Mirror of forum.nim-lang.org

9408 :: Does "pushing" overflowCheck's actually work?

• drkameleon (orginal) [2022-08-24T11:00:36+02:00] view original

  What I need here is the effect of --checks:off with --overflowChecks:on but only for very specific parts of the program.

  So, I thought about leaving the --overflowChecks:on part out when compiling and just add .push/.pop for the error, only when I need it:

  {.push overflowChecks: on.}
  
  # here we do want to catch OverflowDefect errors
  
  {.pop.}
  Run

  The problem is... the way Nim's code is structured, I doubt this construct (^above) has any effect whatsoever.

  I mean... let's say I use * within the aforementioned block... I don't think it'll respect my pushed overflowChecks: on at all. As a matter of fact, by looking through Nim's code, I've found more than a few instances where the only thing being taken into account is the options that were passed when compiling. E.g.:

  https://github.com/nim-lang/Nim/blob/devel/compiler/ccgexprs.nim#L581

  So... the question is: is there any way I can selectively enable overflowChecks, only for some specific block of code? Or... would I have to re-implement even the most basic arithmetic routines for that purpose?

---

• drkameleon (orginal) [2022-08-24T11:39:50+02:00] view original

  The code that was not working (= that was inside a {.push overflowChecks:on.} - {.pop.} block, but was not triggering OverflowDefect's) was mainly ^ and *= (but not * (!?))... so what I did is:

  • repeat the implementation of these 2 operations' implementation, within the aforementioned push/pop block, with a different name
  • use that one whenever needed, within that same block

  So...:

  proc safeMulI*[T: SomeInteger](x: var T, y: T) {.inline, noSideEffect.} =
      x = x * y
  
  func safePow*[T: SomeNumber](x: T, y: Natural): T =
      case y
      of 0: result = 1
      of 1: result = x
      of 2: result = x * x
      of 3: result = x * x * x
      else:
          var (x, y) = (x, y)
          result = 1
          while true:
              if (y and 1) != 0:
                  safeMulI(result, x)
              y = y shr 1
              if y == 0:
                  break
              safeMulI(x, x)
  Run

  That looks kind of messy, but it definitely works.

---