[thumb,framelowering] Reset NoVRegs in Thumb1FrameLowering::emitPrologue.

[fhahn]

Summary

In this function, virtual registers can be introduced (for example
through calls to emitThumbRegPlusImmInReg). doScavengeFrameVirtualRegs
will replace those virtual registers with concrete registers later on
in PrologEpilogInserter, which sets NoVRegs again.

This patch fixes the Codegen/Thumb/segmented-stacks.ll test case which
failed with expensive checks.
https://llvm.org/bugs/show_bug.cgi?id=27484

[rengolin]

This makes sense to me, but I'll let Oliver have the final say, as this is his area.

Also, can you add a test, please?

cheers,
--renato

[fhahn]

Thanks for having a look! Unfortunately I have no idea how to add a test. The problem is only exposed because ARMFrameLowering::adjustForSegmentedStacks verifies the machine function if expensive checks are enabled (which makes sure no virtual registers are present in functions marked as NoVRegs). The NoVRegs property is only violated inside the PrologEpilogInserter pass until doScavengeFrameVirtualRegs is called.

Cheers,
Florian

[olista01]

I also can't see any way to test this other than turning on expensive checks (which already catches it), so LGTM.

lib/CodeGen/PrologEpilogInserter.cpp

for (MachineBasicBlock::iterator I = MBB.begin(); I != MBB.end(); ) {
         assert(RS->getCurrentPosition() == I &&
           "The register scavenger has an unexpected position");
         I = P;
         RS->unprocess(P);
       } else
         ++I;
     }
   }
+
+  MF.getProperties().set(MachineFunctionProperties::Property::NoVRegs);
 }

lib/Target/ARM/Thumb1FrameLowering.cpp

BuildMI(MBB, MBBI, dl, TII.get(ARM::tMOVr), BasePtr)
         .addReg(ARM::SP)
         .add(predOps(ARMCC::AL));
 
   // If the frame has variable sized objects then the epilogue must restore
   // the sp from fp. We can assume there's an FP here since hasFP already
   // checks for hasVarSizedObjects.
   if (MFI.hasVarSizedObjects())
     AFI->setShouldRestoreSPFromFP(true);
+
+  // In some cases, virtual registers have been introduced, e.g. by uses of
+  // emitThumbRegPlusImmInReg.
+  MF.getProperties().reset(MachineFunctionProperties::Property::NoVRegs);
 }
 
 static bool isCSRestore(MachineInstr &MI, const MCPhysReg *CSRegs) {
   if (MI.getOpcode() == ARM::tLDRspi && MI.getOperand(1).isFI() &&
       isCalleeSavedRegister(MI.getOperand(0).getReg(), CSRegs))
     return true;
   else if (MI.getOpcode() == ARM::tPOP) {
     return true;