[LLVMdev] possible thumb bug in constant islands

[Jim Grosbach]

On Nov 18, 2013, at 6:49 PM, reed kotler <rkotler at mips.com> wrote:

> On 11/18/2013 06:34 PM, Jim Grosbach wrote:
>> On Nov 18, 2013, at 3:49 PM, reed kotler <rkotler at mips.com> wrote:
>> 
>>> I don't know ARM hardly at all but...
>>> 
>>> This comment does not seem to match the code.
>>> Or maybe tBfar is a BL?
>> What does the definition of tBfar say?
> 
> Okay.. but
> 
>  // Far jump
>  // Just a pseudo for a tBL instruction. Needed to let regalloc know about
>  // the clobber of LR.
>  let Defs = [LR] in
>  def tBfar : tPseudoExpand<(outs), (ins t_bltarget:$target, pred:$p),
>                          4, IIC_Br, [], (tBL pred:$p, t_bltarget:$target)>,
>                          Sched<[WriteBrTbl]>;
> 
> Are you expecting LR to be saved because of this Defs?
> This is all happening way after register allocation and prologue generation.
> 
> This is an unusual situation where LR would not have been saved but it's possible to have this
> and then you will not return from the function.
> 

Not sure about that aspect. Offhand, it certainly seems like a bug. An extremely unlikely to encounter in practice bug, but still real.

>>> Also, how does this work if the destination is greater than 2**21?
>>> 
>> It doesn’t. IIRC, that’s under the category of “if people start having problems with that limit, we’ll consider complicating the code enough to deal with it."
>> 
>>> /// fixupUnconditionalBr - Fix up an unconditional branch whose destination is
>>> /// too far away to fit in its displacement field. If the LR register has been
>>> /// spilled in the epilogue, then we can use BL to implement a far jump.
>>> /// Otherwise, add an intermediate branch instruction to a branch.
>>> bool
>>> ARMConstantIslands::fixupUnconditionalBr(ImmBranch &Br) {
>>>  MachineInstr *MI = Br.MI;
>>>  MachineBasicBlock *MBB = MI->getParent();
>>>  if (!isThumb1)
>>>    llvm_unreachable("fixupUnconditionalBr is Thumb1 only!");
>>> 
>>>  // Use BL to implement far jump.
>>>  Br.MaxDisp = (1 << 21) * 2;
>>>  MI->setDesc(TII->get(ARM::tBfar));
>>>  BBInfo[MBB->getNumber()].Size += 2;
>>>  adjustBBOffsetsAfter(MBB);
>>>  HasFarJump = true;
>>>  ++NumUBrFixed;
>>> 
>>>  DEBUG(dbgs() << "  Changed B to long jump " << *MI);
>>> 
>>>  return true;
>>> }
>>> 
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> 
>