[LLVMdev] Instructions that cannot be duplicated

[Eli Friedman]

On Thu, Oct 8, 2009 at 2:11 PM, Reid Kleckner <rnk at mit.edu> wrote:
> IMO Jeff's solution is the cleanest, simplest way to get code that
> works.  Just generate a separate function for every barrier in the
> program, and mark it noinline.  This way the instruction pointers will
> be unique to the barrier.

No, this gets rather nasty: to support an instruction like this, it
isn't legal to duplicate calls to functions containing a barrier
instruction.

Another proposal: add an executebarrier function attribute for
functions which directly or indirectly contain an execution barrier,
and adjust all the relevant transformation passes, like jump threading
and loop unswitching, to avoid duplicating calls to such functions.
This puts a slight burden on the frontend to mark functions
appropriately, but I don't see any other solution which doesn't affect
code which doesn't use execute barriers.

-Eli