[LLVMdev] [Bug 3756] __attribute__((always_inline)) and __builtin_constant_p

[Dan Gohman]

On Mar 15, 2009, at 6:16 PM, Nick Lewycky wrote:

> Pierre Habouzit wrote:
>> [ please CC: me as I'm not subscribed ]
>>
>> On Wed, Mar 11, 2009 at 04:13:34AM +0000, bugzilla- 
>> daemon at cs.uiuc.edu wrote:
>>> http://llvm.org/bugs/show_bug.cgi?id=3756
>>>
>>> Chris Lattner <clattner at apple.com> changed:
>>>
>>>           What    |Removed                     |Added
>>> ----------------------------------------------------------------------------
>>>             Status|NEW                         |RESOLVED
>>>         Resolution|                            |WONTFIX
>>>
>>> --- Comment #6 from Chris Lattner <clattner at apple.com>  2009-03-10  
>>> 23:13:33 ---
>>> Unfortunately, this will never be fixed in either llvm-gcc or clang.
>>> __builtin_constant_p is a "best effort" constant folding tester,  
>>> which is
>>> allowed to fail.  You should never write code that assumes that
>>> __builtin_constant_p can't fail (if you need that, just don't use
>>> __builtin_constant_p).
>>
>>
>>> It would be interesting and useful to bring this up on the glibc  
>>> list
>>> to see if they are willing to fix their headers.
>>
>> Hahahaha, you never talked to Uli before did you ?
>>
>>> Barring that, if this is really important to you, please raise the  
>>> issue on the
>>> llvmdev list.  We will need IR extensions to support this among  
>>> other things.
>>
>> Well, I don't expect __builtin_constant_p to works always. That's not
>> precisely the issue. I expect though __attribute__((always_inline))
>> functions to be expanded prior to any optimization pass, because it's
>> what it means. Those functions are basically type-safe macros.
>>
>> If you do that, then my problem go away at once.
>
> No, that's not the problem.

I disagree; I think incomplete support for always_inline is the  
primary issue
here.

>
> Before any optimizations can be applied the function must be converted
> into LLVM IR. In LLVM IR we have no equivalent for  
> "builtin_constant_p".
> Thus the __builtin_constant_p is evaluated and found false as part of
> converting the GCC trees into LLVM IR, then the always_inliner is
> applied on the IR.

Supporting always_inline as a type-safe macro which is always expanded
early is a feature of GCC that people have found useful.  It is a  
limitation of
LLVM that this happens to be complicated to implement there.

Dan