RegAllocGreedy: Fix crash when setting CostPerUse

[arsenm]

Summary

When CostPerUse is non-zero and tryAssign was attempting
to assign an undef VirtReg, the empty VirtReg LiveInterval would
assert in intervalIsInOneMBB.

This happens when I attempt to make the cost of all registers
except VCC have a CostPerUse of 1 on AMDGPU.

I'm not sure I will ever commit that change, because I'm not sure
it's exactly what I want. This results in overly aggressive
VCC usage instead of the relatively few operands where it is
advantageous to use it.

The testcase is the simplest case I've found where this happens
from a branch on undef, but only with the CopyCost change applied.

[qcolombet]

Hi Matt,

I am not sure I understand how we end up in this situation.
How can we try to select a register for an empty live-range. Those cases are supposed to be sorted out before we try selectOrSplit.

Could you describe how we end up in this situation?

Thanks,
-Quentin

[qcolombet]

Hi Matt,

Is this change still relevant?

If not, could you close it?

Thanks,
-Quentin

[arsenm]

Closing for now. I'll reopen if I hit this again when trying to improve vcc usage

lib/CodeGen/RegAllocGreedy.cpp

 ///                when returning true.
 /// @returns True when interference can be evicted cheaper than MaxCost.
 bool RAGreedy::canEvictInterference(LiveInterval &VirtReg, unsigned PhysReg,
                                     bool IsHint, EvictionCost &MaxCost) {
   // It is only possible to evict virtual register interference.
   if (Matrix->checkInterference(VirtReg, PhysReg) > LiveRegMatrix::IK_VirtReg)
     return false;
 
-  bool IsLocal = LIS->intervalIsInOneMBB(VirtReg);
+  bool IsLocal = VirtReg.empty() || LIS->intervalIsInOneMBB(VirtReg);
 
   // Find VirtReg's cascade number. This will be unassigned if VirtReg was never
   // involved in an eviction before. If a cascade number was assigned, deny
   // evicting anything with the same or a newer cascade number. This prevents
   // infinite eviction loops.
   //
   // This works out so a register without a cascade number is allowed to evict
   // anything, and it can be evicted by anything.

test/CodeGen/AMDGPU/branch-on-undef.ll

+; RUN: llc -march=amdgcn -verify-machineinstrs < %s
+
+define void @undef_br(i32 addrspace(1)* %ptr) nounwind {
+entry:
+  br i1 undef, label %exit, label %bb
+
+bb:
+  store volatile i32 0, i32 addrspace(1)* %ptr
+  br label %exit
+
+exit:
+  ret void
+}
+