[LLVMdev] Alignment of pointee

[Hal Finkel]

----- 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