[Gretl-devel] Re: some devel news: using function pointers

Here's something I've been working on lately. tl;dr: gretl has just become substantially more efficient in respect of basic functions applied to each element of a big matrix or series. If you'd like the full story, read on. By "basic functions" I mean those that take a single floating-point value as argument and return a floating-point value, such as log, exp, fabs, all the C-library trigonometric functions, and a bunch of libgretl functions such as cnorm, dnorm, round, etc. Suppose you want to apply such functions to a big matrix. You can of course do that in hansl -- as in "matrix logM = log(M)" for a matrix M -- but what's going on in the background? (The C library has no "matrix" type and its log function applies strictly to single numeric values, with the sole exception of complex numbers if we consider those pairs as "not single"). We have to do the following (in pseudo-code): for each element x = M(i,j) of original matrix M   apply function f to x and get result y   set element R(i,j) of result matrix R to y end The question is, how do we implement the "apply function f" part? That is, how is "function f" identified? If it were identified by name ("log", "sin" and so on) we'd have to look up the function by name on each iteration of the loop. That's possible under POSIX but not, in general, on MS Windows (or not without severe contortions), and it would be horribly inefficient. Up till now we've identified f using a fixed ID number, associated with the name of the function via a look-up table. And the inner "apply" bit included a loop across these IDs. So, for example, if "log" mapped to ID 287 and "exp" to 288, that loop would include lines resembling   if (f == 287) return log(x);   if (f == 288) return exp(x); along with many other clauses of the same sort. That's much better than look-up by name, but gretl still has to wade through a bunch of "if (f == ID)" clauses to find what should be done per element of the matrix (or entry in the series). Solution: Instead of making "f" a symbolic ID of any sort that has to be looked up, make it an actual pointer-to-function (the memory address at which the function can be found). That's what we now do for such functions. In my experiments this reduces the time taken for scripts that are heavy on calls to "basic functions", as defined above, by roughly 25 to 45 percent. Artificial example: <hansl> matrix m = muniform(50, 50) set stopwatch loop 5000 -q   res = log(m)   res = exp(m)   res = sqrt(m) endloop printf "elapsed: %gs\n", $stopwatch </hansl> In current git you can turn off the use of function pointers, to compare with old-style behavior, by commenting out line 1340 of lib/src/gensyntax.c (with comment "attach function pointer, if available"). Allin _______________________________________________ Gretl-devel mailing list -- gretl-devel(a)gretlml.univpm.it To unsubscribe send an email to gretl-devel-leave(a)gretlml.univpm.it Website: https://gretlml.univpm.it/postorius/lists/gretl-devel.gretlml.univpm.it/