[llvm-dev] Changing Alignment of global variables in LLVM

[John Criswell via llvm-dev]

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

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