[LLVMdev] Passes calling

[John Criswell]

On 6/12/14, 11:41 AM, Chandler Carruth wrote:
>
> On Thu, Jun 12, 2014 at 5:23 PM, John Criswell <criswell at illinois.edu 
> <mailto:criswell at illinois.edu>> wrote:
>
>     On 6/12/14, 11:03 AM, Chandler Carruth wrote:
>>
>>     On Thu, Jun 12, 2014 at 4:45 PM, Hanbing Li <hanbing.li at inria.fr
>>     <mailto:hanbing.li at inria.fr>> wrote:
>>
>>         Hi,
>>
>>         I know when a pass needs other passes, it will use something
>>         like this: "AU.addRequired<LoopInfo>();". But this is usually
>>         used for the analysis passes.
>>         When a pass needs another transform pass, can it call it
>>         automatically?
>>         For example:
>>         Some loop optimizations may need loop-simplify, I need add
>>         -loop-simplify before calling these passes manually, or they
>>         can call the pass -loop-simplify automatically?
>>
>>
>>     Currently, the pass manager allows you (somewhat) to use the
>>     addRequired mechanism even with analyses. However, I strongly
>>     encourage you to not leverage it as it will eventually go away.
>
>     Just out of curiosity, why is this feature going away?
>
>
> Because it makes understanding the actual pass sequence horribly complex.
>
> With analyses, things are simple. Whatever order you run the analyses 
> in doesn't matter because they don't mutate anything. Each analysis 
> can depend on other analyses and you either have a cycle (fast to 
> detect and error on) or you have a DAG and there is a natural walk 
> that produces the set of desired analyses.
>
> Now consider when these passes are *transformations*. Now, the 
> relative order in which you run them can dramatically change the 
> results in unpredictable ways. There is no way for multiple "required" 
> transformation passes to be ordered cleanly without a tremendous 
> amount of machinery. All this is what led to LoopSimplify being run 4x 
> as many times as was necessary for a long time. =/

If I understand what you're saying, you're keeping the current 
functionality of being able to schedule *analysis* passes using 
AU.addRequired<>() but removing the ability to schedule *transform* 
passes via AU.addRequired<>(), correct?

Your original email implied that analysis passes would, in the future, 
not be scheduled via AU.addRequired<>().  I completely understand not 
scheduling transform passes that way.

Regards,

John Criswell

>
> Instead, everyone I've discussed this with (there have bene several 
> email threads about this) prefers that there is a single place (the 
> builder) which spells out the exact sequence of transforms applied.

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