[LLVMdev] Alignment of pointee

[Hal Finkel]

----- Original Message -----
> From: "Arnaud Allard de Grandmaison" <arnaud.adegm at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Frank Winter" <fwinter at jlab.org>, "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Tuesday, March 25, 2014 12:21:53 PM
> Subject: Re: [LLVMdev] Alignment of pointee
> 
> 
> 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.

Also, FWIW, at least one user of mine is a big fan of Intel's new align_value attribute (which is, at least, discussed here: http://software.intel.com/en-us/articles/data-alignment-to-assist-vectorization), because it plays well with TMP and other related C++ programming techniques (better than __builtin_assume_aligned).

 -Hal

> 
> 
> 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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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory