[llvm-dev] Ok with mismatch between dead-markings in BUNDLE and bundled instructions?

[Mikael Holmén via llvm-dev]

Hi Quentin and llvm-dev,

I've got a regalloc-related question that you might have an opinion or 
answer about.

In our out-of-tree target we've been doing some bundling before register 
allocation for quite some time now, and last night a new problem popped 
up. What the fix should be depends on if this bundle is legal or not:

     BUNDLE %vreg39<imp-def,dead>
       * %vreg39:hiAcc<def> = mv_ar16_ar16_lo16In32 %vreg37
       [...]

%vreg39 isn't used after the bundle so the dead-marking in the BUNDLE is 
correct. However, the def in the actual bundled instruction defining 
%vreg39 is not marked with dead.

Is the above bundle ok or not?


When the register allocator later tries to spill/reload %vreg39 it 
thinks that %vreg39 is live, so it inserts a reload after the bundle, 
but then the verifier pukes on it and says that there is a use of the 
register after it's dead.

The dead-marking on the BUNDLE is introduced by 
LiveIntervals::computeDeadValues which does

       // This is a dead def. Make sure the instruction knows.
       MachineInstr *MI = getInstructionFromIndex(Def);
       assert(MI && "No instruction defining live value");
       MI->addRegisterDead(LI.reg, TRI);

and the reload after the BUNDLE is introduced by 
InlineSpiller::spillAroundUses which has determined that the register is 
live:

     // Rewrite instruction operands.
     bool hasLiveDef = false;
     for (const auto &OpPair : Ops) {
       MachineOperand &MO = OpPair.first->getOperand(OpPair.second);
       MO.setReg(NewVReg);
       if (MO.isUse()) {
         if (!OpPair.first->isRegTiedToDefOperand(OpPair.second))
           MO.setIsKill();
       } else {
         if (!MO.isDead())
           hasLiveDef = true;
       }


So I suppose we could either make LiveIntervals::computeDeadValues mark 
the individual defs dead as well, or we could change 
InlineSpiller::spillAroundUses so that if we only look at the BUNDLE 
instruction (if it exists) in case of bundled instructions when looking 
for defs.

Any opinions on what way to go?

Thanks,
Mikael