Revert "ReMat: fix overly cavalier attitude to sub-register indices"

Very sorry, this was a premature patch that I still need to investigate and
finish off (for some reason beyond me at the moment it doesn't actually fix the
issue in all cases).

This reverts commit r199091.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199093 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 24 insertions, 4 deletions

diff --git a/lib/CodeGen/RegisterCoalescer.cpp b/lib/CodeGen/RegisterCoalescer.cpp
index f0a4b28306..dd86c1f010 100644
--- a/lib/CodeGen/RegisterCoalescer.cpp
+++ b/lib/CodeGen/RegisterCoalescer.cpp
@@ -816,11 +816,31 @@ bool RegisterCoalescer::reMaterializeTrivialDef(CoalescerPair &CP,
   }
 
   if (TargetRegisterInfo::isVirtualRegister(DstReg)) {
-    MRI->setRegClass(DstReg, CP.getNewRC());
-
     unsigned NewIdx = NewMI->getOperand(0).getSubReg();
-    updateRegDefsUses(DstReg, DstReg, DstIdx);
-    NewMI->getOperand(0).setSubReg(NewIdx);
+    const TargetRegisterClass *RCForInst;
+    if (NewIdx)
+      RCForInst = TRI->getMatchingSuperRegClass(MRI->getRegClass(DstReg), DefRC,
+                                                NewIdx);
+
+    if (MRI->constrainRegClass(DstReg, DefRC)) {
+      // The materialized instruction is quite capable of setting DstReg
+      // directly, but it may still have a now-trivial subregister index which
+      // we should clear.
+      NewMI->getOperand(0).setSubReg(0);
+    } else if (NewIdx && RCForInst) {
+      // The subreg index on NewMI is essential; we still have to make sure
+      // DstReg:idx is in a class that NewMI can use.
+      MRI->constrainRegClass(DstReg, RCForInst);
+    } else {
+      // DstReg is actually incompatible with NewMI, we have to move to a
+      // super-reg's class. This could come from a sequence like:
+      //     GR32 = MOV32r0
+      //     GR8 = COPY GR32:sub_8
+      MRI->setRegClass(DstReg, CP.getNewRC());
+      updateRegDefsUses(DstReg, DstReg, DstIdx);
+      NewMI->getOperand(0).setSubReg(
+          TRI->composeSubRegIndices(SrcIdx, DefMI->getOperand(0).getSubReg()));
+    }
   } else if (NewMI->getOperand(0).getReg() != CopyDstReg) {
     // The New instruction may be defining a sub-register of what's actually
     // been asked for. If so it must implicitly define the whole thing.