[FastISel] Reuse register for bitcast that does not change MVT

The current FastISel code reuses the register for a bitcast that
doesn't change the IR type, but uses a reg-to-reg copy if it
changes the IR type without changing the MVT. However, we can
simply reuse the register in that case as well.

In particular, this avoids unnecessary reg-to-reg copies for pointer
bitcasts. This was found while inspecting O0 codegen differences
between typed and opaque pointers.

Differential Revision: https://reviews.llvm.org/D119432

Author: nikic

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1408,16 +1408,6 @@ bool FastISel::selectCast(const User *I, unsigned Opcode) {
 }
 
 bool FastISel::selectBitCast(const User *I) {
-  // If the bitcast doesn't change the type, just use the operand value.
-  if (I->getType() == I->getOperand(0)->getType()) {
-    Register Reg = getRegForValue(I->getOperand(0));
-    if (!Reg)
-      return false;
-    updateValueMap(I, Reg);
-    return true;
-  }
-
-  // Bitcasts of other values become reg-reg copies or BITCAST operators.
   EVT SrcEVT = TLI.getValueType(DL, I->getOperand(0)->getType());
   EVT DstEVT = TLI.getValueType(DL, I->getType());
   if (SrcEVT == MVT::Other || DstEVT == MVT::Other ||
@@ -1431,18 +1421,14 @@ bool FastISel::selectBitCast(const User *I) {
   if (!Op0) // Unhandled operand. Halt "fast" selection and bail.
     return false;
 
-  // First, try to perform the bitcast by inserting a reg-reg copy.
-  Register ResultReg;
+  // If the bitcast doesn't change the type, just use the operand value.
   if (SrcVT == DstVT) {
-    ResultReg = createResultReg(TLI.getRegClassFor(DstVT));
-    BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc,
-            TII.get(TargetOpcode::COPY), ResultReg).addReg(Op0);
+    updateValueMap(I, Op0);
+    return true;
   }
 
-  // If the reg-reg copy failed, select a BITCAST opcode.
-  if (!ResultReg)
-    ResultReg = fastEmit_r(SrcVT, DstVT, ISD::BITCAST, Op0);
-
+  // Otherwise, select a BITCAST opcode.
+  Register ResultReg = fastEmit_r(SrcVT, DstVT, ISD::BITCAST, Op0);
   if (!ResultReg)
     return false;