[PATCH] D18798: New code hoisting pass based on GVN

[Daniel Berlin via llvm-commits]

On Fri, Apr 8, 2016 at 10:36 PM, Daniel Berlin <dberlin at dberlin.org> wrote:

>
>
> On Fri, Apr 8, 2016 at 3:03 PM, Sebastian Pop <sebpop at gmail.com> wrote:
>
>> sebpop removed rL LLVM as the repository for this revision.
>> sebpop updated this revision to Diff 53095.
>> sebpop added a comment.
>>
>> In http://reviews.llvm.org/D18798#394160, @chandlerc wrote:
>>
>> > - We also would likely need to see a reasonable analysis of the effect
>> this has on compile time so we understand the tradeoff this is making. My
>> suspicion is that it won't be the correct tradeoff for O2 if it is correct
>> at any level. (See below.) [...]  It makes very little sense IMO to do
>> early-cse and then early-gvn... The only point of early-cse was to be a
>> substantially lighter weight CSE than GVN. If you're going to run GVN
>> anyways to do hoisting, just run GVN.
>>
>>
>> I think there is a misconception of "GVN == PRE":
>
>
> No, it's GVN == GVN + elimination in LLVM.
>
> this is because GVN.cpp was
>> around for a while and the "GVN pass" is a "redundancy elimination" pass.
>>
>> That is because it is in LLVM :)
>
>
>> To clarify matters, GVN is an analysis pass,
>
>
> In the theoretical sense, yes.
> But, as you know, it's not, as currently implemented in LLVM.
> Analysis passes do not modify code.
> GVN modifies code.
> As it runs, it deletes code, it simplifies instruction, etc.
> What you are using is *not* GVN.
> You have taken a very small part of GVN, and reused it.
>
> In particular, you've taken the hash table.  But the hash table only
> barely performs value numbering.
>
> In fact, current GVN relies *very* heavily on it's current model of
> simplification + elimination as it goes to perform *value numbering*.
>
> What you are using will miss *very* obvious cases.
> It's a very trivial value numberer.
>
> (For example, without elimination-as-it-goes, given
>
> c = a + 1
> d = c + 1
> e = a + 2
>
> It will never discover e and c have the same value, etc)
>
>

^^^ e and d
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