llvm/lib/CodeGen/SwiftErrorValueTracking.cpp

   1 //===-- SwiftErrorValueTracking.cpp --------------------------------------===//
   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 implements a limited mem2reg-like analysis to promote uses of function
  10 // arguments and allocas marked with swiftalloc from memory into virtual
  11 // registers tracked by this class.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/CodeGen/SwiftErrorValueTracking.h"
  16 #include "llvm/ADT/PostOrderIterator.h"
  17 #include "llvm/ADT/SmallSet.h"
  18 #include "llvm/CodeGen/MachineInstrBuilder.h"
  19 #include "llvm/CodeGen/MachineRegisterInfo.h"
  20 #include "llvm/CodeGen/TargetInstrInfo.h"
  21 #include "llvm/CodeGen/TargetLowering.h"
  22 #include "llvm/IR/Value.h"
  23
  24 using namespace llvm;
  25
  26 Register SwiftErrorValueTracking::getOrCreateVReg(const MachineBasicBlock *MBB,
  27                                                   const Value *Val) {
  28   auto Key = std::make_pair(MBB, Val);
  29   auto It = VRegDefMap.find(Key);
  30   // If this is the first use of this swifterror value in this basic block,
  31   // create a new virtual register.
  32   // After we processed all basic blocks we will satisfy this "upwards exposed
  33   // use" by inserting a copy or phi at the beginning of this block.
  34   if (It == VRegDefMap.end()) {
  35     auto &DL = MF->getDataLayout();
  36     const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
  37     auto VReg = MF->getRegInfo().createVirtualRegister(RC);
  38     VRegDefMap[Key] = VReg;
  39     VRegUpwardsUse[Key] = VReg;
  40     return VReg;
  41   } else
  42     return It->second;
  43 }
  44
  45 void SwiftErrorValueTracking::setCurrentVReg(const MachineBasicBlock *MBB,
  46                                              const Value *Val, Register VReg) {
  47   VRegDefMap[std::make_pair(MBB, Val)] = VReg;
  48 }
  49
  50 Register SwiftErrorValueTracking::getOrCreateVRegDefAt(
  51     const Instruction *I, const MachineBasicBlock *MBB, const Value *Val) {
  52   auto Key = PointerIntPair<const Instruction *, 1, bool>(I, true);
  53   auto It = VRegDefUses.find(Key);
  54   if (It != VRegDefUses.end())
  55     return It->second;
  56
  57   auto &DL = MF->getDataLayout();
  58   const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
  59   Register VReg = MF->getRegInfo().createVirtualRegister(RC);
  60   VRegDefUses[Key] = VReg;
  61   setCurrentVReg(MBB, Val, VReg);
  62   return VReg;
  63 }
  64
  65 Register SwiftErrorValueTracking::getOrCreateVRegUseAt(
  66     const Instruction *I, const MachineBasicBlock *MBB, const Value *Val) {
  67   auto Key = PointerIntPair<const Instruction *, 1, bool>(I, false);
  68   auto It = VRegDefUses.find(Key);
  69   if (It != VRegDefUses.end())
  70     return It->second;
  71
  72   Register VReg = getOrCreateVReg(MBB, Val);
  73   VRegDefUses[Key] = VReg;
  74   return VReg;
  75 }
  76
  77 /// Set up SwiftErrorVals by going through the function. If the function has
  78 /// swifterror argument, it will be the first entry.
  79 void SwiftErrorValueTracking::setFunction(MachineFunction &mf) {
  80   MF = &mf;
  81   Fn = &MF->getFunction();
  82   TLI = MF->getSubtarget().getTargetLowering();
  83   TII = MF->getSubtarget().getInstrInfo();
  84
  85   if (!TLI->supportSwiftError())
  86     return;
  87
  88   SwiftErrorVals.clear();
  89   VRegDefMap.clear();
  90   VRegUpwardsUse.clear();
  91   VRegDefUses.clear();
  92   SwiftErrorArg = nullptr;
  93
  94   // Check if function has a swifterror argument.
  95   bool HaveSeenSwiftErrorArg = false;
  96   for (Function::const_arg_iterator AI = Fn->arg_begin(), AE = Fn->arg_end();
  97        AI != AE; ++AI)
  98     if (AI->hasSwiftErrorAttr()) {
  99       assert(!HaveSeenSwiftErrorArg &&
 100              "Must have only one swifterror parameter");
 101       (void)HaveSeenSwiftErrorArg; // silence warning.
 102       HaveSeenSwiftErrorArg = true;
 103       SwiftErrorArg = &*AI;
 104       SwiftErrorVals.push_back(&*AI);
 105     }
 106
 107   for (const auto &LLVMBB : *Fn)
 108     for (const auto &Inst : LLVMBB) {
 109       if (const AllocaInst *Alloca = dyn_cast<AllocaInst>(&Inst))
 110         if (Alloca->isSwiftError())
 111           SwiftErrorVals.push_back(Alloca);
 112     }
 113 }
 114
 115 bool SwiftErrorValueTracking::createEntriesInEntryBlock(DebugLoc DbgLoc) {
 116   if (!TLI->supportSwiftError())
 117     return false;
 118
 119   // We only need to do this when we have swifterror parameter or swifterror
 120   // alloc.
 121   if (SwiftErrorVals.empty())
 122     return false;
 123
 124   MachineBasicBlock *MBB = &*MF->begin();
 125   auto &DL = MF->getDataLayout();
 126   auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
 127   bool Inserted = false;
 128   for (const auto *SwiftErrorVal : SwiftErrorVals) {
 129     // We will always generate a copy from the argument. It is always used at
 130     // least by the 'return' of the swifterror.
 131     if (SwiftErrorArg && SwiftErrorArg == SwiftErrorVal)
 132       continue;
 133     Register VReg = MF->getRegInfo().createVirtualRegister(RC);
 134     // Assign Undef to Vreg. We construct MI directly to make sure it works
 135     // with FastISel.
 136     BuildMI(*MBB, MBB->getFirstNonPHI(), DbgLoc,
 137             TII->get(TargetOpcode::IMPLICIT_DEF), VReg);
 138
 139     setCurrentVReg(MBB, SwiftErrorVal, VReg);
 140     Inserted = true;
 141   }
 142
 143   return Inserted;
 144 }
 145
 146 /// Propagate swifterror values through the machine function CFG.
 147 void SwiftErrorValueTracking::propagateVRegs() {
 148   if (!TLI->supportSwiftError())
 149     return;
 150
 151   // We only need to do this when we have swifterror parameter or swifterror
 152   // alloc.
 153   if (SwiftErrorVals.empty())
 154     return;
 155
 156   // For each machine basic block in reverse post order.
 157   ReversePostOrderTraversal<MachineFunction *> RPOT(MF);
 158   for (MachineBasicBlock *MBB : RPOT) {
 159     // For each swifterror value in the function.
 160     for (const auto *SwiftErrorVal : SwiftErrorVals) {
 161       auto Key = std::make_pair(MBB, SwiftErrorVal);
 162       auto UUseIt = VRegUpwardsUse.find(Key);
 163       auto VRegDefIt = VRegDefMap.find(Key);
 164       bool UpwardsUse = UUseIt != VRegUpwardsUse.end();
 165       Register UUseVReg = UpwardsUse ? UUseIt->second : Register();
 166       bool DownwardDef = VRegDefIt != VRegDefMap.end();
 167       assert(!(UpwardsUse && !DownwardDef) &&
 168              "We can't have an upwards use but no downwards def");
 169
 170       // If there is no upwards exposed use and an entry for the swifterror in
 171       // the def map for this value we don't need to do anything: We already
 172       // have a downward def for this basic block.
 173       if (!UpwardsUse && DownwardDef)
 174         continue;
 175
 176       // Otherwise we either have an upwards exposed use vreg that we need to
 177       // materialize or need to forward the downward def from predecessors.
 178
 179       // Check whether we have a single vreg def from all predecessors.
 180       // Otherwise we need a phi.
 181       SmallVector<std::pair<MachineBasicBlock *, Register>, 4> VRegs;
 182       SmallSet<const MachineBasicBlock *, 8> Visited;
 183       for (auto *Pred : MBB->predecessors()) {
 184         if (!Visited.insert(Pred).second)
 185           continue;
 186         VRegs.push_back(std::make_pair(
 187             Pred, getOrCreateVReg(Pred, SwiftErrorVal)));
 188         if (Pred != MBB)
 189           continue;
 190         // We have a self-edge.
 191         // If there was no upwards use in this basic block there is now one: the
 192         // phi needs to use it self.
 193         if (!UpwardsUse) {
 194           UpwardsUse = true;
 195           UUseIt = VRegUpwardsUse.find(Key);
 196           assert(UUseIt != VRegUpwardsUse.end());
 197           UUseVReg = UUseIt->second;
 198         }
 199       }
 200
 201       // We need a phi node if we have more than one predecessor with different
 202       // downward defs.
 203       bool needPHI =
 204           VRegs.size() >= 1 &&
 205           llvm::any_of(
 206               VRegs,
 207               [&](const std::pair<const MachineBasicBlock *, Register> &V)
 208                   -> bool { return V.second != VRegs[0].second; });
 209
 210       // If there is no upwards exposed used and we don't need a phi just
 211       // forward the swifterror vreg from the predecessor(s).
 212       if (!UpwardsUse && !needPHI) {
 213         assert(!VRegs.empty() &&
 214                "No predecessors? The entry block should bail out earlier");
 215         // Just forward the swifterror vreg from the predecessor(s).
 216         setCurrentVReg(MBB, SwiftErrorVal, VRegs[0].second);
 217         continue;
 218       }
 219
 220       auto DLoc = isa<Instruction>(SwiftErrorVal)
 221                       ? cast<Instruction>(SwiftErrorVal)->getDebugLoc()
 222                       : DebugLoc();
 223       const auto *TII = MF->getSubtarget().getInstrInfo();
 224
 225       // If we don't need a phi create a copy to the upward exposed vreg.
 226       if (!needPHI) {
 227         assert(UpwardsUse);
 228         assert(!VRegs.empty() &&
 229                "No predecessors?  Is the Calling Convention correct?");
 230         Register DestReg = UUseVReg;
 231         BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc, TII->get(TargetOpcode::COPY),
 232                 DestReg)
 233             .addReg(VRegs[0].second);
 234         continue;
 235       }
 236
 237       // We need a phi: if there is an upwards exposed use we already have a
 238       // destination virtual register number otherwise we generate a new one.
 239       auto &DL = MF->getDataLayout();
 240       auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
 241       Register PHIVReg =
 242           UpwardsUse ? UUseVReg : MF->getRegInfo().createVirtualRegister(RC);
 243       MachineInstrBuilder PHI =
 244           BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc,
 245                   TII->get(TargetOpcode::PHI), PHIVReg);
 246       for (auto BBRegPair : VRegs) {
 247         PHI.addReg(BBRegPair.second).addMBB(BBRegPair.first);
 248       }
 249
 250       // We did not have a definition in this block before: store the phi's vreg
 251       // as this block downward exposed def.
 252       if (!UpwardsUse)
 253         setCurrentVReg(MBB, SwiftErrorVal, PHIVReg);
 254     }
 255   }
 256 }
 257
 258 void SwiftErrorValueTracking::preassignVRegs(
 259     MachineBasicBlock *MBB, BasicBlock::const_iterator Begin,
 260     BasicBlock::const_iterator End) {
 261   if (!TLI->supportSwiftError() || SwiftErrorVals.empty())
 262     return;
 263
 264   // Iterator over instructions and assign vregs to swifterror defs and uses.
 265   for (auto It = Begin; It != End; ++It) {
 266     if (auto *CB = dyn_cast<CallBase>(&*It)) {
 267       // A call-site with a swifterror argument is both use and def.
 268       const Value *SwiftErrorAddr = nullptr;
 269       for (const auto &Arg : CB->args()) {
 270         if (!Arg->isSwiftError())
 271           continue;
 272         // Use of swifterror.
 273         assert(!SwiftErrorAddr && "Cannot have multiple swifterror arguments");
 274         SwiftErrorAddr = &*Arg;
 275         assert(SwiftErrorAddr->isSwiftError() &&
 276                "Must have a swifterror value argument");
 277         getOrCreateVRegUseAt(&*It, MBB, SwiftErrorAddr);
 278       }
 279       if (!SwiftErrorAddr)
 280         continue;
 281
 282       // Def of swifterror.
 283       getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);
 284
 285       // A load is a use.
 286     } else if (const LoadInst *LI = dyn_cast<const LoadInst>(&*It)) {
 287       const Value *V = LI->getOperand(0);
 288       if (!V->isSwiftError())
 289         continue;
 290
 291       getOrCreateVRegUseAt(LI, MBB, V);
 292
 293       // A store is a def.
 294     } else if (const StoreInst *SI = dyn_cast<const StoreInst>(&*It)) {
 295       const Value *SwiftErrorAddr = SI->getOperand(1);
 296       if (!SwiftErrorAddr->isSwiftError())
 297         continue;
 298
 299       // Def of swifterror.
 300       getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);
 301
 302       // A return in a swiferror returning function is a use.
 303     } else if (const ReturnInst *R = dyn_cast<const ReturnInst>(&*It)) {
 304       const Function *F = R->getParent()->getParent();
 305       if (!F->getAttributes().hasAttrSomewhere(Attribute::SwiftError))
 306         continue;
 307
 308       getOrCreateVRegUseAt(R, MBB, SwiftErrorArg);
 309     }
 310   }
 311 }