[llvm-exegesis] Create a proper LLVM IR Function for MachineFunctions

[aeubanks]

Summary

I have upcoming changes break with invalid Function definitions.

[aidengrossman]

llvm/tools/llvm-exegesis/lib/Assembler.cpp

86–87

I'm thinking we should be able to get rid of this comment and function call now that there will be IR within the function?

[aeubanks]

address comment

[aeubanks]

ping

[aidengrossman]

I'm not really the code owner here (most of my changes have been refactorings), but this is pretty small and I've verified that everything works as expected locally. I'd expect @courbet and @gchatelet to not have any problems with this.

llvm/tools/llvm-exegesis/lib/Assembler.cpp

 #include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/ExecutionEngine/SectionMemoryManager.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Instructions.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/MC/MCInstrInfo.h"
 #include "llvm/Support/Alignment.h"
 #include "llvm/Support/MemoryBuffer.h"
 
 namespace llvm {
 namespace exegesis {
 
MachineFunction &createVoidVoidPtrMachineFunction(StringRef FunctionName,
                                                   MachineModuleInfo *MMI) {
   Type *const ReturnType = Type::getInt32Ty(Module->getContext());
   Type *const MemParamType = PointerType::get(
       Type::getInt8Ty(Module->getContext()), 0 /*default address space*/);
   FunctionType *FunctionType =
       FunctionType::get(ReturnType, {MemParamType}, false);
   Function *const F = Function::Create(
       FunctionType, GlobalValue::InternalLinkage, FunctionName, Module);
-  // Making sure we can create a MachineFunction out of this Function even if it
-  // contains no IR.
-  F->setIsMaterializable(true);
+  BasicBlock *BB = BasicBlock::Create(Module->getContext(), "", F);
+  new UnreachableInst(Module->getContext(), BB);
   return MMI->getOrCreateMachineFunction(*F);
 }
 
 BasicBlockFiller::BasicBlockFiller(MachineFunction &MF, MachineBasicBlock *MBB,
                                    const MCInstrInfo *MCII)
     : MF(MF), MBB(MBB), MCII(MCII) {}
 
 void BasicBlockFiller::addInstruction(const MCInst &Inst, const DebugLoc &DL) {