[llvm] r204857 - Simplify PBQP graph removeAdjEdgeId implementation.

[David Blaikie]

On Wed, Mar 26, 2014 at 3:16 PM, Lang Hames <lhames at gmail.com> wrote:
> Correct on all points. Well - I made the last cosmetic fixes - I think it's
> your turn to make these. ;)

r204983

> It's about time you got your name on the blame list for this thing anyway.

And now I am eternally tainted.

As an aside, you could write setAdjEdgeIdx like this:

ThisEdgeAdjIdxs[NId == NIds[0]] = NewIdx;

Skipping the conditionals. Not sure if LLVM's smart enough to do that
optimization for you - I hope so. (& guess if it does, you prefer the
way it's currently written, for legibility - which I'm OK with)

I've seen this sort of "boolean test as index into two element array"
as semi-idiomatic in various tree structures, for example.

- David

> On Wed, Mar 26, 2014 at 2:55 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Wed, Mar 26, 2014 at 2:21 PM, Lang Hames <lhames at gmail.com> wrote:
>> > Author: lhames
>> > Date: Wed Mar 26 16:21:53 2014
>> > New Revision: 204857
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=204857&view=rev
>> > Log:
>> > Simplify PBQP graph removeAdjEdgeId implementation.
>> >
>> > Modified:
>> >     llvm/trunk/include/llvm/CodeGen/PBQP/Graph.h
>> >
>> > Modified: llvm/trunk/include/llvm/CodeGen/PBQP/Graph.h
>> > URL:
>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/CodeGen/PBQP/Graph.h?rev=204857&r1=204856&r2=204857&view=diff
>> >
>> > ==============================================================================
>> > --- llvm/trunk/include/llvm/CodeGen/PBQP/Graph.h (original)
>> > +++ llvm/trunk/include/llvm/CodeGen/PBQP/Graph.h Wed Mar 26 16:21:53
>> > 2014
>> > @@ -67,16 +67,16 @@ namespace PBQP {
>> >          return Idx;
>> >        }
>> >
>> > -      // If a swap is performed, returns the new EdgeId that must be
>> > -      // updated, otherwise returns invalidEdgeId().
>> > -      EdgeId removeAdjEdgeId(AdjEdgeIdx Idx) {
>> > -        EdgeId EIdToUpdate = Graph::invalidEdgeId();
>> > -        if (Idx < AdjEdgeIds.size() - 1) {
>> > -          std::swap(AdjEdgeIds[Idx], AdjEdgeIds.back());
>> > -          EIdToUpdate = AdjEdgeIds[Idx];
>> > -        }
>> > +      void removeAdjEdgeId(Graph &G, NodeId ThisNId, AdjEdgeIdx Idx) {
>> > +        // Swap-and-pop for fast removal.
>> > +        //   1) Update the adj index of the edge currently at back().
>> > +        //   2) Swap Edge at Idx with back().
>> > +        //   3) pop_back()
>> > +        // If Idx == size() - 1 then the updateAdjEdgeIdx and swap are
>> > +        // redundant, but both operations are cheap.
>> > +        G.getEdge(AdjEdgeIds.back()).updateAdjEdgeIdx(ThisNId, Idx);
>>
>> Is there more to this "updateAdjEdgeIdx" function or is it just a
>> trivial (indexed/mapped) setter? "update" rather than "set" makes me
>> wonder what work it's doing...
>>
>> > +        std::swap(AdjEdgeIds[Idx], AdjEdgeIds.back());
>>
>> Since you're about to pop the back anyway, this could just be
>> AdjEdgeIds[Idx] = AdjEdgeIds.back(); right? (if they were movable, you
>> could add a std::move() around the RHS, but they're just indexes,
>> right?)
>>
>> (& comment 2 would be "move back() down to fill the hole at Idx" or
>> something like that)
>>
>> >          AdjEdgeIds.pop_back();
>> > -        return EIdToUpdate;
>> >        }
>> >
>> >        const AdjEdgeList& getAdjEdgeIds() const { return AdjEdgeIds; }
>> > @@ -138,9 +138,7 @@ namespace PBQP {
>> >          assert(ThisEdgeAdjIdxs[NIdx] !=
>> > NodeEntry::getInvalidAdjEdgeIdx() &&
>> >                 "Edge not connected to NIds[NIdx].");
>> >          NodeEntry &N = G.getNode(NIds[NIdx]);
>> > -        EdgeId EIdToUpdate = N.removeAdjEdgeId(ThisEdgeAdjIdxs[NIdx]);
>> > -        if (EIdToUpdate != Graph::invalidEdgeId())
>> > -          G.getEdge(EIdToUpdate).updateAdjEdgeIdx(NIds[NIdx],
>> > ThisEdgeAdjIdxs[NIdx]);
>> > +        N.removeAdjEdgeId(G, NIds[NIdx], ThisEdgeAdjIdxs[NIdx]);
>> >          ThisEdgeAdjIdxs[NIdx] = NodeEntry::getInvalidAdjEdgeIdx();
>> >        }
>> >
>> >
>> >
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>