[llvm-dev] Controlling parameter alignment

[Momchil Velikov via llvm-dev]

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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