved sext operation may have not
    286         // been moved yet.
    287         while (!Inst->use_empty()) {
    288           Use &U = *Inst->use_begin();
    289           Instruction *User = dyn_cast<Instruction>(U.getUser());
    290           assert(User && "User of sext is not an Instruction!");
    291           User->setOperand(U.getOperandNo(), SExt);
    292         }
    293         ToRemove.insert(Inst);
    294         SExt->setOperand(0, Inst->getOperand(0));
    295         SExt->moveBefore(Inst);
    296         continue;
    297       }
    298 
    299       // Get through the Instruction:
    300       // 1. Update its type.
    301       // 2. Replace the uses of SExt by Inst.
    302       // 3. Sign extend each operand that needs to be sign extended.
    303 
    304       // Step #1.
    305       Inst->mutateType(SExt->getType());
    306       // Step #2.
    307       SExt->replaceAllUsesWith(Inst);
    308       // Step #3.
    309       Instruction *SExtForOpnd = SExt;
    310 
    311       DEBUG(dbgs() << "Propagate SExt to operands\n");
    312       for (int OpIdx = 0, EndOpIdx = Inst->getNumOperands(); OpIdx != EndOpIdx;
    313            ++OpIdx) {
    314         DEBUG(dbgs() << "Operand:\n" << *(Inst->getOperand(OpIdx)) << '\n');
    315         if (Inst->getOperand(OpIdx)->getType() == SExt->getType() ||
    316             !shouldSExtOperand(Inst, OpIdx)) {
    317           DEBUG(dbgs() << "No need to propagate\n");
    318           continue;
    319         }
    320         // Check if we can statically sign extend the operand.
    321         Value *Opnd = Inst->getOperand(OpIdx);
    322         if (const ConstantInt *Cst = dyn_cast<ConstantInt>(Opnd)) {
    323           DEBUG(dbgs() << "Statically sign extend\n");
    324           Inst->setOperand(OpIdx, ConstantInt::getSigned(SExt->getType(),
    325                                                          Cst->getSExtValue()));
    326           continue;
    327         }
    328         // UndefValue are typed, so we have to statically sign extend them.
    329         if (isa<UndefValue>(Opnd)) {
    330           DEBUG(dbgs() << "Statically sign extend\n");
    331           Inst->setOperand(OpIdx, UndefValue::get(SExt->getType()));
    332           continue;
    333         }
    334 
    335         // Otherwise we have to explicity sign extend it.
    336         assert(SExtForOpnd &&
    337                "Only one operand should have been sign extended");
    338 
    339         SExtForOpnd->setOperand(0, Opnd);
    340 
    341         DEBUG(dbgs() << "Move before:\n" << *Inst << "\nSign extend\n");
    342         // Move the sign extension before the insertion point.
    343         SExtForOpnd->moveBefore(Inst);
    344         Inst->setOperand(OpIdx, SExtForOpnd);
    345         // If more sext are required, new instructions will have to be created.
    346         SExtForOpnd = nullptr;
    347       }
    348       if (SExtForOpnd == SExt) {
    349         DEBUG(dbgs() << "Sign extension is useless now\n");
    350         ToRemove.insert(SExt);
    351         break;
    352       }
    353     }
    354 
    355     // If the use is already of the right type, connect its uses to its argument
    356     // and delete it.
    357     // This can happen for an Instruction all uses of which are sign extended.
    358     if (!ToRemove.count(SExt) &&
    359         SExt->getType() == SExt->getOperand(0)->getType()) {
    360       DEBUG(dbgs() << "Sign extension is useless, attach its use to "
    361                       "its argument\n");
    362       SExt->replaceAllUsesWith(SExt->getOperand(0));
    363       ToRemove.insert(SExt);
    364     } else
    365       ValToSExtendedUses[SExt->getOperand(0)].push_back(SExt);
    366   }
    367 
    368   if (EnableMerge)
    369     mergeSExts(ValToSExtendedUses, ToRemove);
    370 
    371   // Remove all instructions marked as ToRemove.
    372   for (Instruction *I: ToRemove)
    373     I->eraseFromParent();
    374   return LocalChange;
    375 }
    376 
    377 void AArch64AddressTypePromotion::mergeSExts(ValueToInsts &ValToSExtendedUses,
    378                                              SetOfInstructions &ToRemove) {
    379   DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    380 
    381   for (auto &Entry : ValToSExtendedUses) {
    382     Instructions &Insts = Entry.second;
    383     Instructions CurPts;
    384     for (Instruction *Inst : Insts) {
    385       if (ToRemove.count(Inst))
    386         continue;
    387       bool inserted = false;
    388       for (auto &Pt : CurPts) {
    389         if (DT.dominates(Inst, Pt)) {
    390           DEBUG(dbgs() << "Replace all uses of:\n" << *Pt << "\nwith:\n"
    391                        << *Inst << '\n');
    392           Pt->replaceAllUsesWith(Inst);
    393           ToRemove.insert(Pt);
    394           Pt = Inst;
    395           inserted = true;
    396           break;
    397         }
    398         if (!DT.dominates(Pt, Inst))
    399           // Give up if we need to merge in a common dominator as the
    400           // expermients show it is not profitable.
    401           continue;
    402 
    403         DEBUG(dbgs() << "Replace all uses of:\n" << *Inst << "\nwith:\n"
    404                      << *Pt << '\n');
    405         Inst->replaceAllUsesWith(Pt);
    406         ToRemove.insert(Inst);
    407         inserted = true;
    408         break;
    409       }
    410       if (!inserted)
    411         CurPts.push_back(Inst);
    412     }
    413   }
    414 }
    415 
    416 void AArch64AddressTypePromotion::analyzeSExtension(Instructions &SExtInsts) {
    417   DEBUG(dbgs() << "*** Analyze Sign Extensions ***\n");
    418 
    419   DenseMap<Value *, Instruction *> SeenChains;
    420 
    421   for (auto &BB : *Func) {
    422     for (auto &II : BB) {
    423       Instruction *SExt = &II;
    424 
    425       // Collect all sext operation per type.
    426       if (!isa<SExtInst>(SExt) || !shouldConsiderSExt(SExt))
    427         continue;
    428 
    429       DEBUG(dbgs() << "Found:\n" << (*SExt) << '\n');
    430 
    431       // Cases where we actually perform the optimization:
    432       // 1. SExt is used in a getelementptr with more than 2 operand =>
    433       //    likely we can merge some computation if they are done on 64 bits.
    434       // 2. The beginning of the SExt chain is SExt several time. =>
    435       //    code sharing is possible.
    436 
    437       bool insert = false;
    438       // #1.
    439       for (const User *U : SExt->users()) {
    440         const Instruction *Inst = dyn_cast<GetElementPtrInst>(U);
    441         if (Inst && Inst->getNumOperands() > 2) {
    442           DEBUG(dbgs() << "Interesting use in GetElementPtrInst\n" << *Inst
    443                        << '\n');
    444           insert = true;
    445           break;
    446         }
    447       }
    448 
    449       // #2.
    450       // Check the head of the chain.
    451       Instruction *Inst = SExt;
    452       Value *Last;
    453       do {
    454         int OpdIdx = 0;
    455         const BinaryOperator *BinOp = dyn_cast<BinaryOperator>(Inst);
    456         if (BinOp && isa<ConstantInt>(BinOp->getOperand(0)))
    457           OpdIdx = 1;
    458         Last = Inst->getOperand(OpdIdx);
    459         Inst = dyn_cast<Instruction>(Last);
    460       } while (Inst && canGetThrough(Inst) && shouldGetThrough(Inst));
    461 
    462       DEBUG(dbgs() << "Head of the chain:\n" << *Last << '\n');
    463       DenseMap<Value *, Instruction *>::iterator AlreadySeen =
    464           SeenChains.find(Last);
    465       if (insert || AlreadySeen != SeenChains.end()) {
    466         DEBUG(dbgs() << "Insert\n");
    467         SExtInsts.push_back(SExt);
    468         if (AlreadySeen != SeenChains.end() && AlreadySeen->second != nullptr) {
    469           DEBUG(dbgs() << "Insert chain member\n");
    470           SExtInsts.push_back(AlreadySeen->second);
    471           SeenChains[Last] = nullptr;
    472         }
    473       } else {
    474         DEBUG(dbgs() << "Record its chain membership\n");
    475         SeenChains[Last] = SExt;
    476       }
    477     }
    478   }
    479 }
    480 
    481 bool AArch64AddressTypePromotion::runOnFunction(Function &F) {
    482   if (!EnableAddressTypePromotion || F.isDeclaration())
    483     return false;
    484   Func = &F;
    485   ConsideredSExtType = Type::getInt64Ty(Func->getContext());
    486 
    487   DEBUG(dbgs() << "*** " << getPassName() << ": " << Func->getName() << '\n');
    488 
    489   Instructions SExtInsts;
    490   analyzeSExtension(SExtInsts);
    491   return propagateSignExtension(SExtInsts);
    492 }
    493