llvm dev - Jun 2014 - [LLVMdev] A question about the BasicBlock::hasAddressTaken() implementation.

Hi Fellows,

    BasicBlock::hasAddressTaken has the following implementation:

              bool hasAddressTaken() const { return
getSubclassDataFromValue() != 0; }

     and then, in llvm/IR/Value.h, we have

               unsigned short getSubclassDataFromValue() const { return
SubclassData; }

     But it's not clear to me at all, how is (SubclassData != 0) equivalent
to the basic block has its address taken? What's the logic behind all
these? Thanks so  much.


Best Regards,
Paul
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20140613/1b2cf521/attachment.html>

Hi Paul,

On 13 June 2014 22:58, Paul Vario <paul.paul.mit at gmail.com>
wrote:
>                unsigned short getSubclassDataFromValue() const { return
> SubclassData; }
>
>      But it's not clear to me at all, how is (SubclassData != 0)
equivalent
> to the basic block has its address taken? What's the logic behind all
these?
That field in Value is there as a convenience for anything that
inherits from it to use to store a few bits of information. For
example, Atomic instructions store what ordering they've been given
there; I think arithmetic instructions use it for the wrapping
behaviour flags.

BasicBlock appears to use it as a reference count: whenever a
BasicBlockAddress is created, that value gets incremented by a call to
AdjustBlockAddressRefCount. So assuming all BasicBlockAddresses that
have been created are actually still in use, it should be accurate.
Just don't let them leak, I suppose!

Cheers.

Tim.