RegisterCoalescer: Avoid unnecessary register class widening for some rematerializations

I couldn't provide a testcase as none of the public targets has wide
register classes with alot of subregisters and at the same time an
instruction which "ReMaterializable" and "AsCheapAsAMove" (could
probably be added for R600).

llvm-svn: 235668

1 files changed, 25 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/RegisterCoalescer.cpp b/llvm/lib/CodeGen/RegisterCoalescer.cpp
index 5d958a61e6a..a8fcd950268 100644
--- a/llvm/lib/CodeGen/RegisterCoalescer.cpp
+++ b/llvm/lib/CodeGen/RegisterCoalescer.cpp
@@ -194,7 +194,7 @@ namespace {
 
     /// If the source of a copy is defined by a
     /// trivial computation, replace the copy by rematerialize the definition.
-    bool reMaterializeTrivialDef(CoalescerPair &CP, MachineInstr *CopyMI,
+    bool reMaterializeTrivialDef(const CoalescerPair &CP, MachineInstr *CopyMI,
                                  bool &IsDefCopy);
 
     /// Return true if a copy involving a physreg should be joined.
@@ -851,7 +851,7 @@ static bool definesFullReg(const MachineInstr &MI, unsigned Reg) {
   return false;
 }
 
-bool RegisterCoalescer::reMaterializeTrivialDef(CoalescerPair &CP,
+bool RegisterCoalescer::reMaterializeTrivialDef(const CoalescerPair &CP,
                                                 MachineInstr *CopyMI,
                                                 bool &IsDefCopy) {
   IsDefCopy = false;
@@ -929,6 +929,29 @@ bool RegisterCoalescer::reMaterializeTrivialDef(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
+  const TargetRegisterClass *NewRC = CP.getNewRC();
+  if (DstIdx != 0) {
+    MachineOperand &DefMO = NewMI->getOperand(0);
+    if (DefMO.getSubReg() == DstIdx) {
+      assert(SrcIdx == 0 && CP.isFlipped()
+             && "Shouldn't have SrcIdx+DstIdx at this point");
+      const TargetRegisterClass *DstRC = MRI->getRegClass(DstReg);
+      const TargetRegisterClass *CommonRC =
+        TRI->getCommonSubClass(DefRC, DstRC);
+      if (CommonRC != nullptr) {
+        NewRC = CommonRC;
+        DstIdx = 0;
+        DefMO.setSubReg(0);
+      }
+    }
+  }
+
   LIS->ReplaceMachineInstrInMaps(CopyMI, NewMI);
   CopyMI->eraseFromParent();
   ErasedInstrs.insert(CopyMI);
@@ -948,7 +971,6 @@ bool RegisterCoalescer::reMaterializeTrivialDef(CoalescerPair &CP,
   }
 
   if (TargetRegisterInfo::isVirtualRegister(DstReg)) {
-    const TargetRegisterClass *NewRC = CP.getNewRC();
     unsigned NewIdx = NewMI->getOperand(0).getSubReg();
 
     if (DefRC != nullptr) {