[LLVMdev] Alignment of pointee

[Arnaud Allard de Grandmaison]

Related to this subject is the __attribute__(aligned(X)) that can be set on
a type in C/C++. It is being used when generating the load / stores /
memcpy / ... but is lost with respect to the type's attribute. In many a
case this could help various analysis or transforms to provide more
accurate results when such a type is used. The __builtin_assume_aligned
could be an way to solve this.

Cheers,
--
Arnaud


On Tue, Mar 25, 2014 at 4:22 PM, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
> > From: "Frank Winter" <fwinter at jlab.org>
> > To: llvmdev at cs.uiuc.edu
> > Sent: Tuesday, March 25, 2014 9:23:59 AM
> > Subject: Re: [LLVMdev] Alignment of pointee
> >
> > On 03/25/2014 10:08 AM, Krzysztof Parzyszek wrote:
> > > On 3/25/2014 8:53 AM, Frank Winter wrote:
> > >>
> > >> One can see that if the initial alignment of the pointee of %arg0
> > >> was 32
> > >> bytes and since the vectorizer operates on a loop with a fixed
> > >> trip
> > >> count of 4 and the size of double is 8 bytes, the vector loads and
> > >> stores could be ideally aligned with 32 bytes (which on my target
> > >> architecture would result in vector loads without additional
> > >> permutations.
> > >>
> > >> Is it somehow possible to achieve this? I am generating the IR
> > >> with the
> > >> builder, i.e. I am not coming from C or clang.
> > >
> > > If you are generating the loads and stores, you could just set the
> > > alignment to whatever you want, i.e. 32 bytes in your case.
> > I can't. Take a look again at the first piece of code. The loads
> > occur
> > in the 'inner' loop. Only for the first iteration the alignment of 32
> > bytes is true, not for iteration 2, 3 and 4. So, the alignment
> > information cannot enter at the point of loading. Thus, the idea of
> > attaching the information right at the pointer's definition, i.e. as
> > the
> > argument.
>
> I had a patchset that I had posted for review some months (a year?) ago to
> implement __builtin_assume_aligned. This ended up side-tracked in a
> discussion re: generalized invariants. We should, however, certainly
> revisit this soon.
>
>  -Hal
>
> > >
> > > I have wondered about it in a general case, when you simply want to
> > > have an alignment information on the pointer, and not on
> > > loads/stores.  My idea was to invent a builtin, something like
> > > "assert_aligned", that does nothing, other than manifest the
> > > alignment
> > > by the fact of its existence. For example:
> > >   %argp = call i8* llvm.assert.aligned(%arg0, 32)
> > > would state that the pointer %argp is aligned to 32 bytes, and the
> > > value of it is the same as %arg0 at the place of the "call".
> > >
> > > That was a while ago and maybe there are other ways of doing it
> > > now.
> > Should be doable this way. Although I am not sure whether a assertion
> > or
> > an annotation would be cleaner.
> >
> > There should already be a solution.
> > >
> > > -Krzysztof
> > >
> >
> >
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>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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