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Postings from August 2008
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. AllisonThread Previous