[llvm-dev] Controlling parameter alignment (original) (raw)

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 10 13:50:58 PST 2021

On Wed, Mar 10, 2021 at 6:35 AM Momchil Velikov <momchil.velikov at gmail.com> wrote:

Just to be clear, the suggestion is to introduce alignstack to affect argument alignment, while retaining the current semantics for align?

Thus, a pointer argument having both align(A) and alignstack(B) would itself be allocated at B boundary (if it happens to be passed in memory), while it would contain an A-aligned address?

Yes, that's the proposal as I understand it.

> Using alignstack for byval would also eliminate the special rule (for > align) there I guess.

So, byval arguments adopt the stackalign attribute with the same semantics as the current align and then align is invalid for byval arguments ?

I think there is a backwards compatibility consideration here. It also seems reasonable to allow align to appear with byval as an optimization hint: the optimizer can assume the low bits of such a pointer are zero.

Looks good to me.

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