[LLVMdev] pragmas

[Nick Lewycky]

Anthony Danalis wrote:
> Thanks for the quick responses.  "disappearing function calls" is by  
> far the preferred way for me, as I want my pass to work with standard  
> LLVM and not a hacked version that supports extra pragmas, or  
> intrinsics.  I am just new to LLVM and wanted to make sure that there  
> isn't already a mechanism for passing meta-data between the user and  
> the optimizer.

I wrote a patch added embedded metadata here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20090316/075433.html

which I'm sure will land "soon" (this weekend?). Unfortunately it 
doesn't permit the metadata to reference registers in your function (so, 
if you have "%x = load ..." then you can't have metadata on %x.

There's always a phase two I suppose.

The design is here: http://nondot.org/sabre/LLVMNotes/EmbeddedMetadata.txt

Nick

> I am planning to contribute to the wiki soon, especially little howtos  
> for things that take me a day to figure out how to do and turn out to  
> be < 50 lines of code that I could have copy pasted from ... a wiki!
> 
> Anthony.
> 
> On Mar 25, 2009, at 2:31 PM, someguy wrote:
> 
>> Pre-empting Chris's inevitable response: don't add intrinsics!
>>
>> I really like the 'disappearing function calls' idea. Chris suggested
>> practically the same thing for a previous question about adding BigInt
>> support.
>>
>> Anthony, whichever route you take in the end, please consider
>> documenting your 'code adventure' on the wiki so others can learn from
>> your experience.
>>
>> Justing
>> Registered Wiki Pimp.
>>
>> On Wed, Mar 25, 2009 at 8:02 PM, Luke Dalessandro
>> <luked at cs.rochester.edu> wrote:
>>> You could encode this information as simple library function calls  
>>> and
>>> then find them again in the generated LLVM IR. The client then just
>>> needs a header declaring the functions and information on what they
>>> mean. Since there are never any definitions of them they won't end up
>>> going anywhere.
>>>
>>> A more ambitious plan would be to modify llvm-gcc with new __builtins
>>> and create intrinsics in LLVM to map them to. There's really no
>>> advantage to this other than not needing the header file while
>>> compiling. There's a big disadvantage in that you end up mucking with
>>> both the front end and the llvm intrinsics.
>>>
>>> Finally, you can modify llvm-gcc pragma handling to insert things  
>>> that
>>> you want, but this is more work. You have to deal with the c-parser  
>>> and
>>> c++ parser, and understand more of the front end internals. I would
>>> avoid this unless you feel like you want pragmas that have some  
>>> sort of
>>> lexical semantics, and don't want to force people to use BEGIN and  
>>> END
>>> macros.
>>>
>>> Hope this is helpful,
>>> Luke
>>>
>>> Anthony Danalis wrote:
>>>> Hello
>>>>
>>>> Is there a "generic" pragma that is supported by LLVM and is visible
>>>> by optimization passes?  or any other way for a programmer to pass
>>>> meta-data information to the compiler?
>>>> I am writing an analysis pass that could benefit from user provided
>>>> information.  At this stage, I could like to keep the kind of
>>>> information that the user can provide as general as possible.
>>>> Examples would be "x, y are (not) aliased", "loop trip count =
>>>> x" (where x is either a static constant, or an expression", "branch
>>>> cond. is true most of the time" (or x% of the time), and other
>>>> information that might be statically undecidable, but the developer
>>>> knows the answer to.
>>>>
>>>> thanks,
>>>> Anthony
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