[Rd] JIT compiler does not compile closures with custom environments

[Taras Zakharko]

I have encountered a behavior of R’s JIT compiler that I can’t quite figure 
out. Consider the following code:


   f_global <- function(x) {
     for(i in 1:10000) x <- x + 1
     x
   }

   f_env <- local({
    function(x) {
      for(i in 1:10000) x <- x + 1
      x
    }
   })

   compiler::enableJIT(3)

  bench::mark(f_global(0), f_env(0))
  # 1 f_global(0)    103µs 107.61µs     8770.    11.4KB      0    4384     0
  # 2 f_env(0)       1.1ms   1.42ms      712.        0B     66.3   290    27
  
Inspecting the closures shows that f_global has been byte-compiled while f_env 
has not been byte-compiled. Furthermore, if I assign a new environment to 
f_global (e.g. via environment(f_global) <- new.env()), it won’t be 
byte-compiled either. 

However, if I have a function returning a closure, that closure does get 
byte-compiled:

  f_closure <- (function() {
    function(x) {
      for(i in 1:10000) x <- x + 1
     x
   }
  })()

  bench::mark(f_closure(0))
  # 1 f_closure(0)    105µs    109µs     8625.        0B     2.01  4284     1   
   497ms

What is going on here? Both f_closure and f_env have non-global environments. 
Why is one JIT-compiled, but not the other? Is there a way to ensure that 
functions defined in environments will be JIT-compiled? 

Thanks, 

Taras
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