llvm dev - Feb 2021 - Controlling parameter alignment

+Johannes, are there other folks who care about LLVM IR attributes that we
can add?

WDYT about trying to separate actual argument memory alignment from pointer
argument alignment?

On Wed, Feb 17, 2021 at 4:24 AM Momchil Velikov <momchil.velikov at
gmail.com>
wrote:
> Currently we don't have a way to express in LLVM IR the alignment
> requirements of the function arguments. The `align` attribute is
> applicable to pointers only, and only for some special ways of passing
> arguments (e..g `byval`). When implementing AAPCS32/AAPCS64, clang
> resorts to dubious hacks of coercing to types, which naturally have
> the needed alignment.
>
> But we don't have enough types to cover all the cases.
>
> The problem appeared when passing over-aligned Homogeneous
> Floating-Point Aggregates (HFAs). When we pass a type with increased
> alignment requirements, for example
>
> struct S {
>   __attribute__ ((__aligned__(16))) double v[4];
> };
>
> Clang uses `[4 x double]` for the parameter, which is passed on the stack
> at alignment 8, whereas it should be at alignment 16.
>
> The trick of coercing to, say, `[i128, i128]` does not work, because
> the argument may end up in GP registers. A hypothetical coercion to
> `[f128, f128]` won't work either, because argument needs to be
> expanded to consecutive FP registers and they aren't overlapping on
> AArch64 (e.g. Q1 does not overlap D2/D3).
>
> There was a deficiency in the ABI
> which is addressed in a proposed fix
> (https://github.com/ARM-software/abi-aa/pull/67), which matches GCC
> behaviour and the original intent.
>
> With this ABI fix, we would need alignments of 8 and 16 to pass HFA
> arguments, although we should be ideally looking at a generic
> solution.
>
> There are similar issues with AAPCS32.
>
> One proposal was to adopt the `stackalign` attribute to convey these
> alignment requirement, https://reviews.llvm.org/D75903.
>
> Another option is to extend the `align` attribute semantics to apply
> to non-pointer arguments (I have a patch for that, which looks very
> much as the one above).
>
> To which Reid Kleckner commented like:
>
> > Mostly I think I meant that this will be a big change in the meaning
> > of either the align or the alignstack attributes, and that should be
> > hashed out on llvm-dev.
> >
> > Right now align is kind of a hybrid between an optimization annotation
> > attribute, like dereferenceable or nonnull, and an ABI attribute, like
> > byval or inreg. When align is used with byval, it affects argument
> > memory layout. When byval is not present, it is just an optimizer
> > hint. IMO, ideally, we should separate those two roles.
> >
> > I should be able to control the alignment of the memory used to pass a
> > pointer argument, at the same time that I annotate which low bits of
> > the pointer are known to be zero.
>
> Thoughts?
>
> ~chill
> --
> Compiler scrub, Arm
>
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To double check, the problem is the last argument here
https://godbolt.org/z/Gqva4Y does not carry the align(16)
information anymore, correct?

If so, I don't see an immediate problem with allowing an
"alignment"
be applied to non-pointer values. The semantics would probably say
that if the value is demoted to memory/stack, the associated alignment
has to be guaranteed. As noted before [0], `align` is actually a hint
and not a requirement. I'm unsure if we could not make it binding TBH,
will think about that, interested in opinions.

I take it we can't/don't want to legalize the constraints in the
backend, right? I mean, use `%sturct.S* align(16) byval(%sturct.S) %p`
in the IR. On that thought, what happens if we have a `struct S *`
argument in source which we then promote to pass it by value?

~ Johannes


[0] https://reviews.llvm.org/D75903#2549569


On 2/25/21 4:21 PM, Reid Kleckner wrote:
> +Johannes, are there other folks who care about LLVM IR attributes that we
> can add?
>
> WDYT about trying to separate actual argument memory alignment from pointer
> argument alignment?
>
> On Wed, Feb 17, 2021 at 4:24 AM Momchil Velikov <momchil.velikov at
gmail.com>
> wrote:
>
>> Currently we don't have a way to express in LLVM IR the alignment
>> requirements of the function arguments. The `align` attribute is
>> applicable to pointers only, and only for some special ways of passing
>> arguments (e..g `byval`). When implementing AAPCS32/AAPCS64, clang
>> resorts to dubious hacks of coercing to types, which naturally have
>> the needed alignment.
>>
>> But we don't have enough types to cover all the cases.
>>
>> The problem appeared when passing over-aligned Homogeneous
>> Floating-Point Aggregates (HFAs). When we pass a type with increased
>> alignment requirements, for example
>>
>> struct S {
>>    __attribute__ ((__aligned__(16))) double v[4];
>> };
>>
>> Clang uses `[4 x double]` for the parameter, which is passed on the
stack
>> at alignment 8, whereas it should be at alignment 16.
>>
>> The trick of coercing to, say, `[i128, i128]` does not work, because
>> the argument may end up in GP registers. A hypothetical coercion to
>> `[f128, f128]` won't work either, because argument needs to be
>> expanded to consecutive FP registers and they aren't overlapping on
>> AArch64 (e.g. Q1 does not overlap D2/D3).
>>
>> There was a deficiency in the ABI
>> which is addressed in a proposed fix
>> (https://github.com/ARM-software/abi-aa/pull/67), which matches GCC
>> behaviour and the original intent.
>>
>> With this ABI fix, we would need alignments of 8 and 16 to pass HFA
>> arguments, although we should be ideally looking at a generic
>> solution.
>>
>> There are similar issues with AAPCS32.
>>
>> One proposal was to adopt the `stackalign` attribute to convey these
>> alignment requirement, https://reviews.llvm.org/D75903.
>>
>> Another option is to extend the `align` attribute semantics to apply
>> to non-pointer arguments (I have a patch for that, which looks very
>> much as the one above).
>>
>> To which Reid Kleckner commented like:
>>
>>> Mostly I think I meant that this will be a big change in the
meaning
>>> of either the align or the alignstack attributes, and that should
be
>>> hashed out on llvm-dev.
>>>
>>> Right now align is kind of a hybrid between an optimization
annotation
>>> attribute, like dereferenceable or nonnull, and an ABI attribute,
like
>>> byval or inreg. When align is used with byval, it affects argument
>>> memory layout. When byval is not present, it is just an optimizer
>>> hint. IMO, ideally, we should separate those two roles.
>>>
>>> I should be able to control the alignment of the memory used to
pass a
>>> pointer argument, at the same time that I annotate which low bits
of
>>> the pointer are known to be zero.
>> Thoughts?
>>
>> ~chill
>> --
>> Compiler scrub, Arm
>>