[fastregalloc] Fix bug when there is no register class.

[LuoYuanke]

Summary

In global Isel there may be no register class for a virtual register. In
that case we can invoke MRI->getRegClass(). We should invoke
MRI->getRegClassOrNull() instead and check if it returns nullptr.

[DavidSpickett]

I can confirm this fixes the reproducer I posted on https://reviews.llvm.org/D126771.

[arsenm]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

This should not happen. This is a bug in x86 globalisel. Virtual registers without a class should not get past the selector. The verifier should catch this

[LuoYuanke]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

I'm not familiar with globalisel, but there is no harm for this patch. It just trigger the bug of globalisel. Can we land it first to let the lit test pass? We can debug and create another patch for globalisel if there is a bug in it.

[LuoYuanke]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

We can't trigger the bug with -verify-machineinstrs. There may be a bug in handling DBG_VALUE in globalisel.
DBG_VALUE %0:gpr, $noreg, !"argc", !DIExpression(), debug-location !19; foo.c:11:14 line no:11

However I think it is irrelevant to this patch.

[arsenm]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

No, this is a hack. The virtual register here should have been constrained to a real class

[LuoYuanke]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

That's the bug of globalisel, but we need more time to root cause it and fix it. Here we just avoid triggerring the bug and don't block others' work. When globalisel bug is fixed we can change back to getRegClass(). What do you think?

[LuoYuanke]

llvm/lib/CodeGen/RegAllocFast.cpp

307–310

How about add "FIXME" for the patch?

[MatzeB]

When there is a bug producing invalid IR then we better crash than hide it.

[MatzeB]

Are you sure your bot touble actually comes from GlobalIsel? While GIsel is unfinished and likely broken on x86 it also shouldn‘t be enabled anywhere except a few dedicated globalisel tests…

llvm/lib/CodeGen/RegAllocFast.cpp

 
 char RegAllocFast::ID = 0;
 
 INITIALIZE_PASS(RegAllocFast, "regallocfast", "Fast Register Allocator", false,
                 false)
 
 bool RegAllocFast::shouldAllocateRegister(const Register Reg) const {
   assert(Register::isVirtualRegister(Reg));
-  const TargetRegisterClass &RC = *MRI->getRegClass(Reg);
-  return ShouldAllocateClass(*TRI, RC);
+  const TargetRegisterClass *RC = MRI->getRegClassOrNull(Reg);
+  // Follow the default behaviour to return true if there is no
+  // register class for the register. This may happen in gloal ISel.
+  if (!RC)
+    return true;
+  return ShouldAllocateClass(*TRI, *RC);
 }
 
 void RegAllocFast::setPhysRegState(MCPhysReg PhysReg, unsigned NewState) {
   for (MCRegUnitIterator UI(PhysReg, TRI); UI.isValid(); ++UI)
     RegUnitStates[*UI] = NewState;
 }
 
 bool RegAllocFast::isPhysRegFree(MCPhysReg PhysReg) const {

llvm/lib/Target/X86/X86FastPreTileConfig.cpp

bool X86FastPreTileConfig::runOnMachineFunction(MachineFunction &MFunc) {
   TRI = ST->getRegisterInfo();
   CfgSS = -1;
 
   unsigned NumVirtRegs = MRI->getNumVirtRegs();
   // Abandon early if there is no tile register to config.
   bool HasVirtTileReg = false;
   for (unsigned I = 0, E = NumVirtRegs; I != E; ++I) {
     Register VirtReg = Register::index2VirtReg(I);
-    if (MRI->getRegClass(VirtReg)->getID() == X86::TILERegClassID) {
+    const TargetRegisterClass *RC = MRI->getRegClassOrNull(VirtReg);
+    if (!RC)
+      continue;
+    if (RC->getID() == X86::TILERegClassID) {
       HasVirtTileReg = true;
       break;
     }
   }
   if (!HasVirtTileReg)
     return false;
 
   StackSlotForVirtReg.resize(NumVirtRegs);

llvm/test/DebugInfo/Generic/two-cus-from-same-file.ll

 ; For http://llvm.org/bugs/show_bug.cgi?id=12942
 ;   There are two CUs coming from /tmp/foo.c in this module. Make sure it doesn't
 ;   blow llc up and produces something reasonable.
 ;
 
 ; RUN: %llc_dwarf %s -o %t -filetype=obj -O0
 ; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
+; RUN: %llc_dwarf %s -o %t --global-isel -filetype=obj -O0
+; RUN: llvm-dwarfdump -debug-info %t | FileCheck %s
 
 ; ModuleID = 'test.bc'
 
 @str = private unnamed_addr constant [4 x i8] c"FOO\00"
 @str1 = private unnamed_addr constant [6 x i8] c"Main!\00"
 
 define void @foo() nounwind !dbg !5 {
 entry: