[LLVMdev] Instructions that cannot be duplicated

[Villmow, Micah]

> -----Original Message-----
> From: Eli Friedman [mailto:eli.friedman at gmail.com]
> Sent: Wednesday, October 07, 2009 5:50 PM
> To: Villmow, Micah
> Cc: LLVM Developers Mailing List
> Subject: Re: [LLVMdev] Instructions that cannot be duplicated
> 
> On Wed, Oct 7, 2009 at 11:20 AM, Villmow, Micah
<Micah.Villmow at amd.com>
> wrote:
> > Is there a current way to specify that an instruction or function
> call
> > cannot be duplicated and thus any optimizations that might want to
> duplicate
> > this instruction would fail?
> 
> No.  Anything can be duplicated.  That could change, but you would
> need to make a strong case for why other solutions won't work.
[Villmow, Micah] Well the problem is that the function in question
cannot get duplicated because it has side-effects that duplicating
causes undefined behavior on vector hardware. Also, moving the
instruction inside of flow control when it is originally outside of flow
control produces undefined behavior. There currently is no way to
specify this in LLVM that I know of. We've tried lowering it to an
intrinsic and setting MayWriteMem and this does not solve the problem.
After looking at the llvm IR, there is no equivalent method of
representing an instruction that is an execution barrier(not a memory
barrier, which llvm.barrier.[ss|ll|ls|sl] is). If you have any idea's,
we would be willing to give them a try.


On the unique barrier issue, even if the barrier is given a unique
global identifier, it is the function duplication that causes the
problem. A unique global identifier lets us identify that invalid
optimizations have occurred, but it does not guarantee correctness since
the barrier function is unique per function call. So any sort of
duplication is invalid.
Micah