[LLVMdev] Instructions that cannot be duplicated

[Eli Friedman]

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