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

Momchil Velikov via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 31 01:51:44 PDT 2021

On Wed, Mar 24, 2021 at 5:46 PM Momchil Velikov <momchil.velikov at gmail.com> wrote:

On Wed, Mar 24, 2021 at 11:28 AM Momchil Velikov <_ _momchil.velikov at gmail.com> wrote: > > On Wed, Mar 10, 2021 at 9:51 PM Reid Kleckner <rnk at google.com> wrote: > > > > 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. > > Something is not quite right here. > > We have up to three relevant alignment properties for a parameter: > * the alignment of the parameter itself (if it happenes to be passed in memory) > * if it's a pointer, the actual alignment of the pointed to memory (as an optimisation aid) > * if it's a byval or a byref argument, the minimum alignment of the storage, allocated > for the original argument value (ABI affecting). > > For non-pointer arguments alignstack(N) gives stack slot alignment. > For pointer arguments, we retain that use of alignstack(N) and also have align(M) to give > the actual alignment of the contained pointer. > > Now when we add byval or byref to the above, there is no attribute left to give the alignment > of the allocated memory. We thought of using alignstack(N), but that would leave us without a way > to specify the pointer alignment itself.

I hope I'm not missing something obvious, and if I don't here's an idea: Extend the byval(Ty) attribute to byval(Ty [, Align]) (same for byref). (Most of the attributes take zero or one parameters, but there's a precedent with allocsize(<EltSizeParam>[, <NumEltsParam>])) So we end up with : * align(N) for pointer content * stackalign(N) for the minimum alignment for of the actual argument, if it ends up in memory * byval(Ty[, N]) and byref(Ty[, N]) for the original argument value Thus something like call %f(%struct.S * alignstack(8) align(32)_ _byval(%struct.S, 16) p); would mean: * the caller has allocated a slot for struct S, that slot is at least 16 bytes aligned * the caller is passing a pointer to that slot, which pointer itself should be 8 bytes aligned, if it ends up in memory * that pointer happens to have the lower five bits clear

Hello,

Any comments, suggestions, ideas ?

~chill

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