[PATCH] D15676: [attrs] Extract the pure inference of function attributes into a standalone pass.

[Philip Reames via llvm-commits]

I agree w/James perspective here.  In particular, I feel like the issue 
with norecurse is how we chose to specify it and not the notion itself 
or the implementation thereof.  We should raise some of the issues on 
llvm-dev and hammer through them, but I don't think we need to revert 
anything in the meantime.  This doesn't feel like a major problem or the 
fundamentally wrong approach.

Philip

On 12/21/2015 11:51 AM, James Molloy via llvm-commits wrote:
> Hey Chandler,
>
> I'll think this through and give a more reasoned reply in the morning. 
> However I would just say that I'm not opposed to an alternative 
> implementation that meets the same ends.
>
> I did put a lot of effort into this current solution though and it is 
> *significantly* less broken than what was in LLVM before (main is non 
> recursive, unconditionally!!). It currently gives nontrivial speed ups 
> to workloads we care about, so ripping it all out before designing 
> something new (likely taking at least a month given the season) isn't 
> massively appealing.
>
> Cheers,
>
> James
> On Mon, 21 Dec 2015 at 19:40, Chandler Carruth <chandlerc at gmail.com 
> <mailto:chandlerc at gmail.com>> wrote:
>
>     I'm getting the strong indication that norecurse isn't actually a
>     good idea, and hasn't been hammered through enough.
>
>     It turns out that I also need to build brand new infrastructure to
>     keep supporting it.
>
>     I'm ripping it out of this patch clearly, but I am wondering
>     whether we want to rip it out entirely and start a new discussion
>     on the actual problem James is trying to solve and the best way to
>     go about it.
>
>     Note that, for example, if strlen and memcpy can't be marked as
>     norecurse (and I agree that they can't in a strict sense as it is
>     conforming to implement both of them as recursive algorithms) then
>     *any* inference of norecurse in LLVM is actually broken today
>     because we may add calls to non-norecurse functions (namely
>     memcpy) at a later point. Given that, I think the spec for
>     norecurse is too narrow to be a terribly useful property. I
>     suspect that James will need to use some other strategy to get the
>     useful optimizations he's after (which center around *main* not
>     being recursive, a very different beast).
>
>     -Chandler
>
>     On Mon, Dec 21, 2015 at 10:09 AM Mehdi Amini
>     <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>
>
>
>         Sent from my iPhone
>
>         On Dec 21, 2015, at 12:07 AM, James Molloy
>         <james at jamesmolloy.co.uk <mailto:james at jamesmolloy.co.uk>> wrote:
>
>>         Hi Mehdi,
>>
>>         > For strlen: you’re right that it is not correct in the
>>         absolute, but for what we use the information I’m not sure it
>>         matters. We’re interested in the fact that it won’t call back
>>         in the user code in any way, so that we can infer the
>>         norecurse on the callers.
>>
>>         I understand where you're coming from, but I think
>>         second-guessing how an attribute may be used is a dangerous
>>         route to go down. "norecurse" could be used for something
>>         else in the future, so unless we have a good reason it would
>>         be good not to make assumptions.
>>
>
>         Agree, but if we go on the "safe way" now it means we should
>         either redefine the norecurse attribute (like the
>         almostreadnone for example) or have another one.
>
>         Mehdi
>
>
>
>>         James
>>
>>         On Mon, 21 Dec 2015 at 10:06 James Molloy via llvm-commits
>>         <llvm-commits at lists.llvm.org
>>         <mailto:llvm-commits at lists.llvm.org>> wrote:
>>
>>             jmolloy accepted this revision.
>>             jmolloy added a comment.
>>             This revision is now accepted and ready to land.
>>
>>             Hi Chandler,
>>
>>             Thanks for taking the time to do this.
>>
>>             I agree with Philip - while I like that you've added
>>             extra norecurse attributes, I don't like that it's
>>             smuggled in as part of this patch. I'd highly prefer this
>>             patch to be NFC and then a simple update patch adding the
>>             norecurse attributes.
>>
>>             Apart from that and the specific comment below, it LGTM.
>>
>>             Cheers,
>>
>>             James
>>
>>
>>             ================
>>             Comment at:
>>             include/llvm/Transforms/IPO/InferFunctionAttrs.h:25
>>             @@ +24,3 @@
>>             +/// A pass which infers function attributes from the
>>             names and signatures of
>>             +/// function declarations in a module..
>>             +class InferFunctionAttrsPass {
>>             ----------------
>>             Double period.
>>
>>             ================
>>             Comment at:
>>             include/llvm/Transforms/IPO/InferFunctionAttrs.h:26
>>             @@ +25,3 @@
>>             +/// function declarations in a module..
>>             +class InferFunctionAttrsPass {
>>             +public:
>>             ----------------
>>             Unrelated: Damn, new style passes look so much nicer!
>>
>>
>>             http://reviews.llvm.org/D15676
>>
>>
>>
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>
>
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