[PATCH] D30810: Preserve vec3 type.

[Bruno Cardoso Lopes via Phabricator via cfe-commits]

bruno added a comment.

> Thanks for your comment. We have a pass to undo the vec4 to vec3. I wondered why clang generates the vec4 for vec3 load/store. As you can see the comment on clang's code, they are generated for better performance. I had a questions at this point. clang should consider vector load/store aligned by 4 regardless of target???

I believe the assumption is more practical: most part of upstream llvm targets only support vectors with even sized number of lanes. And in those cases you would have to expand to a 4x vector and leave the 4th element as undef anyway, so it was done in the front-end to get rid of it right away. Probably GPU targets do some special tricks here during legalization.

> llvm's codegen could handle vec3 according to targets' vector load/store with their alignment. I agree the flag looks odd but I was concerned some llvm's targets depend on the vec4 so I suggested to add the flag. If I missed something, please let me know.

My guess here is that targets that do care are looking through the vector shuffle and customizing to whatever seems the best to them. If you wanna change the default behavior you need some data to show that your model solves a real issue and actually brings benefits; do you have any real examples on where that happens, or why GPU targets haven't yet tried to change this? Maybe other custom front-ends based on clang do? Finding the historical reason (if any) should be a good start point.


https://reviews.llvm.org/D30810