Fix load-store optimizer on thumbv4t

[jroelofs]

Summary

This is another instance of the lo->lo copy problem. Unfortunately, I don't have a testcase.

[jroelofs]

ping

[t.p.northover]

So why couldn't you create a test? It doesn't seem like a terribly difficult issue to trigger: just a generic load/store merge operation on Thumb1.

llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp

574

Isn't the whole block already isThumb1?

[jroelofs]

Add a testcase. Address Tim's comments.

[t.p.northover]

Hi Jon,

Thanks for adding the test. Just one debugging line that crept in. Feel free to commit with that removed.

Tim.

lib/Target/ARM/ARMLoadStoreOptimizer.cpp

564 ↗

(On Diff #18554)

Probably best without this.

[jroelofs]

Good catch, thanks Tim! r226711

[jroelofs]

@t.p.northover, @hansw Is r226711 appropriate for the 3.6.0 branch?

[hans]

If Tim is ok with it, I'm happy to merge it.

llvm/lib/Target/ARM/ARMLoadStoreOptimizer.cpp

       //   MOV  NewBase, Base
       //   ADDS NewBase, #imm8.
       if (Base != NewBase && Offset >= 8) {
+        const ARMSubtarget &Subtarget = MBB.getParent()->getTarget()
+           .getSubtarget<ARMSubtarget>();
         // Need to insert a MOV to the new base first.
-        BuildMI(MBB, MBBI, dl, TII->get(ARM::tMOVr), NewBase)
-          .addReg(Base, getKillRegState(BaseKill))
-          .addImm(Pred).addReg(PredReg);
+        if (isARMLowRegister(NewBase) && isARMLowRegister(Base) &&
+            Subtarget.isThumb1Only() && !Subtarget.hasV6Ops()) {
+          // thumbv4t doesn't have lo->lo copies that don't clobber cpsr,
+          // and we can't predicate tMOVSr
+          if (Pred != ARMCC::AL)
+            return false;
+          BuildMI(MBB, MBBI, dl, TII->get(ARM::tMOVSr), NewBase)
+            .addReg(Base, getKillRegState(BaseKill));
+        } else
+          BuildMI(MBB, MBBI, dl, TII->get(ARM::tMOVr), NewBase)
+            .addReg(Base, getKillRegState(BaseKill))
+            .addImm(Pred).addReg(PredReg);
+
         // Set up BaseKill and Base correctly to insert the ADDS/SUBS below.
         Base = NewBase;
         BaseKill = false;