[cfe-dev] Alignment of bitfield structs

[Renato Golin]

On 20 April 2015 at 14:57, Bradley Smith <bradley.smith at arm.com> wrote:
> From what I can tell there are only 2 ways to fix this, either have clang
> not combine the bitfield into an i40, or somehow pass the bitfield layout
> down to LLVM. Neither of these seem particularly easy to do, what are others
> thoughts on this, have I missed something? Thanks.

Hi Bradley,

We had similar issues when we did te EDG bridge and IIRC, the issue
depends on how you lower the bitfield access logic.

Your two proposals are similar in that they suggest keeping the
information down the pipeline. Either way, keeping that information
valid for too long is going to be a pain, mainly because the current
passes don't account for anything special from bitfields. In theory,
bitfields are just a blob with complicated accessors, and no pass
should break that, unless the pass itself is broken.

Can you give an example of a piece of IR that breaks and maybe finding
if there is an optimisation pass that, if omitted, produces good code?

cheers,
--renato