Optimize:

  r1025 = s/zext r1024, 4
  r1026 = extract_subreg r1025, 4
to:
  r1026 = copy r1024

llvm-svn: 122925

1 files changed, 39 insertions, 23 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
index 51cbb7f29b4..977cd63844b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/InstrEmitter.cpp
@@ -428,31 +428,47 @@ void InstrEmitter::EmitSubregNode(SDNode *Node,
 
     // Figure out the register class to create for the destreg.
     unsigned VReg = getVR(Node->getOperand(0), VRBaseMap);
-    const TargetRegisterClass *TRC = MRI->getRegClass(VReg);
-    const TargetRegisterClass *SRC = TRC->getSubRegisterRegClass(SubIdx);
-    assert(SRC && "Invalid subregister index in EXTRACT_SUBREG");
-
-    // Figure out the register class to create for the destreg.
-    // Note that if we're going to directly use an existing register,
-    // it must be precisely the required class, and not a subclass
-    // thereof.
-    if (VRBase == 0 || SRC != MRI->getRegClass(VRBase)) {
-      // Create the reg
-      assert(SRC && "Couldn't find source register class");
-      VRBase = MRI->createVirtualRegister(SRC);
-    }
+    MachineInstr *DefMI = MRI->getVRegDef(VReg);
+    unsigned SrcReg, DstReg, DefSubIdx;
+    if (DefMI &&
+        TII->isCoalescableExtInstr(*DefMI, SrcReg, DstReg, DefSubIdx) &&
+        SubIdx == DefSubIdx) {
+      // Optimize these:
+      // r1025 = s/zext r1024, 4
+      // r1026 = extract_subreg r1025, 4
+      // to a copy
+      // r1026 = copy r1024
+      const TargetRegisterClass *TRC = MRI->getRegClass(SrcReg);
+      VRBase = MRI->createVirtualRegister(TRC);
+      BuildMI(*MBB, InsertPos, Node->getDebugLoc(),
+              TII->get(TargetOpcode::COPY), VRBase).addReg(SrcReg);
+    } else {
+      const TargetRegisterClass *TRC = MRI->getRegClass(VReg);
+      const TargetRegisterClass *SRC = TRC->getSubRegisterRegClass(SubIdx);
+      assert(SRC && "Invalid subregister index in EXTRACT_SUBREG");
+
+      // Figure out the register class to create for the destreg.
+      // Note that if we're going to directly use an existing register,
+      // it must be precisely the required class, and not a subclass
+      // thereof.
+      if (VRBase == 0 || SRC != MRI->getRegClass(VRBase)) {
+        // Create the reg
+        assert(SRC && "Couldn't find source register class");
+        VRBase = MRI->createVirtualRegister(SRC);
+      }
 
-    // Create the extract_subreg machine instruction.
-    MachineInstr *MI = BuildMI(*MF, Node->getDebugLoc(),
-                               TII->get(TargetOpcode::COPY), VRBase);
+      // Create the extract_subreg machine instruction.
+      MachineInstr *MI = BuildMI(*MF, Node->getDebugLoc(),
+                                 TII->get(TargetOpcode::COPY), VRBase);
 
-    // Add source, and subreg index
-    AddOperand(MI, Node->getOperand(0), 0, 0, VRBaseMap, /*IsDebug=*/false,
-               IsClone, IsCloned);
-    assert(TargetRegisterInfo::isVirtualRegister(MI->getOperand(1).getReg()) &&
-           "Cannot yet extract from physregs");
-    MI->getOperand(1).setSubReg(SubIdx);
-    MBB->insert(InsertPos, MI);
+      // Add source, and subreg index
+      AddOperand(MI, Node->getOperand(0), 0, 0, VRBaseMap, /*IsDebug=*/false,
+                 IsClone, IsCloned);
+      assert(TargetRegisterInfo::isVirtualRegister(MI->getOperand(1).getReg())&&
+             "Cannot yet extract from physregs");
+      MI->getOperand(1).setSubReg(SubIdx);
+      MBB->insert(InsertPos, MI);
+    }
   } else if (Opc == TargetOpcode::INSERT_SUBREG ||
              Opc == TargetOpcode::SUBREG_TO_REG) {
     SDValue N0 = Node->getOperand(0);