[llvm] [Thumb1] Resolve FIXME: Use 'mov hi, $src; mov $dst, hi' (PR #81908)

[Tim Northover via llvm-commits]

================
@@ -47,24 +49,63 @@ void Thumb1InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
   assert(ARM::GPRRegClass.contains(DestReg, SrcReg) &&
          "Thumb1 can only copy GPR registers");
 
-  if (st.hasV6Ops() || ARM::hGPRRegClass.contains(SrcReg)
-      || !ARM::tGPRRegClass.contains(DestReg))
+  if (st.hasV6Ops() || ARM::hGPRRegClass.contains(SrcReg) ||
+      !ARM::tGPRRegClass.contains(DestReg))
     BuildMI(MBB, I, DL, get(ARM::tMOVr), DestReg)
         .addReg(SrcReg, getKillRegState(KillSrc))
         .add(predOps(ARMCC::AL));
   else {
-    // FIXME: Can also use 'mov hi, $src; mov $dst, hi',
-    // with hi as either r10 or r11.
-
     const TargetRegisterInfo *RegInfo = st.getRegisterInfo();
-    if (MBB.computeRegisterLiveness(RegInfo, ARM::CPSR, I)
-        == MachineBasicBlock::LQR_Dead) {
+    LiveRegUnits UsedRegs(*RegInfo);
+    UsedRegs.addLiveOuts(MBB);
+
+    auto InstUpToI = MBB.end();
+    while (InstUpToI != I)
+      // The pre-decrement is on purpose here.
+      // We want to have the liveness right before I.
+      UsedRegs.stepBackward(*--InstUpToI);
+
+    if (UsedRegs.available(ARM::CPSR)) {
       BuildMI(MBB, I, DL, get(ARM::tMOVSr), DestReg)
           .addReg(SrcReg, getKillRegState(KillSrc))
           ->addRegisterDead(ARM::CPSR, RegInfo);
       return;
     }
 
+    BitVector Allocatable = RegInfo->getAllocatableSet(
+        MF, RegInfo->getRegClass(ARM::hGPRRegClassID));
+
+    // LR is techically caller-save, not callee-save. This means that as long as
+    // it is saved on stack and killed, it is safe to use as a temporary high
+    // register, as it will be restored anyway.
+    Allocatable.reset(ARM::SP);
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TNorthover wrote:

Is this necessary? I'd expect `getAllocatableSet` to already block out reserved registers (which these had better be!)

https://github.com/llvm/llvm-project/pull/81908