llvm dev - Oct 2017 - Changing Alignment of global variables in LLVM

If I know for sure I am accessing 32 byte chunks at a time, how can I go
about changing the alignment of @u?

Should I use DataLayout's reset method? I couldn't find a method to
change
alignment of one global variable.

Thanks

On Tue, Oct 3, 2017 at 6:34 PM, Matthias Braun <mbraun at apple.com>
wrote:
> The effective alignment is part of the load and store operations. Updating
> those would mean that you need to know which ones are based on the @u
> address.
>
> You can come up with some heuristics that track values through
> getelementptr and similar constructs in llvm IR and which will work in most
> cases.
>
> However as you can have arbitrary computations to create the address for a
> load/store tracking the source of such computations is actually undecidable
> in general and you may have cases where you cannot determine the origin
> when allowing arbitrary IR.
> Because of this I'd recommend to rather adjust the frontends type
system
> so the loads and stores are created with the correct alignment in the first
> place.
>
> - Matthias
>
> > On Oct 3, 2017, at 3:25 PM, Charith Mendis via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> >
> > What is the best way to change the alignment of global variables and
> allocated structures in LLVM during one of its optimization passes?
> >
> >
> > For example, I want to change,
> >
> > @u = internal unnamed_addr global [5 x [65 x [65 x [65 x double]]]]
> zeroinitializer, align 16
> >
> > to align to 32 bytes.
> >
> > How can this be accomplished so that all other references in the code
> accessing this structure are also updated.
> >
> > Thank You
> >
> > --
> > Kind regards,
> > Charith Mendis
> >
> > Graduate Student,
> > CSAIL,
> > Massachusetts Institute of Technology
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>

-- 
Kind regards,
Charith Mendis

Graduate Student,
CSAIL,
Massachusetts Institute of Technology
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> On Oct 3, 2017, at 3:39 PM, Charith Mendis <char.mendis1989 at
gmail.com> wrote:
> 
> If I know for sure I am accessing 32 byte chunks at a time, how can I go
about changing the alignment of @u?
> 
> Should I use DataLayout's reset method? I couldn't find a method to
change alignment of one global variable.
GlobalObject::setAlignment()?

- Matthias
> 
> Thanks
> 
> On Tue, Oct 3, 2017 at 6:34 PM, Matthias Braun <mbraun at apple.com
<mailto:mbraun at apple.com>> wrote:
> The effective alignment is part of the load and store operations. Updating
those would mean that you need to know which ones are based on the @u address.
> 
> You can come up with some heuristics that track values through
getelementptr and similar constructs in llvm IR and which will work in most
cases.
> 
> However as you can have arbitrary computations to create the address for a
load/store tracking the source of such computations is actually undecidable in
general and you may have cases where you cannot determine the origin when
allowing arbitrary IR.
> Because of this I'd recommend to rather adjust the frontends type
system so the loads and stores are created with the correct alignment in the
first place.
> 
> - Matthias
> 
> > On Oct 3, 2017, at 3:25 PM, Charith Mendis via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> >
> >
> > What is the best way to change the alignment of global variables and
allocated structures in LLVM during one of its optimization passes?
> >
> >
> > For example, I want to change,
> >
> > @u = internal unnamed_addr global [5 x [65 x [65 x [65 x double]]]]
zeroinitializer, align 16
> >
> > to align to 32 bytes.
> >
> > How can this be accomplished so that all other references in the code
accessing this structure are also updated.
> >
> > Thank You
> >
> > --
> > Kind regards,
> > Charith Mendis
> >
> > Graduate Student,
> > CSAIL,
> > Massachusetts Institute of Technology
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> 
> 
> 
> 
> -- 
> Kind regards,
> Charith Mendis
> 
> Graduate Student,
> CSAIL,
> Massachusetts Institute of Technology
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On 10/3/17 6:41 PM, Matthias Braun via llvm-dev wrote:
>
>> On Oct 3, 2017, at 3:39 PM, Charith Mendis <char.mendis1989 at
gmail.com
>> <mailto:char.mendis1989 at gmail.com>> wrote:
>>
>> If I know for sure I am accessing 32 byte chunks at a time, how can I 
>> go about changing the alignment of @u?
>>
>> Should I use DataLayout's reset method? I couldn't find a
method to
>> change alignment of one global variable.
> GlobalObject::setAlignment()?
This is what we used in BBAC and PAMD (ISMM 2017); these systems 
transform globals to align them on a power-of-two boundary.

Regards,

John Criswell
>
> - Matthias
>
>>
>> Thanks
>>
>> On Tue, Oct 3, 2017 at 6:34 PM, Matthias Braun<mbraun at apple.com 
>> <mailto:mbraun at apple.com>>wrote:
>>
>>     The effective alignment is part of the load and store operations.
>>     Updating those would mean that you need to know which ones are
>>     based on the @u address.
>>
>>     You can come up with some heuristics that track values through
>>     getelementptr and similar constructs in llvm IR and which will
>>     work in most cases.
>>
>>     However as you can have arbitrary computations to create the
>>     address for a load/store tracking the source of such computations
>>     is actually undecidable in general and you may have cases where
>>     you cannot determine the origin when allowing arbitrary IR.
>>     Because of this I'd recommend to rather adjust the frontends
type
>>     system so the loads and stores are created with the correct
>>     alignment in the first place.
>>
>>     - Matthias
>>
>>     > On Oct 3, 2017, at 3:25 PM, Charith Mendis via llvm-dev
>>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>     >
>>     >
>>     > What is the best way to change the alignment of global
>>     variables and allocated structures in LLVM during one of its
>>     optimization passes?
>>     >
>>     >
>>     > For example, I want to change,
>>     >
>>     > @u = internal unnamed_addr global [5 x [65 x [65 x [65 x
>>     double]]]] zeroinitializer, align 16
>>     >
>>     > to align to 32 bytes.
>>     >
>>     > How can this be accomplished so that all other references in
>>     the code accessing this structure are also updated.
>>     >
>>     > Thank You
>>     >
>>     > --
>>     > Kind regards,
>>     > Charith Mendis
>>     >
>>     > Graduate Student,
>>     > CSAIL,
>>     > Massachusetts Institute of Technology
>>     > _______________________________________________
>>     > LLVM Developers mailing list
>>     >llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>     >http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>
>>
>>
>>
>> --
>> Kind regards,
>> Charith Mendis
>>
>> Graduate Student,
>> CSAIL,
>> Massachusetts Institute of Technology
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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