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<p class=MsoNormal>Hi,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>One of PlumHall’s testcases generates a function with
half a million SDNodes. This brings out the O(n) complexity in DAGCombiner::AddToWorkList()
and means that in debug mode the test has not been seen to complete compilation,
and in release mode it takes several hours. KCacheGrind showed 99% of the time
is being spent in std::remove, doing linear scans of the DAGCombiner’s
worklist during AddToWorkList.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The problem the algorithm is trying to solve is: When
inserting node N into the worklist, check the worklist to see if it contains N.
If so, remove it. Then add N to the end of the worklist (first to be popped).<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Reducing this to log(n) time requires use of a set and a
vector, as essentially you have two orderings. I wanted to use SetVector for
this purpose but SetVector::insert has a different semantic to what I need. On
insert, if N already exists it will do nothing (i.e. N will not be the first
item to be popped from the vector). This breaks the DAGCombiner.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>My solution was to allow duplicate items in the vector, and
on every pop to query the set to see if the item should actually be in the
vector. If not, discard and grab the next item. This means that the vector
could potentially grow larger than it would previously.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>I’ve implemented this directly in DAGCombiner without adding
a separate ADT. If people feel this is worthwhile, I could add another ADT or
template-specialise SetVector to have this behaviour? (I’m not too keen
on template specialization changing semantics however).<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>This change means that in release mode the testcase with
half a million nodes compiles in 30secs, and around 2 minutes in debug mode.<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Please review!<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Cheers,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>James<o:p></o:p></p>
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