[Rd] Why is as.function() slower than eval(call("function"())?

[Duncan Murdoch]

On 04/08/2017 12:32 AM, Gregory Werbin wrote:
> (Apologies if this is better suited for R-help.)
>
> On my system (macOS Sierra, late 2014 MacBook Pro; R 3.4.1, Homebrew build), I found that it is faster to construct a function using eval(call("function", ...)) than using as.function(list(...)). Example:
>
>     make_fn_1 <- function(a, b) eval(call("function", a, b), env = parent.frame())
>     make_fn_2 <- function(a, b) as.function(c(a, list(b)), env = parent.frame())
>
>     a <- as.pairlist(alist(x = , y = ))
>     b <- quote(x + y)
>
>     library("microbenchmark")
>     microbenchmark(make_fn_1(a, b), make_fn_2(a, b))
>
>     # Unit: microseconds
>     #             expr   min     lq    mean median     uq    max neval cld
>     #  make_fn_1(a, b) 1.671 1.8855 2.13297  2.039 2.1950  9.852   100  a
>     #  make_fn_2(a, b) 3.541 3.7230 4.13400  3.906 4.1055 23.153   100   b
>
> At first I thought the gap was due to the overhead of calling c(a, list(b)). But this turns out not to be the case:
>
>     make_fn_weird <- function(a, b) as.function(c(a, b), env = parent.frame())
>     b_wrapped <- list(b)
>
>     make_fn_weirder <- function(a_b) as.function(a_b, env = parent.frame())
>     a_b <- c(a, b_wrapped)
>
>     microbenchmark(make_fn_1(a, b), make_fn_2(a, b),
>                    make_fn_weird(a, b_wrapped), make_fn_weirder(a_b))
>
>     # Unit: microseconds
>     #                         expr   min     lq    mean median     uq    max neval cld
>     #              make_fn_1(a, b) 1.718 1.8990 2.12119 1.9860 2.1605  8.057   100 a
>     #              make_fn_2(a, b) 3.393 3.5865 4.03029 3.6655 3.9615 27.499   100   c
>     #  make_fn_weird(a, b_wrapped) 3.354 3.5005 3.77190 3.6405 3.9425  6.839   100   c
>     #         make_fn_weirder(a_b) 2.488 2.6290 2.83352 2.7215 2.8800  7.007   100  b
>
> One IRC user pointed out that as.function() takes its own path through the code, namely do_asfunction() (in src/main/coerce.c). What is it about this code path that's 50% slower than whatever happens during eval(call("function", a, b))?
>
> Obviously this is a trivial micro-optimization and it doesn't matter to 99% of users. Mostly asking out of curiosity, but also wondering if there's a more general lesson to be learned here.

The main difference is that `function` is a primitive, while 
as.function() is a generic.  You will get much closer timing if you skip 
the method dispatch by calling as.function.default() directly.

The next part of the difference is that as.function.default is a regular 
R closure:

as.function.default <- function (x, envir = parent.frame(), ...)
if (is.function(x)) x else .Internal(as.function.default(x, envir))

If I skip the is.function(x) test and call .Internal directly, I find it 
is about 10% faster than `function`.  But that is an extremely risky 
optimization; it wouldn't be accepted in a CRAN package.

Duncan Murdoch