I noticed in soundmanager.cpp that two inner functions use copies rather than references:
QHash<AudioOutput, const CSAMPLE*> requestBuffer(QList<AudioOutput> outputs, unsigned long iFramesPerBuffer, SoundDevice *device, double streamTime = 0);
void pushBuffer(QList<AudioInput> inputs, short *inputBuffer, unsigned long iFramesPerBuffer, unsigned int iFrameSize);
Note that a copy of the QList is made on every call.
changing these to const references appears to have a nice impact on performance:
Before: Debug [Main]: Stat("SoundDevicePortAudio::callbackProcess output 0, HDA Intel: STAC92xx Analog (hw:0,0)","count=478,sum=2.15742e+08ns,average=451343ns,min=132001ns,max=1.77272e+06ns,variance=7.36831e+10ns^2,stddev=271446ns")
After:
Debug [Main]: Stat("SoundDevicePortAudio::callbackProcess output 0, HDA Intel: STAC92xx Analog (hw:0,0)","count=452,sum=1.35353e+08ns,average=299454ns,min=79619ns,max=1.36219e+06ns,variance=4.88806e+10ns^2,stddev=221089ns")
patch included
Huh - assuming this is not just noise that's a nice win. Qt has a feature called implicit sharing which prevents copying the QList. Instead, it keeps a reference count and the copy constructor shares data across all the QLists until one of them is modified at which point it transparently copies the list right before the modification. So unless Qt is mistakenly thinking we're modifying the list and detaching it, I think this doesn't actually copy the elements of the list.
What this does save on is incrementing the reference count and creating a new QList wrapper on the stack. If the measurements aren't noise then this could warrant a quick scan through the engine code to get rid of other uses of implicit sharing.
Patch looks good for 1.11.0 to me -- logic-wise it's a no-op.