[llvm-dev] [GVN] same sequence of instructions in if and else branch

[Daniel Berlin via llvm-dev]

This GVN does not do that, this is correct. It is a very simple GVN. All
phi nodes are considered to have unique values.
GCC's GVN, and the one i'm developing for LLVM at
https://github.com/dberlin/llvm-gvn-rewrite  does the right thing.



On Tue, Feb 9, 2016 at 11:42 AM, Taewook Oh via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> There is a phi-node "%phi = phi i64 [%cast1, %if], [%cast2, %else]" in the
> common successor. The actual control flow is a bit more complex, but there
> is a common successor block, and %cast1 and %cast2 are the two values that
> the phi node in the common successor takes.
>
> Does the existence of the phi node affect optimization?
>
> Thanks,
> Taewook
>
>
> From: <mats.o.petersson at googlemail.com> on behalf of mats petersson <
> mats at planetcatfish.com>
> Date: Tuesday, February 9, 2016 at 11:34 AM
> To: Taewook Oh <twoh at fb.com>
> Cc: "llvm-dev at lists.llvm.org" <llvm-dev at lists.llvm.org>
> Subject: Re: [llvm-dev] [GVN] same sequence of instructions in if and
> else branch
>
> And there's no PHI node after this that depends on the condition?
>
> --
> Mats
>
> On 9 February 2016 at 19:30, Taewook Oh via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello,
>>
>>
>> I found that GVN doesn't promote identical sequence of instructions in if
>> and else branch to their common predecessors. For example, for the
>> following code snippet
>>
>>
>> pred:
>>
>> …
>>
>> br i1 %cmp, label %if, label %else
>>
>> if:
>>
>> %incdec.ptr.1 = getelementptr inbounds i8, i8* %ptr, i64 1
>>
>> %cast1 = ptrtoint i8* %incdec.ptr.1 to i64
>>
>>     …
>>
>> else:
>>
>> %incdec.ptr.2 = getelementptr inbounds i8, i8* %ptr, i64 1
>>
>> %cast2 = ptrtoint i8* %incdec.ptr.2 to i64
>>
>>
>> GVN doesn't move instructions in 'if' and 'else' blocks to 'pred' block
>> even though it knows that incdec.ptr.1/case1 has a same value number with
>> incdec.ptr.2/cast2. I see a case where this kind of redundancy confuses
>> following optimization passes and ends up generating suboptimal code.
>>
>>
>> From the GVN implementation, it seems that transformation is performed
>> only when the "leader" value dominates the other value, so it cannot handle
>> a case like the above example. I wonder if this is by design or just a
>> missing feature.
>>
>>
>> Thanks,
>>
>> Taewook
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__lists.llvm.org_cgi-2Dbin_mailman_listinfo_llvm-2Ddev&d=CwMFaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=kOsLCgQzH7N8ptZ7diJD9g&m=Xf5AAq_dBp5IcStlnft7nao-p-fDTN5AH6oItVXC3BA&s=4VUE3_dUQQ8AKzkWv5Tu6nJ979NtsOIq3qVC7CipHL8&e=>
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160209/982f7a70/attachment.html>