[PATCH] D36562: [Bitfield] Make the bitfield a separate location if it has width of legal integer type and its bit offset is naturally aligned for the type

Xinliang David Li via cfe-commits cfe-commits at lists.llvm.org
Tue Aug 22 19:03:07 PDT 2017

On Tue, Aug 22, 2017 at 6:37 PM, Chandler Carruth via Phabricator <
reviews at reviews.llvm.org> wrote:

> chandlerc added a comment.
> I'm really not a fan of the degree of complexity and subtlety that this
> introduces into the frontend, all to allow particular backend optimizations.
> I feel like this is Clang working around a fundamental deficiency in LLVM
> and we should instead find a way to fix this in LLVM itself.
> As has been pointed out before, user code can synthesize large integers
> that small bit sequences are extracted from, and Clang and LLVM should
> handle those just as well as actual bitfields.
> Can we see how far we can push the LLVM side before we add complexity to
> Clang here? I understand that there remain challenges to LLVM's stuff, but
> I don't think those challenges make *all* of the LLVM improvements off the
> table, I don't think we've exhausted all ways of improving the LLVM changes
> being proposed, and I think we should still land all of those and
> re-evaluate how important these issues are when all of that is in place.

The main challenge of doing  this in LLVM is that inter-procedural analysis
(and possibly cross module) is needed (for store forwarding issues).

Wei, perhaps you can provide concrete test case to illustrate the issue so
that reviewers have a good understanding.


> Repository:
>   rL LLVM
> https://reviews.llvm.org/D36562
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