[llvm-commits] Dead argument elimination rewrite

[Dan Gohman]

On Jun 17, 2008, at 1:44 AM, Matthijs Kooijman wrote:

> Hi Dan,
>
>> I don't think it's a high priority, but a "flatten return values"  
>> pass
>> would be nice to have.
> I have one lying around that is general enough IIRC, I'll only have  
> to pull it
> out of our local namespace into llvm :-)

Cool, thanks!

>
>
>>
>> UseMap is a std::multimap that maps every argument and return value
>> to every argument or return value that it "uses". That caught my
>> eye as a potential scalability suspect. Have you done any testing
>> on large inputs?
> I've done some testing on the basictest.ll testcase, copied about a  
> hundred
> times (in the same file). Execution times are very similar to the  
> old version
> (around 43ms according to -time-passes). I've also did the same  
> thing to a new
> testcase that has a lot of unused return values (parts of struct  
> returns).
> Here, the new code needs over twice as much time, but that's mainly  
> due to it
> removing stuff that the old code didn't.
>
> I also tried using a hash_multimap instead of a normal multimap, but  
> that
> didn't seem to make much of a difference. Perhaps on even larger  
> inputs, but
> I'll leave it at multimap for now since the hash version is non- 
> standard.

Ok. Would you mind taking a look at timings on
MultiSource/Applications/kimwitu++
also?

>
>
>> The InspectedFunctions member variable looks redundant. Can it be
>> removed?
> I added it, to prevent infinite looping. When removing stuff from a  
> function,
> a new function is created with the new function type. This variable  
> helps to
> prevent the new function from being stripped as well.
>
> Alternatively, I could make a copy of the function list and iterate  
> over that,
> which should have the same effect.

When DAE creates a new function, it inserts the function into the list  
in the
position of the old function, so as far as I can tell DAE shouldn't  
ever end
up calling RemoveDeadStuffFromFunction on one of the new functions.

Dan