[PATCH] D30810: Preserve vec3 type.

[Anastasia Stulova via Phabricator via cfe-commits]

Anastasia added a comment.

I don't think there is anything wrong with the generation of vec3->vec4 in Clang. I believe the motivation for this was the OpenCL spec treating vec3 as vec4 aligned type (see section 6.1.5: https://www.khronos.org/registry/OpenCL/specs/opencl-2.0-openclc.pdf#12). So in terms of memory layout vec3 wouldn't be any different to vec4. But in terms of operations (including loads/stores) there can be potential gain from not handling the 4th element.  This can be exploited by some targets. I think generating the vec3 from the frontend would be a better choice in the first place. Because backend can decide how to handle this. Including for architectures with no SIMD support it would just generate 3 separate loads. Right now it seems that it will be forced to generate 4 loads.

But considering that transformation to vec4 has been the default implementation for quite a while in the frontend, I think we would need a stronger motivation for switching to original vec3. So current approach with a special flag for preserving vec3 should be good enough to fit all needs.


https://reviews.llvm.org/D30810