[LLVMdev] possible thumb bug in constant islands

Jim Grosbach
Tue, 19 Nov 2013 02:34:38 -0000

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?

>
> 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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