[llvm-dev] RFC: Remove inaccessiblememonly from 3.8 branch

Pete Cooper via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 10 13:29:32 PST 2016

Hi Philip
> On Feb 10, 2016, at 1:12 PM, Philip Reames via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> A while back, we introduced new attributes to model functions which read and wrote from locations not otherwise visible in the IR.  The motivation was to enable much more aggressive aliasing around such calls.
> Since that time, we've reverted the original patch and AFAIK we have no transformation or analysis passes in tree which use this information.  Given this, I don't think it's really fair to say the attribute has baked at all.  We have no reason to believe that we picked the right semantics, and last I remember, there was some legitimate question about that.  Worse, AFAIK, all active development has stopped.  I don't know of any active reviews for this attribute.
> These attributes have not yet shipped in any LLVM release, but this is about to change in 3.8.  Once that changes, we will have accepted a backwards compatibility guarantee with bitcode which is potentially concerning for a new attribute in such a half developed state.
> Given the preceding, I'd like to propose that we remove the inaccessiblememonly and inaccessiblemem_or_argmemonly attributes from the 3.8 release branch.  This could be done by either a) actually removing the code, or b) simply disabling it.  If we remove the documentation from the LangRef, disable the LL and bitcode parsing for the attribute, and explicitly document that we are not reserving the attribute index numbers, I think that prevents any assumption of forward compatibility.  Another option would be to mark them experimental, which would have effectively the same effect.
I don’t mind which of the above options we choose, but +1 for doing something.

The most important point to me is that of forward compatibility.  If we ship it then I feel like we are more or less accepting the current semantics of the attribute, even if that later turns out to not be what we want.  

For that reason, I like that you pointed out attributes being experimental.  Perhaps being experimental should be the default for new attributes (default doesn’t mean we can’t add new non-experimental ones), as then at least we are free to change attributes as needed.

The same thing kind of happened with convergent which was added but then changed slightly in behaviour.  Luckily I don’t think we had a release in that timeframe so it wasn’t a problem, but had we had a release with someone relying on the initial behaviour of convergent, we could have had trouble changing it later.

> Philip
> The original GlobalsAA:
> http://reviews.llvm.org/D15605
> The revert:
> http://reviews.llvm.org/D15919
> The last active revision I know of: (Abandoned Jan 6)
> http://reviews.llvm.org/D15665
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