Merge pull request #4672 from bnbarham/revert-reuse-register

[stable/20220421] Revert "[FastISel] Reuse register for bitcast that does not change MVT"

Author: bnbarham

llvm/lib/CodeGen/SelectionDAG/FastISel.cpp

@@ -1406,6 +1406,16 @@ 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 ||
@@ -1419,14 +1429,18 @@ bool FastISel::selectBitCast(const User *I) {
   if (!Op0) // Unhandled operand. Halt "fast" selection and bail.
     return false;
 
-  // If the bitcast doesn't change the type, just use the operand value.
+  // First, try to perform the bitcast by inserting a reg-reg copy.
+  Register ResultReg;
   if (SrcVT == DstVT) {
-    updateValueMap(I, Op0);
-    return true;
+    ResultReg = createResultReg(TLI.getRegClassFor(DstVT));
+    BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc,
+            TII.get(TargetOpcode::COPY), ResultReg).addReg(Op0);
   }
 
-  // Otherwise, select a BITCAST opcode.
-  Register ResultReg = fastEmit_r(SrcVT, DstVT, ISD::BITCAST, Op0);
+  // If the reg-reg copy failed, select a BITCAST opcode.
+  if (!ResultReg)
+    ResultReg = fastEmit_r(SrcVT, DstVT, ISD::BITCAST, Op0);
+
   if (!ResultReg)
     return false;