[PATCH] D12310: Introducing llvm.invariant.group.barrier intrinsic & Global Opt handling

[Nick Lewycky via llvm-commits]

On 8 September 2015 at 11:28, Piotr Padlewski <prazek at google.com> wrote:

> Prazek marked an inline comment as done.
>
> ================
> Comment at: lib/CodeGen/CodeGenPrepare.cpp:1414-1417
> @@ -1413,2 +1413,6 @@
>      }
> +    case Intrinsic::invariant_group_barrier:
> +      II->replaceAllUsesWith(II->getArgOperand(0));
> +      II->eraseFromParent();
> +      return true;
>      }
> ----------------
> nlewycky wrote:
> > Do we also discard the !invariant.group metadata on instructions?
> >
> > The backend has pointers to the Value*'s, it would be bad if they used
> the !invariant.group markers but all the invariant.group.barrier calls had
> been removed.
> The question is, does it make any difference? What I understand, is that
> CodeGenPrepare is done just before machine code generation, which means
> that additional metadata in load/stores doesn't make any difference.
>

Today it does not. My point is that machine code generation does look at
Value*. It does things like alias analysis on the IR. Leaving the
!invariant.group there is a trap for a future developer.

================
> Comment at: lib/Transforms/IPO/GlobalOpt.cpp:2507-2508
> @@ -2506,1 +2506,4 @@
>            continue;
> +        } else if (II->getIntrinsicID() ==
> Intrinsic::invariant_group_barrier) {
> +          setVal(II, getVal(II->getOperand(0)));
> +          DEBUG(dbgs() << "Passing through invariant.group.barrier
> intrinsic.\n");
> ----------------
> nlewycky wrote:
> > Why is this safe? We have
> >   %p2 = call @invariant.group.barrier(%p1)
> > and we set that %p2 just plain *is* %p1. Then nothing happens until
> evaluation completes and we commit the changes, which means replacing %p2
> with %p1. At the time, why won't that operation miscompile?
> >
> > I can think of two possible reasons, one is that %p1 may be constrained
> (like, it may be a ConstantExpr) and the other is that we may know that
> there are no remaining loads of either %p1 or %p2 with !invariant.group on
> them.
> >
> > In any event, please answer in the form of a comment in this code. :)
> What do you mean about miscompile?
> I think this is safe because global-opt doesn't care about
> !invariant.group metadata, which means that getting rid of
> invariant.group.barrier will not break anything.
>

That's a good start, but why is it impossible to get a case where replacing
the pointer is something globalopt does, then the pointer is used by a load
in a function that globalopt didn't look at?
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