Besides being able to assign the function result to constants, the main benefit is that, for constant inputs, the result is deterministic and so the compiler can calculate this beforehand and replace a function call with an assignment. For example, if tan was a regular function, x := tan(Pi / 2) might look like this in assembly:
MOVAPD XMM0, Pi DIVSD XMM0, TwoConstant CALL tan ; Result is now in XMM0, which is 1 It won't be exactly that, but you get the idea. If tan is defined as a pure function, then the result can be pre- calculated and the entire block changed to a simple assignment: MOVAPD XMM0, OneConstant ; value of tan(Pi / 2) It doesn't take a genius to know that that is faster by several orders of magnitude. More subtly, if it turns out that the function is never called with variable arguments, the entire function can be removed from the compiled binary, reducing its size. Gareth aka. Kit On Mon 09/07/18 10:25 , Sven Barth via fpc-devel fpc-devel@lists.freepascal.org sent: > Ryan Joseph schrieb am Mo., 9. Juli 2018, 08:45: > > > On Jul 8, 2018, at 8:50 AM, J. Gareth Moreton wrote: > > > > With some blessing from Florian on the concept, I've set up a Wiki page > discussing the design proposals for the support of pure functions, as well > as some explanation on what they actually are. > > > > What are the performance benefits? It sounds like this is a proposal for a > compiler optimization which we can explicitly opt in to, but what exactly > is the optimization? > > If nothing else I like the idea as a way to enforce a function is not > accessing global state. Kind of like const for functions. > > It would allow you to declare constants that use those functions with the > compiler evaluating them at compile time. Also the compiler might be able > to optimize such functions better especially if they're inlined. *shrugs* > Regards, Sven __________________________________________ _____ > fpc-devel maillist - fpc- de...@lists.freepascal.org > http://lists.freepascal.org/cgi- bin/mailman/listinfo/fpc-devel [1] > > > > Links: > ------ > [1] > http://secureweb.fast.net.uk/parse.php? redirect=http://lists.freepascal.org > /cgi-bin/mailman/listinfo/fpc-devel > _______________________________________________ fpc-devel maillist - fpc-devel@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
