[llvm-dev] LLVM Pass Managers

[Son Tuan VU via llvm-dev]

Hi Vedant,

Thanks for your reply. More comments inline.

Son Tuan Vu

On Fri, Apr 20, 2018 at 10:19 PM, Vedant Kumar <vsk at apple.com> wrote:

> Hi,
>
> + Chandler, who has a lot more experience with our pass managers.
>
> On Apr 20, 2018, at 12:56 PM, Son Tuan VU <sontuan.vu119 at gmail.com> wrote:
>
> + Vedant: what do you think about the last point, since Debugify is also
> related?
>
>
> Could you clarify the question? I'm not sure what it is you'd like me to
> chime in about.
>
> I guess you now know what I wanted to ask?

>
> 2, I've asked this question once but have had no answer, so I'm gonna
> revive it here: in *opt*, the *verify-each* mode is only activated when
> we provide a pass name or a pipeline (list of pass names).  I guess this is
> intended, but I expect that *verify-each* to insert *Verifier* to the
> whole pipeline after each optimization pass, even when I pass -O1/2/3/s/z
> to *opt*.  And I assume others do expect the same thing.
>
>
> I'd expect the same thing. I'm pretty sure it's a missing feature that
> -verify-each doesn't apply at opt -O1/etc.
>
>
> The 2nd question lead me to another idea: if we really want to make
> *verify-each* to work on any pipeline (not only user-provided ones), and
> we'll soon have a *debugify-each* option, should we refactor the code to
> have a *apply-after-each=pass-name* option that automatically inserts
> *pass-name *to the pipeline after each optimization pass?  We can even
> replace *print-after-all* option with
> *apply-after-each=PrintModulePassWrapper *for example?
>
>
> Speaking for myself, I'd rather type "opt -foo-each" over "opt
> -apply-after-each=foo".
>

I agree that "opt -foo-each" would be easier and more intuitive, but if
someone ever wants to apply foo after each pass, she will have to hack into
opt and write her own custom PassManager. Or maybe this situation is not
likely to happen?

>
> best,
> vedant
>
>
>
> Son Tuan Vu
>
> On Thu, Apr 19, 2018 at 6:14 PM, Philip Pfaffe <philip.pfaffe at gmail.com>
> wrote:
>
>> Hi Son,
>>
>> PassManagerBuilder is used to populate legacy PassManagers.  That role is
>> taken over by PassBuilder for new-PM passes.
>>
>> Cheers,
>> Philip
>>
>> 2018-04-18 13:40 GMT+02:00 Son Tuan VU <sontuan.vu119 at gmail.com>:
>>
>>> Hi Philip,
>>>
>>> Thank you for your reply. So what would be the right way/API to write
>>> out-of-tree pass? I've been using *PassManagerBuilder*, which requires
>>> a callback refering to *legacy::PassManagerBase*, so I guess this will
>>> be no longer the good way to go?
>>>
>>> Son Tuan Vu
>>>
>>> On Tue, Apr 17, 2018 at 6:06 PM, Philip Pfaffe <philip.pfaffe at gmail.com>
>>> wrote:
>>>
>>>> Hi Son,
>>>>
>>>> I have an answer to your first question:
>>>>
>>>> 1, What are the differences between *LegacyPassManager* and
>>>>> *PassManager*?  I see that *opt* uses the former most of the times
>>>>> while the latter is used via *PassBuilder* API when an user wants to
>>>>> build her own pipeline, but I have no idea why so. What to use and when to
>>>>> use it is not clear to me.
>>>>>
>>>> PassManager is the result of a long going effort to replace the default
>>>> pass manager of opt and clang. Here's the original RFC containing the
>>>> motivation: http://lists.llvm.org/pipermai
>>>> l/llvm-dev/2012-July/051643.html
>>>>
>>>> LegacyPassManager is currently still the default, but we are really
>>>> close to switching over the the new one. Both opt and clang have switches
>>>> to enable the new one, by calling `opt -passes="...."` or `clang
>>>> -fexperimental-new-pass-manager`. PassBuilder is essentially a big
>>>> factory to produce PassManager and AnalysisManager objects for the passes
>>>> and analyses included in core LLVM. If you develop analyses and passes, I'd
>>>> suggest doing so against the new PM, especially if you plan on upstreaming
>>>> your work.
>>>>
>>>> Cheers,
>>>> Philip
>>>>
>>>
>>>
>>
>
>
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