RegisterCoalescer: Improve a comment.

Explain the relation of the example to the variables in the code,
explain what bad behaviour the code avoids in this case.

llvm-svn: 237706

1 files changed, 5 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index c2d1620c552..9c3d075ff0f 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -955,12 +955,11 @@ bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
   TII->reMaterialize(*MBB, MII, DstReg, SrcIdx, DefMI, *TRI);
   MachineInstr *NewMI = std::prev(MII);
 
-  // A situation like the following:
-  //     %vreg0:subX = instr           ; DefMI
-  //     %vregY      = copy %vreg:subX ; CopyMI
-  // does not need subregisters/regclass widening after rematerialization, just
-  // do:
-  //     %vregY = instr
+  // In a situation like the following:
+  //     %vreg0:subreg = instr              ; DefMI, subreg = DstIdx
+  //     %vreg1        = copy %vreg0:subreg ; CopyMI, SrcIdx = 0
+  // instead of widening %vreg1 to the register class of %vreg0 simply do:
+  //     %vreg1 = instr
   const TargetRegisterClass *NewRC = CP.getNewRC();
   if (DstIdx != 0) {
     MachineOperand &DefMO = NewMI->getOperand(0);