[LLVMdev] Dead Code Elimination and undef values

[Cristianno Martins]

Hi Duncan,

Sorry for my late reply.

First, since I had already the information about which instructions should
be deleted, I ended up writing a function that just erase all the useless
instructions. Besides that, I actually think the problem with dce was being
cause by my pass: I forgot to return "true" at the end of it, and I guess
the PassManager was not calling dce after it because my pass was telling
the PassManager the code was not being modified. If this is the case, there
is no bug to report, right?

Either way, thank you for your reply =),


--
Cristianno Martins
PhD Student of Computer Science
University of Campinas
cmartins at ic.unicamp.br
<cristiannomartins at hotmail.com>


On Fri, May 31, 2013 at 10:26 PM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Cristianno,
>
>
> On 01/06/13 01:49, Cristianno Martins wrote:
>
>> Hello there,
>>
>> I'm writing a transformation pass for LLVM, and I hoped to use dce to
>> clean up
>> the resulting code after my pass. I just have some questions about LLVM's
>> dce
>> implementation.
>>
>> Well, my transformation is a function pass, and, after the changes are
>> made,
>> some instructions are not needed anymore. In order to easily get rid of
>> those
>> instructions, I'm setting all their uses to UndefValue. This is necessary
>> because some of the instructions I want to erase are inside loops, and,
>> therefore, there might be a circular dependence between the instruction I
>> want
>> to delete and a PHINode, for example, when both of them are not useful
>> anymore.
>>
>> Ok, the problem is: I've always had explicitly invoked opt enabling, at
>> least,
>> -dce -adce -globaldce and -die, and I'm still getting a resultant code
>> with some
>> instructions like:
>>
>> store i8 undef, i8* %out.1107, align 1, !tbaa !1
>>
>> and
>>
>> %storemerge = phi i8 [ %conv46, %if.else ], [ %call, %if.then ],
>> where %storemerge has no uses.
>>
>> So, is there any other dce variant I should be enabling for opt? Better
>> yet, is
>> this the correct behavior for all the dce variants I enabled in opt?
>>
>
> try running the instcombine pass too.  I'm surprised that dce didn't get
> the phi
> node though, want to open a bugreport about that?
>
> Ciao, Duncan.
>
>
>> Thanks,
>>
>> --
>> Cristianno Martins
>> PhD Student of Computer Science
>> University of Campinas
>> cmartins at ic.unicamp.br <mailto:cmartins at ic.unicamp.br**>
>> <mailto:cristiannomartins@**hotmail.com <cristiannomartins at hotmail.com>>
>>
>>
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