[RISCV][MoveMerge] Don't copy kill flag when moving past an instruction that reads the register.

If we're moving the second copy before another instruction that
reads the copied register, we need to clear the kill flag on the
combined move.

Fixes #153598.

Author: topperc

llvm/lib/Target/RISCV/RISCVMoveMerger.cpp

@@ -137,6 +137,11 @@ RISCVMoveMerge::mergePairedInsns(MachineBasicBlock::iterator I,
     NextI = next_nodbg(NextI, E);
   DebugLoc DL = I->getDebugLoc();
 
+  // Make a copy so we can update the kill flag in the MoveFromAToS case. The
+  // copied operand needs to be scoped outside the if since we make a pointer
+  // to it.
+  MachineOperand PairedSource = *PairedRegs.Source;
+
   // The order of S-reg depends on which instruction holds A0, instead of
   // the order of register pair.
   // e,g.
@@ -147,8 +152,15 @@ RISCVMoveMerge::mergePairedInsns(MachineBasicBlock::iterator I,
   //   mv a1, s1    =>  cm.mva01s s2,s1
   bool StartWithX10 = ARegInFirstPair == RISCV::X10;
   if (isMoveFromAToS(Opcode)) {
-    Sreg1 = StartWithX10 ? FirstPair.Source : PairedRegs.Source;
-    Sreg2 = StartWithX10 ? PairedRegs.Source : FirstPair.Source;
+    // We are moving one of the copies earlier so its kill flag may become
+    // invalid. Clear the copied kill flag if there are any reads of the
+    // register between the new location and the old location.
+    for (auto It = std::next(I); It != Paired && PairedSource.isKill(); ++It)
+      if (It->readsRegister(PairedSource.getReg(), TRI))
+        PairedSource.setIsKill(false);
+
+    Sreg1 = StartWithX10 ? FirstPair.Source : &PairedSource;
+    Sreg2 = StartWithX10 ? &PairedSource : FirstPair.Source;
   } else {
     Sreg1 = StartWithX10 ? FirstPair.Destination : PairedRegs.Destination;
     Sreg2 = StartWithX10 ? PairedRegs.Destination : FirstPair.Destination;