[LLVMdev] llvm.memory.barrier does not work

[Eli Friedman]

On Wed, Sep 28, 2011 at 3:27 PM, Junjie Gu <jgu222 at gmail.com> wrote:
> Instrinsic llvm.memory.barrier does not work as expected.  Is it a bug
> or it has not been implemented yet ?

It's going away in favor of the new fence instruction (and I'll remove
it as soon as dragonegg catches up).  It should still work at the
moment, though.

> (1) false arguments do not work
>
> // pesudo code
> void foo(int *x) {
>  x[2] = 10;
>  llvm.memory.barrier(0, 0, 0, 0, 0);
>   x[2] = 20;
>  return void
> }
>
>
> The barrier is actually noop,  but it prevents "x[2] = 10" from being deleted.

Don't do that. :)  Really, why are you using a noop barrier?

> (2) True arguments do not work.
>
> // pesudo code
> void foo(int * restrict x) {
>  x[2] = 10;
>  llvm.memory.barrier(1, 1, 1, 1, 1);
>   x[2] = 20;
>  return void
> }
>
> "x[2] = 10' should not be deleted because barrier is present.  But it
> is deleted anyway.

The pointer is "restrict", therefore the compiler assumes nothing else
can touch it while the function runs.

Actually, the transformation in question is probably valid even if the
pointer isn't restrict (although LLVM won't actually do that); your
use of a barrier here doesn't really make sense.

-Eli