[cfe-dev] Bitwidth attribute in clang

[Steve Naroff]

On Aug 8, 2008, at 9:03 AM, Nadav wrote:

> Hi Steve,
>
> I enjoyed watching your presentation from the devmtg. It was very  
> good.
>

Glad you enjoyed it!

> I started with a different solution. I added a new 'unsigned bitwidth'
> member to the BuiltinType. If it is set to zero then the default type
> behaviour is used, otherwise the bitwidth field is used. With this
> implementation I don't have to implement the signed/unsigned float/int
> behaviour details.
> Would this also work ?
>

I imagine so, however it doesn't seem as clean (since the behavior  
isn't a built-in property of C).

> I have another question,  For this code :
> typedef int int4 __attribute__((bitwidth(4)));
>
> I added a handle for the new attribute. From the handle I am given a
> "TypedefDecl" object. From this I would like to get the definition of
> the new type ("int4") and change it. However, the getUnderlyingType()
> API returns the Type object for *int. I could not find the API to get
> the Type object for "int4".
>

Have you looked at SemaDeclAttr.cpp:HandleModeAttr()? Is this GCC-ism  
close to what you are trying to accomplish?

snaroff

>
> Thank you,
> Nadav
>
>
> On Fri, Aug 8, 2008 at 2:35 PM, Steve Naroff <snaroff at apple.com>  
> wrote:
>>
>> On Aug 8, 2008, at 2:49 AM, Nadav wrote:
>>
>>> Hello,
>>>
>>> I am working on adding attribute support for arbitrary integer bit
>>> width for clang. This is useful for hardware synthesis where smaller
>>> integers translate to smaller circuits.
>>>
>>> There is a similar open bug against llvm-gcc which explains this.
>>> http://llvm.org/bugs/show_bug.cgi?id=1284
>>>
>>> Example:
>>> typedef int int5 __attribute__((bitwidth(5)));
>>>
>>> I followed efriedma's 51577 patch and added the new attribute syntax
>>> without a problem.
>>> http://llvm.org/viewvc/llvm-project?view=rev&revision=51577
>>>
>>> I wanted to consult with the list on the best way to implement this.
>>> It looks like the current type classes cannot support arbitrary
>>> bitlength integers. The BuiltinType class supports only the standard
>>> types (UChar, UInt, Int, etc).  Would you suggest deriving another  
>>> new
>>> type from Type (similar to complex, pointer, etc). After I do this I
>>> will have to change many other places in the code.
>>>
>>
>> To create the new type, you will have to modify Type, ASTContext,  
>> and Sema
>> (ProcessDeclAttribute). See VectorType/ExtVectorType for a couple  
>> types
>> created from attributes. Once the type is created, you will need to  
>> hack
>> Sema for type checking/analysis.
>>
>>> Is there another better way to implement this ?
>>
>> Not really...you have the right idea.
>>
>> snaroff
>>
>>>
>>>
>>> Thank you,
>>> Nadav Rotem
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>
>>
>
>
>
> -- 
> Cheers,
> Nadav