llvm dev - Jan 2013 - [LLVMdev] Inconsistent label syntax in LLVM assembly

Hi all,

i'm writing a TXL (http://www.txl.ca) grammar and a revamp of  
bison/flex grammar for LLVM.

I've noticed an inconsistency regarding label naming conventions.

For instance, the following is a segment of legit LLVM assembly  
(human-readable) IR:

   br i1 %38, label %17, label %39
; <label>:39                                      ; preds = %._crit_edge
   ret void

However, ";" is a comment-line character. How is this interpreted, as
a meta-comment? (a semantically important comment)?

Other branches appear using regular conventions:

   br i1 %exitcond, label %._crit_edge, label %.lr.ph
._crit_edge:             ; preds = %.lr.ph, %.preheader

So, naturally I have some questions:

1) What is the differences among the branch labels? Differences in the  
formation (e.g. emitted by different passes?)

2) How to normalize the appearance of branch labels? I.e. is it  
possible to not emit the strange ; <label>:39 like ones?

3) Why <label>:39 and not 39: ?

4) Why not prefixing all labels with <label>?

Best regards
Nikolaos Kavvadias

Hi Nikolaos,

On 20/01/13 12:00, nkavv at physics.auth.gr wrote:
> Hi all,
>
> i'm writing a TXL (http://www.txl.ca) grammar and a revamp of
bison/flex grammar
> for LLVM.
>
> I've noticed an inconsistency regarding label naming conventions.
>
> For instance, the following is a segment of legit LLVM assembly
(human-readable)
> IR:
>
>    br i1 %38, label %17, label %39
> ; <label>:39                                      ; preds =
%._crit_edge
>    ret void
>
> However, ";" is a comment-line character. How is this
interpreted, as a
> meta-comment? (a semantically important comment)?
it's just a comment and has no semantic comment.  You can delete it, it
won't
make any difference.

Ciao, Duncan.
>
> Other branches appear using regular conventions:
>
>    br i1 %exitcond, label %._crit_edge, label %.lr.ph
> ._crit_edge:             ; preds = %.lr.ph, %.preheader
>
> So, naturally I have some questions:
>
> 1) What is the differences among the branch labels? Differences in the
formation
> (e.g. emitted by different passes?)
>
> 2) How to normalize the appearance of branch labels? I.e. is it possible to
not
> emit the strange ; <label>:39 like ones?
>
> 3) Why <label>:39 and not 39: ?
>
> 4) Why not prefixing all labels with <label>?
>
> Best regards
> Nikolaos Kavvadias
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Duncan
>>   br i1 %38, label %17, label %39
>> ; <label>:39                                      ; preds =
%._crit_edge
>>   ret void
>>
>> However, ";" is a comment-line character. How is this
interpreted, as a
>> meta-comment? (a semantically important comment)?
>
> it's just a comment and has no semantic comment.  You can delete it, it
won't
> make any difference.
OK. However, in this case, how is the branch resolved? I mean that  
there is no obvious %39 label declared in the LLVM translation unit.


Best regards
Nikolaos Kavvadias