[LLVMdev] Instructions that cannot be duplicated

[Jeffrey Yasskin]

Are the platforms with no function calls the same ones that have
optimization-hostile barrier instructions? If the two sets of
platforms are disjoint, OpenCL implementers can use my or Devang's
noinline-function technique on the optimization-hostile platforms, and
inject a unique argument into the barrier() call in the frontend on
the no-function platforms.

On Fri, Oct 9, 2009 at 3:34 AM, Mon Ping Wang <monping at apple.com> wrote:
>
> Point taken :->.  Inlining of these functions containing these
> barriers are required on some platforms.  The only restriction is that
> any control flow optimization must preserve the property that all
> threads will hit the same barrier.
>
> -- Mon Ping
>
> On Oct 9, 2009, at 2:22 AM, Eli Friedman wrote:
>
>> On Fri, Oct 9, 2009 at 12:20 AM, Mon Ping Wang <monping at apple.com>
>> wrote:
>>>
>>> The requirement in OpenCL is that all threads (work-items) are
>>> required to
>>> hit the same barrier.  If one does what you have shown below, it is
>>> not
>>> legal because some threads may go through the block with S1 and
>>> some other
>>> threads will go the other way.  On some hardware, such a program
>>> will cause
>>> a hardware stall. If one is inlining, it is preferable to inline
>>> early
>>> assuming the rest of the transformations don't mess with the
>>> barrier. Eli is
>>> correct that you can't duplicate calls to a function containing
>>> these kind
>>> of barriers for the same reasons.  From the discussions so far, it
>>> would be
>>> nice if such a concept where you don't want to modify the control
>>> flow of a
>>> basic block containing such an execution barrier or a function
>>> containing
>>> such a barrier. This requires that all phases that does such
>>> optimizations
>>> would have to be made aware of it.  Such a concept may be also
>>> useful for
>>> other things like inline assembly where one may not want to
>>> duplicate a
>>> block.
>>
>> It's probably worth noting that I wasn't proposing a general
>> prohibition of duplication; it would be okay for inlining or loop
>> unrolling to duplicate a call to a function marked executebarrier.
>> It's not the same sort of prohibition that one might want for inline
>> assembly.
>>
>> -Eli
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>