Geoffrey Broadwell via RT wrote:
What is the replacement for the old "regular" variants that use a pre-existing destination? A few years ago when I was doing copious Perl 5 PDL work, I found that in certain loops I would get bottlenecked entirely by creation and destruction of temporaries. I might be doing several dozen math operations per iteration, but I as the programmer knew that I only needed a handful of temporaries, and these could be created outside the loop. The vast majority of the object cycling was quite obviously wasted. In some cases, I could work around this by considerably uglifying the code and/or reaching through several layers of abstraction, but sometimes there was no recourse except to drop to PDL::PP (specially preprocessed C) or even C itself. I'd like to be able to write decently-performing math libraries in PIR, instead of having to drop down all the way to C. Being forced to create and destroy loads of temporaries I don't need will make this nigh impossible, let alone putting a major strain on the GC subsystem. Will there be some way to stay in PIR and still optimize away temporary cycling?
Compared to the cost of the multiple dispatch that all the math ops do, the cost of creating a temporary variable to hold the result is very minor.
That said, the semantics of whether the destination PMC is modified or replaced by a new one is determined by the particular sub selected by multiple dispatch. It's even possible (or will be once the MMD branch is merged back in) to define a non-multi vtable function in a particular PMC, to optimize away the cost of multiple dispatch.
Allison
