[LLVMdev] Alignment of pointee

[Hal Finkel]

Hi Frank,

Two things:

 1. For function parameters, you can now provide the align attribute for pointer types so specify the pointer alignments (see the language reference).

 2. The llvm.assume intrinsic will provide this feature. The necessary patches are undergoing review. I expect them to land within the next month.

Thanks again,
Hal

----- Original Message -----
> From: "Frank Winter" <fwinter at jlab.org>
> To: "Hal Finkel" <hfinkel at anl.gov>, "Arnaud Allard de Grandmaison" <arnaud.adegm at gmail.com>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Friday, August 8, 2014 10:55:36 AM
> Subject: Re: [LLVMdev] Alignment of pointee
> 
> What's the status of providing alignment information for a pointer?
> Can
> I build an aligned pointer, e.g. a pointer with its pointee 32 bytes
> aligned (float*) and use it as a function's parameter? This would be
> very helpful for optimization passes to get the alignment info on
> load/stores right. Which in turn is a crucial performance feature
> (and
> on some architectures necessary to generate legal vectorized code).
> 
> Best wishes,
> Frank
> 
> 
> On 03/25/2014 01:29 PM, Hal Finkel wrote:
> > ----- 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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> >>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>>
> >> --
> >> Hal Finkel
> >> Assistant Computational Scientist
> >> Leadership Computing Facility
> >> Argonne National Laboratory
> >>
> >>
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >>
> >>
> 
> 

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