D26130: Simplify return logic

[Kevin Ottens]

ervin added a comment.


  In D26130#582391 <https://phabricator.kde.org/D26130#582391>, @tcanabrava wrote:
  
  > In D26130#582343 <https://phabricator.kde.org/D26130#582343>, @ervin wrote:
  >
  > > This is pointless, most compilers would do NRVO (+ move assign) here... which would be neatly obliterated by the ternary operator. This is a pessimisation you're proposing here, not an optimization.
  >
  >
  > I don't know what you are talking about, this is using return time optimization. have you tested the code or just assumed that ternaries will not do NRVO?
  
  
  OK, let me try again.
  
  I did benchmark old and new before my first reply (and I wonder why I wrote pessimisation previously, I think I got carried away a bit, apologies). What I meant was that: if and when there is a small measurable gain, I don't think it is as dramatic as you make it sound. The point I was trying to make was that since the ternary will prevent NRVO, you're trading one return optimization (NRVO) for another (RVO). In other words, the same amount of objects will be involved. It's far from the "there was no return optimization now there's one" that I understood you're trumping. Thus the gain between the two versions when there is, is unlikely to come from RVO. It is mostly about avoiding the move assignment in the newer version (roughly a couple pointers copy).
  
  Now, the **really** interesting bit you did is wrapping "d->" in QStringLiteral, that's what gives a large gain for the dpointer case (when I did the benchmarking I did it with QStringLiteral in both versions, otherwise I knew it was just unfair).
  
  Obviously I'd be totally cool with just a QStringLiteral change since it would have real value (divides time by three roughly, this is definitely a large gain).

REPOSITORY
  R237 KConfig

REVISION DETAIL
  https://phabricator.kde.org/D26130

To: tcanabrava, patrickelectric, ervin
Cc: ervin, kde-frameworks-devel, LeGast00n, GB_2, michaelh, ngraham, bruns
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