third_party/llvm-10.0/llvm/lib/CodeGen/SwiftErrorValueTracking.cpp - SwiftShader - Git at Google

 //===-- SwiftErrorValueTracking.cpp --------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This implements a limited mem2reg-like analysis to promote uses of function
 // arguments and allocas marked with swiftalloc from memory into virtual
 // registers tracked by this class.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/SwiftErrorValueTracking.h"
 #include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetLowering.h"
 #include "llvm/IR/Value.h"

 using namespace llvm;

 Register SwiftErrorValueTracking::getOrCreateVReg(const MachineBasicBlock *MBB,
                                                   const Value *Val) {
   auto Key = std::make_pair(MBB, Val);
   auto It = VRegDefMap.find(Key);
   // If this is the first use of this swifterror value in this basic block,
   // create a new virtual register.
   // After we processed all basic blocks we will satisfy this "upwards exposed
   // use" by inserting a copy or phi at the beginning of this block.
   if (It == VRegDefMap.end()) {
     auto &DL = MF->getDataLayout();
     const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
     auto VReg = MF->getRegInfo().createVirtualRegister(RC);
     VRegDefMap[Key] = VReg;
     VRegUpwardsUse[Key] = VReg;
     return VReg;
   } else
     return It->second;
 }

 void SwiftErrorValueTracking::setCurrentVReg(const MachineBasicBlock *MBB,
                                              const Value *Val, Register VReg) {
   VRegDefMap[std::make_pair(MBB, Val)] = VReg;
 }

 Register SwiftErrorValueTracking::getOrCreateVRegDefAt(
     const Instruction *I, const MachineBasicBlock *MBB, const Value *Val) {
   auto Key = PointerIntPair<const Instruction *, 1, bool>(I, true);
   auto It = VRegDefUses.find(Key);
   if (It != VRegDefUses.end())
     return It->second;

   auto &DL = MF->getDataLayout();
   const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
   Register VReg = MF->getRegInfo().createVirtualRegister(RC);
   VRegDefUses[Key] = VReg;
   setCurrentVReg(MBB, Val, VReg);
   return VReg;
 }

 Register SwiftErrorValueTracking::getOrCreateVRegUseAt(
     const Instruction *I, const MachineBasicBlock *MBB, const Value *Val) {
   auto Key = PointerIntPair<const Instruction *, 1, bool>(I, false);
   auto It = VRegDefUses.find(Key);
   if (It != VRegDefUses.end())
     return It->second;

   Register VReg = getOrCreateVReg(MBB, Val);
   VRegDefUses[Key] = VReg;
   return VReg;
 }

 /// Set up SwiftErrorVals by going through the function. If the function has
 /// swifterror argument, it will be the first entry.
 void SwiftErrorValueTracking::setFunction(MachineFunction &mf) {
   MF = &mf;
   Fn = &MF->getFunction();
   TLI = MF->getSubtarget().getTargetLowering();
   TII = MF->getSubtarget().getInstrInfo();

   if (!TLI->supportSwiftError())
     return;

   SwiftErrorVals.clear();
   VRegDefMap.clear();
   VRegUpwardsUse.clear();
   VRegDefUses.clear();
   SwiftErrorArg = nullptr;

   // Check if function has a swifterror argument.
   bool HaveSeenSwiftErrorArg = false;
   for (Function::const_arg_iterator AI = Fn->arg_begin(), AE = Fn->arg_end();
        AI != AE; ++AI)
     if (AI->hasSwiftErrorAttr()) {
       assert(!HaveSeenSwiftErrorArg &&
              "Must have only one swifterror parameter");
       (void)HaveSeenSwiftErrorArg; // silence warning.
       HaveSeenSwiftErrorArg = true;
       SwiftErrorArg = &*AI;
       SwiftErrorVals.push_back(&*AI);
     }

   for (const auto &LLVMBB : *Fn)
     for (const auto &Inst : LLVMBB) {
       if (const AllocaInst *Alloca = dyn_cast<AllocaInst>(&Inst))
         if (Alloca->isSwiftError())
           SwiftErrorVals.push_back(Alloca);
     }
 }

 bool SwiftErrorValueTracking::createEntriesInEntryBlock(DebugLoc DbgLoc) {
   if (!TLI->supportSwiftError())
     return false;

   // We only need to do this when we have swifterror parameter or swifterror
   // alloc.
   if (SwiftErrorVals.empty())
     return false;

   MachineBasicBlock *MBB = &*MF->begin();
   auto &DL = MF->getDataLayout();
   auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
   bool Inserted = false;
   for (const auto *SwiftErrorVal : SwiftErrorVals) {
     // We will always generate a copy from the argument. It is always used at
     // least by the 'return' of the swifterror.
     if (SwiftErrorArg && SwiftErrorArg == SwiftErrorVal)
       continue;
     Register VReg = MF->getRegInfo().createVirtualRegister(RC);
     // Assign Undef to Vreg. We construct MI directly to make sure it works
     // with FastISel.
     BuildMI(*MBB, MBB->getFirstNonPHI(), DbgLoc,
             TII->get(TargetOpcode::IMPLICIT_DEF), VReg);

     setCurrentVReg(MBB, SwiftErrorVal, VReg);
     Inserted = true;
   }

   return Inserted;
 }

 /// Propagate swifterror values through the machine function CFG.
 void SwiftErrorValueTracking::propagateVRegs() {
   if (!TLI->supportSwiftError())
     return;

   // We only need to do this when we have swifterror parameter or swifterror
   // alloc.
   if (SwiftErrorVals.empty())
     return;

   // For each machine basic block in reverse post order.
   ReversePostOrderTraversal<MachineFunction *> RPOT(MF);
   for (MachineBasicBlock *MBB : RPOT) {
     // For each swifterror value in the function.
     for (const auto *SwiftErrorVal : SwiftErrorVals) {
       auto Key = std::make_pair(MBB, SwiftErrorVal);
       auto UUseIt = VRegUpwardsUse.find(Key);
       auto VRegDefIt = VRegDefMap.find(Key);
       bool UpwardsUse = UUseIt != VRegUpwardsUse.end();
       Register UUseVReg = UpwardsUse ? UUseIt->second : Register();
       bool DownwardDef = VRegDefIt != VRegDefMap.end();
       assert(!(UpwardsUse && !DownwardDef) &&
              "We can't have an upwards use but no downwards def");

       // If there is no upwards exposed use and an entry for the swifterror in
       // the def map for this value we don't need to do anything: We already
       // have a downward def for this basic block.
       if (!UpwardsUse && DownwardDef)
         continue;

       // Otherwise we either have an upwards exposed use vreg that we need to
       // materialize or need to forward the downward def from predecessors.

       // Check whether we have a single vreg def from all predecessors.
       // Otherwise we need a phi.
       SmallVector<std::pair<MachineBasicBlock *, Register>, 4> VRegs;
       SmallSet<const MachineBasicBlock *, 8> Visited;
       for (auto *Pred : MBB->predecessors()) {
         if (!Visited.insert(Pred).second)
           continue;
         VRegs.push_back(std::make_pair(
             Pred, getOrCreateVReg(Pred, SwiftErrorVal)));
         if (Pred != MBB)
           continue;
         // We have a self-edge.
         // If there was no upwards use in this basic block there is now one: the
         // phi needs to use it self.
         if (!UpwardsUse) {
           UpwardsUse = true;
           UUseIt = VRegUpwardsUse.find(Key);
           assert(UUseIt != VRegUpwardsUse.end());
           UUseVReg = UUseIt->second;
         }
       }

       // We need a phi node if we have more than one predecessor with different
       // downward defs.
       bool needPHI =
           VRegs.size() >= 1 &&
           std::find_if(
               VRegs.begin(), VRegs.end(),
               [&](const std::pair<const MachineBasicBlock *, Register> &V)
                   -> bool { return V.second != VRegs[0].second; }) !=
               VRegs.end();

       // If there is no upwards exposed used and we don't need a phi just
       // forward the swifterror vreg from the predecessor(s).
       if (!UpwardsUse && !needPHI) {
         assert(!VRegs.empty() &&
                "No predecessors? The entry block should bail out earlier");
         // Just forward the swifterror vreg from the predecessor(s).
         setCurrentVReg(MBB, SwiftErrorVal, VRegs[0].second);
         continue;
       }

       auto DLoc = isa<Instruction>(SwiftErrorVal)
                       ? cast<Instruction>(SwiftErrorVal)->getDebugLoc()
                       : DebugLoc();
       const auto *TII = MF->getSubtarget().getInstrInfo();

       // If we don't need a phi create a copy to the upward exposed vreg.
       if (!needPHI) {
         assert(UpwardsUse);
         assert(!VRegs.empty() &&
                "No predecessors?  Is the Calling Convention correct?");
         Register DestReg = UUseVReg;
         BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc, TII->get(TargetOpcode::COPY),
                 DestReg)
             .addReg(VRegs[0].second);
         continue;
       }

       // We need a phi: if there is an upwards exposed use we already have a
       // destination virtual register number otherwise we generate a new one.
       auto &DL = MF->getDataLayout();
       auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
       Register PHIVReg =
           UpwardsUse ? UUseVReg : MF->getRegInfo().createVirtualRegister(RC);
       MachineInstrBuilder PHI =
           BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc,
                   TII->get(TargetOpcode::PHI), PHIVReg);
       for (auto BBRegPair : VRegs) {
         PHI.addReg(BBRegPair.second).addMBB(BBRegPair.first);
       }

       // We did not have a definition in this block before: store the phi's vreg
       // as this block downward exposed def.
       if (!UpwardsUse)
         setCurrentVReg(MBB, SwiftErrorVal, PHIVReg);
     }
   }
 }

 void SwiftErrorValueTracking::preassignVRegs(
     MachineBasicBlock *MBB, BasicBlock::const_iterator Begin,
     BasicBlock::const_iterator End) {
   if (!TLI->supportSwiftError() || SwiftErrorVals.empty())
     return;

   // Iterator over instructions and assign vregs to swifterror defs and uses.
   for (auto It = Begin; It != End; ++It) {
     ImmutableCallSite CS(&*It);
     if (CS) {
       // A call-site with a swifterror argument is both use and def.
       const Value *SwiftErrorAddr = nullptr;
       for (auto &Arg : CS.args()) {
         if (!Arg->isSwiftError())
           continue;
         // Use of swifterror.
         assert(!SwiftErrorAddr && "Cannot have multiple swifterror arguments");
         SwiftErrorAddr = &*Arg;
         assert(SwiftErrorAddr->isSwiftError() &&
                "Must have a swifterror value argument");
         getOrCreateVRegUseAt(&*It, MBB, SwiftErrorAddr);
       }
       if (!SwiftErrorAddr)
         continue;

       // Def of swifterror.
       getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);

       // A load is a use.
     } else if (const LoadInst *LI = dyn_cast<const LoadInst>(&*It)) {
       const Value *V = LI->getOperand(0);
       if (!V->isSwiftError())
         continue;

       getOrCreateVRegUseAt(LI, MBB, V);

       // A store is a def.
     } else if (const StoreInst *SI = dyn_cast<const StoreInst>(&*It)) {
       const Value *SwiftErrorAddr = SI->getOperand(1);
       if (!SwiftErrorAddr->isSwiftError())
         continue;

       // Def of swifterror.
       getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);

       // A return in a swiferror returning function is a use.
     } else if (const ReturnInst *R = dyn_cast<const ReturnInst>(&*It)) {
       const Function *F = R->getParent()->getParent();
       if (!F->getAttributes().hasAttrSomewhere(Attribute::SwiftError))
         continue;

       getOrCreateVRegUseAt(R, MBB, SwiftErrorArg);
     }
   }
 }
	//===-- SwiftErrorValueTracking.cpp --------------------------------------===//
	//
	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
	// See https://llvm.org/LICENSE.txt for license information.
	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
	//
	//===----------------------------------------------------------------------===//
	//
	// This implements a limited mem2reg-like analysis to promote uses of function
	// arguments and allocas marked with swiftalloc from memory into virtual
	// registers tracked by this class.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/SwiftErrorValueTracking.h"
	#include "llvm/ADT/PostOrderIterator.h"
	#include "llvm/ADT/SmallSet.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/CodeGen/TargetInstrInfo.h"
	#include "llvm/CodeGen/TargetLowering.h"
	#include "llvm/IR/Value.h"

	using namespace llvm;

	Register SwiftErrorValueTracking::getOrCreateVReg(const MachineBasicBlock *MBB,
	const Value *Val) {
	auto Key = std::make_pair(MBB, Val);
	auto It = VRegDefMap.find(Key);
	// If this is the first use of this swifterror value in this basic block,
	// create a new virtual register.
	// After we processed all basic blocks we will satisfy this "upwards exposed
	// use" by inserting a copy or phi at the beginning of this block.
	if (It == VRegDefMap.end()) {
	auto &DL = MF->getDataLayout();
	const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
	auto VReg = MF->getRegInfo().createVirtualRegister(RC);
	VRegDefMap[Key] = VReg;
	VRegUpwardsUse[Key] = VReg;
	return VReg;
	} else
	return It->second;
	}

	void SwiftErrorValueTracking::setCurrentVReg(const MachineBasicBlock *MBB,
	const Value *Val, Register VReg) {
	VRegDefMap[std::make_pair(MBB, Val)] = VReg;
	}

	Register SwiftErrorValueTracking::getOrCreateVRegDefAt(
	const Instruction I, const MachineBasicBlock MBB, const Value *Val) {
	auto Key = PointerIntPair<const Instruction *, 1, bool>(I, true);
	auto It = VRegDefUses.find(Key);
	if (It != VRegDefUses.end())
	return It->second;

	auto &DL = MF->getDataLayout();
	const TargetRegisterClass *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
	Register VReg = MF->getRegInfo().createVirtualRegister(RC);
	VRegDefUses[Key] = VReg;
	setCurrentVReg(MBB, Val, VReg);
	return VReg;
	}

	Register SwiftErrorValueTracking::getOrCreateVRegUseAt(
	const Instruction I, const MachineBasicBlock MBB, const Value *Val) {
	auto Key = PointerIntPair<const Instruction *, 1, bool>(I, false);
	auto It = VRegDefUses.find(Key);
	if (It != VRegDefUses.end())
	return It->second;

	Register VReg = getOrCreateVReg(MBB, Val);
	VRegDefUses[Key] = VReg;
	return VReg;
	}

	/// Set up SwiftErrorVals by going through the function. If the function has
	/// swifterror argument, it will be the first entry.
	void SwiftErrorValueTracking::setFunction(MachineFunction &mf) {
	MF = &mf;
	Fn = &MF->getFunction();
	TLI = MF->getSubtarget().getTargetLowering();
	TII = MF->getSubtarget().getInstrInfo();

	if (!TLI->supportSwiftError())
	return;

	SwiftErrorVals.clear();
	VRegDefMap.clear();
	VRegUpwardsUse.clear();
	VRegDefUses.clear();
	SwiftErrorArg = nullptr;

	// Check if function has a swifterror argument.
	bool HaveSeenSwiftErrorArg = false;
	for (Function::const_arg_iterator AI = Fn->arg_begin(), AE = Fn->arg_end();
	AI != AE; ++AI)
	if (AI->hasSwiftErrorAttr()) {
	assert(!HaveSeenSwiftErrorArg &&
	"Must have only one swifterror parameter");
	(void)HaveSeenSwiftErrorArg; // silence warning.
	HaveSeenSwiftErrorArg = true;
	SwiftErrorArg = &*AI;
	SwiftErrorVals.push_back(&*AI);
	}

	for (const auto &LLVMBB : *Fn)
	for (const auto &Inst : LLVMBB) {
	if (const AllocaInst *Alloca = dyn_cast<AllocaInst>(&Inst))
	if (Alloca->isSwiftError())
	SwiftErrorVals.push_back(Alloca);
	}
	}

	bool SwiftErrorValueTracking::createEntriesInEntryBlock(DebugLoc DbgLoc) {
	if (!TLI->supportSwiftError())
	return false;

	// We only need to do this when we have swifterror parameter or swifterror
	// alloc.
	if (SwiftErrorVals.empty())
	return false;

	MachineBasicBlock MBB = &MF->begin();
	auto &DL = MF->getDataLayout();
	auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
	bool Inserted = false;
	for (const auto *SwiftErrorVal : SwiftErrorVals) {
	// We will always generate a copy from the argument. It is always used at
	// least by the 'return' of the swifterror.
	if (SwiftErrorArg && SwiftErrorArg == SwiftErrorVal)
	continue;
	Register VReg = MF->getRegInfo().createVirtualRegister(RC);
	// Assign Undef to Vreg. We construct MI directly to make sure it works
	// with FastISel.
	BuildMI(*MBB, MBB->getFirstNonPHI(), DbgLoc,
	TII->get(TargetOpcode::IMPLICIT_DEF), VReg);

	setCurrentVReg(MBB, SwiftErrorVal, VReg);
	Inserted = true;
	}

	return Inserted;
	}

	/// Propagate swifterror values through the machine function CFG.
	void SwiftErrorValueTracking::propagateVRegs() {
	if (!TLI->supportSwiftError())
	return;

	// We only need to do this when we have swifterror parameter or swifterror
	// alloc.
	if (SwiftErrorVals.empty())
	return;

	// For each machine basic block in reverse post order.
	ReversePostOrderTraversal<MachineFunction *> RPOT(MF);
	for (MachineBasicBlock *MBB : RPOT) {
	// For each swifterror value in the function.
	for (const auto *SwiftErrorVal : SwiftErrorVals) {
	auto Key = std::make_pair(MBB, SwiftErrorVal);
	auto UUseIt = VRegUpwardsUse.find(Key);
	auto VRegDefIt = VRegDefMap.find(Key);
	bool UpwardsUse = UUseIt != VRegUpwardsUse.end();
	Register UUseVReg = UpwardsUse ? UUseIt->second : Register();
	bool DownwardDef = VRegDefIt != VRegDefMap.end();
	assert(!(UpwardsUse && !DownwardDef) &&
	"We can't have an upwards use but no downwards def");

	// If there is no upwards exposed use and an entry for the swifterror in
	// the def map for this value we don't need to do anything: We already
	// have a downward def for this basic block.
	if (!UpwardsUse && DownwardDef)
	continue;

	// Otherwise we either have an upwards exposed use vreg that we need to
	// materialize or need to forward the downward def from predecessors.

	// Check whether we have a single vreg def from all predecessors.
	// Otherwise we need a phi.
	SmallVector<std::pair<MachineBasicBlock *, Register>, 4> VRegs;
	SmallSet<const MachineBasicBlock *, 8> Visited;
	for (auto *Pred : MBB->predecessors()) {
	if (!Visited.insert(Pred).second)
	continue;
	VRegs.push_back(std::make_pair(
	Pred, getOrCreateVReg(Pred, SwiftErrorVal)));
	if (Pred != MBB)
	continue;
	// We have a self-edge.
	// If there was no upwards use in this basic block there is now one: the
	// phi needs to use it self.
	if (!UpwardsUse) {
	UpwardsUse = true;
	UUseIt = VRegUpwardsUse.find(Key);
	assert(UUseIt != VRegUpwardsUse.end());
	UUseVReg = UUseIt->second;
	}
	}

	// We need a phi node if we have more than one predecessor with different
	// downward defs.
	bool needPHI =
	VRegs.size() >= 1 &&
	std::find_if(
	VRegs.begin(), VRegs.end(),
	[&](const std::pair<const MachineBasicBlock *, Register> &V)
	-> bool { return V.second != VRegs[0].second; }) !=
	VRegs.end();

	// If there is no upwards exposed used and we don't need a phi just
	// forward the swifterror vreg from the predecessor(s).
	if (!UpwardsUse && !needPHI) {
	assert(!VRegs.empty() &&
	"No predecessors? The entry block should bail out earlier");
	// Just forward the swifterror vreg from the predecessor(s).
	setCurrentVReg(MBB, SwiftErrorVal, VRegs[0].second);
	continue;
	}

	auto DLoc = isa<Instruction>(SwiftErrorVal)
	? cast<Instruction>(SwiftErrorVal)->getDebugLoc()
	: DebugLoc();
	const auto *TII = MF->getSubtarget().getInstrInfo();

	// If we don't need a phi create a copy to the upward exposed vreg.
	if (!needPHI) {
	assert(UpwardsUse);
	assert(!VRegs.empty() &&
	"No predecessors? Is the Calling Convention correct?");
	Register DestReg = UUseVReg;
	BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc, TII->get(TargetOpcode::COPY),
	DestReg)
	.addReg(VRegs[0].second);
	continue;
	}

	// We need a phi: if there is an upwards exposed use we already have a
	// destination virtual register number otherwise we generate a new one.
	auto &DL = MF->getDataLayout();
	auto const *RC = TLI->getRegClassFor(TLI->getPointerTy(DL));
	Register PHIVReg =
	UpwardsUse ? UUseVReg : MF->getRegInfo().createVirtualRegister(RC);
	MachineInstrBuilder PHI =
	BuildMI(*MBB, MBB->getFirstNonPHI(), DLoc,
	TII->get(TargetOpcode::PHI), PHIVReg);
	for (auto BBRegPair : VRegs) {
	PHI.addReg(BBRegPair.second).addMBB(BBRegPair.first);
	}

	// We did not have a definition in this block before: store the phi's vreg
	// as this block downward exposed def.
	if (!UpwardsUse)
	setCurrentVReg(MBB, SwiftErrorVal, PHIVReg);
	}
	}
	}

	void SwiftErrorValueTracking::preassignVRegs(
	MachineBasicBlock *MBB, BasicBlock::const_iterator Begin,
	BasicBlock::const_iterator End) {
	if (!TLI->supportSwiftError() \|\| SwiftErrorVals.empty())
	return;

	// Iterator over instructions and assign vregs to swifterror defs and uses.
	for (auto It = Begin; It != End; ++It) {
	ImmutableCallSite CS(&*It);
	if (CS) {
	// A call-site with a swifterror argument is both use and def.
	const Value *SwiftErrorAddr = nullptr;
	for (auto &Arg : CS.args()) {
	if (!Arg->isSwiftError())
	continue;
	// Use of swifterror.
	assert(!SwiftErrorAddr && "Cannot have multiple swifterror arguments");
	SwiftErrorAddr = &*Arg;
	assert(SwiftErrorAddr->isSwiftError() &&
	"Must have a swifterror value argument");
	getOrCreateVRegUseAt(&*It, MBB, SwiftErrorAddr);
	}
	if (!SwiftErrorAddr)
	continue;

	// Def of swifterror.
	getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);

	// A load is a use.
	} else if (const LoadInst LI = dyn_cast<const LoadInst>(&It)) {
	const Value *V = LI->getOperand(0);
	if (!V->isSwiftError())
	continue;

	getOrCreateVRegUseAt(LI, MBB, V);

	// A store is a def.
	} else if (const StoreInst SI = dyn_cast<const StoreInst>(&It)) {
	const Value *SwiftErrorAddr = SI->getOperand(1);
	if (!SwiftErrorAddr->isSwiftError())
	continue;

	// Def of swifterror.
	getOrCreateVRegDefAt(&*It, MBB, SwiftErrorAddr);

	// A return in a swiferror returning function is a use.
	} else if (const ReturnInst R = dyn_cast<const ReturnInst>(&It)) {
	const Function *F = R->getParent()->getParent();
	if (!F->getAttributes().hasAttrSomewhere(Attribute::SwiftError))
	continue;

	getOrCreateVRegUseAt(R, MBB, SwiftErrorArg);
	}
	}
	}