[llvm-dev] RFC: Drop support for old style scalar TBAA

[Duncan P. N. Exon Smith via llvm-dev]

Seems like a nice cleanup to me!

> On 2016-Nov-07, at 13:01, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi all,
> 
> I want to clarify / emphasize two things based on an off-list
> conversation:
> 
> - The bitcode and IR parser both auto upgrade old style TBAA to the
>   newer TBAA struct tags.

Shouldn't we drop the textual IR auto-upgrade?  I'd rather fail in the verifier, since we don't aim for textual IR compatibility anyway.  (Could be done as follow-up, and it would be nice to attach an upgrade script to a PR or on the list or something.)

>  So, for instance, if you're writing out IR
>   to disk with the old representation and reading it back in then the
>   auto-upgrade logic will do the right thing for you.  You do *not*
>   need to do anything special here.  However, I'd still appreciate it
>   if can give me a thumbs-up or thumbs-down after doing some basic
>   sanity testing with D26229 applied locally.
> 
> - You'll only be affected by this change if you're creating IR in
>   memory (for instance via the IRBuilder interface), use the old TBAA
>   metadata representation and don't have an intermediate IR or
>   bitcode parsing step that would have auto-upgraded the IR.  In this
>   case, the quickest way to keep your frontend working with the new
>   representation is to change
> 
>     I->setMetadata(LLVMContext::MD_tbaa, MD);
> 
>   to
> 
>     I->setMetadata(LLVMContext::MD_tbaa, llvm::UpgradeTBAANode(*MD));
> 
>   llvm::UpgradeTBAA is present on tip-of-tree.  If your version of
>   LLVM is too old to have that utility then copy/pasting it in from
>   tip-of-tree into your code base should work fine.
> 
> -- Sanjoy
> 
> Sanjoy Das wrote:
> > In https://reviews.llvm.org/D26229 I've proposed dropping support for
> > old style scalar TBAA metadata.
> >
> > Here is the proposed commit message:
> >
> > "We've had support for auto upgrading old style scalar TBAA access
> > metadata into the "new" struct path aware TBAA metadata for 3 years now.
> > The only way to actually generate old style TBAA was explicitly through
> > the IRBuilder API. I think this is a good time for dropping support for
> > old style scalar TBAA."
> >
> >
> > Are there active users of LLVM who _need_ the old style scalar TBAA and
> > cannot use the new struct path TBAA metadata? If so, or if you object
> > the change for other reasons, please respond here.
> >
> > The motivation here is to reduce complexity in our TBAA analysis if
> > feasible.
> >
> > Thanks,
> > -- Sanjoy
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev