GlobalISel: Use a callback to compute constrained reg class for unallocatble registers

Summary:
constrainOperandRegClass() currently fails if it tries to constrain the
register class of an operand that is defeined with an unallocatable register
class.  This patch resolves this by adding a target callback to compute
register constriants in this case.

This is required by the AMDGPU because many of its instructions have source opreands
defined with the unallocatable register classe VS_32 which is a union of two allocatable
register classes VGPR_32 and SReg_32.

Reviewers: dsanders, aditya_nandakumar

Reviewed By: aditya_nandakumar

Subscribers: rovka, kristof.beyls, tpr, llvm-commits

Differential Revision: https://reviews.llvm.org/D45991

llvm-svn: 331485

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -995,6 +995,12 @@ public:
   /// of the set as well.
   bool checkAllSuperRegsMarked(const BitVector &RegisterSet,
       ArrayRef<MCPhysReg> Exceptions = ArrayRef<MCPhysReg>()) const;
+
+  virtual const TargetRegisterClass *
+  getConstrainedRegClassForOperand(const MachineOperand &MO,
+                                   const MachineRegisterInfo &MRI) const {
+    return nullptr;
+  }
 };
 
 //===----------------------------------------------------------------------===//

llvm/lib/CodeGen/GlobalISel/Utils.cpp

@@ -58,6 +58,13 @@ unsigned llvm::constrainOperandRegClass(
   // register class constraints on some of their operands: If it's a use, we can
   // skip constraining as the instruction defining the register would constrain
   // it.
+
+  // We can't constrain unallocatable register classes, because we can't create
+  // virtual registers for these classes, so we need to let targets handled this
+  // case.
+  if (RegClass && !RegClass->isAllocatable())
+    RegClass = TRI.getConstrainedRegClassForOperand(RegMO, MRI);
+
   if (!RegClass) {
     assert((!isTargetSpecificOpcode(II.getOpcode()) || RegMO.isUse()) &&
            "Register class constraint is required unless either the "