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[Patryk27]

Whoopsie, I meant to update the already-existing diff - let me close this one.

llvm/lib/Target/AVR/AVRInstrInfo.cpp

if (STI.hasMOVW() && AVR::DREGSMOVWRegClass.contains(DestReg, SrcReg)) {
       BuildMI(MBB, MI, DL, get(AVR::MOVWRdRr), DestReg)
           .addReg(SrcReg, getKillRegState(KillSrc));
     } else {
       Register DestLo, DestHi, SrcLo, SrcHi;
 
       TRI.splitReg(DestReg, DestLo, DestHi);
       TRI.splitReg(SrcReg, SrcLo, SrcHi);
 
-      bool CopyHasOverlappingRegs = DestLo == SrcHi;
-
-      // Usually, for readability & consistency, we emit:
-      //
-      // ```
-      // mov $DestLo, $SrcLo
-      // mov $DestHi, $SrcHi
-      // ```
-      //
-      // ... but if our registers overlap, then we have to switch the order:
-      //
-      // ```
-      // mov $DestHi, $SrcHi
-      // mov $DestLo, $SrcLo
-      // ```
-      //
-      // ... as not to garble the operation.
-      //
-      // E.g. imagine trying to expand `$r25r24 = COPY $r24r23` - doing it the
-      // first way would yield:
-      //
-      // ```
-      // mov r24, r23
-      // mov r25, r24
-      // ```
-      //
-      // ... which would always set both registers to `r23`.
-      if (CopyHasOverlappingRegs) {
+      if (DestLo == SrcHi) {
         BuildMI(MBB, MI, DL, get(AVR::MOVRdRr), DestHi)
             .addReg(SrcHi, getKillRegState(KillSrc));
         BuildMI(MBB, MI, DL, get(AVR::MOVRdRr), DestLo)
             .addReg(SrcLo, getKillRegState(KillSrc));
       } else {
         BuildMI(MBB, MI, DL, get(AVR::MOVRdRr), DestLo)
             .addReg(SrcLo, getKillRegState(KillSrc));
         BuildMI(MBB, MI, DL, get(AVR::MOVRdRr), DestHi)

llvm/test/CodeGen/AVR/pseudo/COPY.mir

body: |
   bb.0.entry:
     liveins: $r25r24
 
     ; CHECK-LABEL: test_copy_nonoverlapping:
     ; CHECK: mov r22, r24
     ; CHECK-NEXT: mov r23, r25
 
     $r23r22 = COPY $r25r24
-    RCALLk @foo, implicit $sp, implicit killed $r24r23
+    RCALLk @foo, implicit $r24r23
 ...
 
 ---
 name: test_copy_overlapping
 tracksRegLiveness: true
 body: |
   bb.0.entry:
     liveins: $r24r23
 
     ; CHECK-LABEL: test_copy_overlapping:
     ; CHECK: mov r25, r24
     ; CHECK-NEXT: mov r24, r23
 
     $r25r24 = COPY $r24r23
-    RCALLk @foo, implicit $sp, implicit killed $r25r24
+    RCALLk @foo, implicit $r25r24
 ...