1 files changed, 25 insertions, 3 deletions
diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 10b74f5..2d9ada5 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -414,6 +414,31 @@ bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
   if (!UseI.isCopy())
     return false;
 
+  const TargetRegisterClass *CopySrcRC =
+      TRI->getMinimalPhysRegClass(CopySrcReg);
+  const TargetRegisterClass *UseDstRC =
+      TRI->getMinimalPhysRegClass(UseI.getOperand(0).getReg());
+  const TargetRegisterClass *CrossCopyRC = TRI->getCrossCopyRegClass(CopySrcRC);
+
+  // If cross copy register class is not the same as copy source register class
+  // then it is not possible to copy the register directly and requires a cross
+  // register class copy. Fowarding this copy without checking register class of
+  // UseDst may create additional cross register copies when expanding the copy
+  // instruction in later passes.
+  if (CopySrcRC != CrossCopyRC) {
+    const TargetRegisterClass *CopyDstRC =
+        TRI->getMinimalPhysRegClass(Copy.getOperand(0).getReg());
+
+    // Check if UseDstRC matches the necessary register class to copy from
+    // CopySrc's register class. If so then forwarding the copy will not
+    // introduce any cross-class copys. Else if CopyDstRC matches then keep the
+    // copy and do not forward. If neither UseDstRC or CopyDstRC matches then
+    // we may need a cross register copy later but we do not worry about it
+    // here.
+    if (UseDstRC != CrossCopyRC && CopyDstRC == CrossCopyRC)
+      return false;
+  }
+
   /// COPYs don't have register class constraints, so if the user instruction
   /// is a COPY, we just try to avoid introducing additional cross-class
   /// COPYs.  For example:
@@ -430,9 +455,6 @@ bool MachineCopyPropagation::isForwardableRegClassCopy(const MachineInstr &Copy,
   ///
   /// so we have reduced the number of cross-class COPYs and potentially
   /// introduced a nop COPY that can be removed.
-  const TargetRegisterClass *UseDstRC =
-      TRI->getMinimalPhysRegClass(UseI.getOperand(0).getReg());
-
   const TargetRegisterClass *SuperRC = UseDstRC;
   for (TargetRegisterClass::sc_iterator SuperRCI = UseDstRC->getSuperClasses();
        SuperRC; SuperRC = *SuperRCI++)