vmops user guide

[timotheecour]

this should become a paragraph in manual eventually

when writing a new vmops, eg copySign (nim-lang#16406),

# in math.nim
proc c_copysign(x, y: cdouble): cdouble {.importc: "copysignf", header: "<math.h>".}

func copySign*[T: SomeFloat](x, y: T): T {.inline, since: (1, 5, 1).} =
  template impl() = # ... fallback implementation
  when not defined(js):
    result = c_copysign(x, y)
  else:
    impl()

if you run vm tests (eg: nim r --lib:lib tests/stdlib/tmath.nim) it may fail with:

math.nim(187, 14) Error: cannot 'importc' variable at compile time; c_copysign

this is because you're running using a nim binary that wasn't compiled with copySign vmops, so it won't use the registerCallback in this case.

It will succeed if you first bootstrap nim then run the test, but that's not good enough.

solutions (depending on the situation)

• option 1: if there's a fallback (eg impl) available (eg for js), you can use it in vm for old nim:

  when defined(js):
    impl()
  else:
    when nimvm:
      # this won't be used in VM if nim was compiled with vmops (ie for recent enough nim after merging PR)
      impl() # possibly slower or buggy for some edge cases
    else:
      result = c_copysign(x, y)

(eg as in b23d13b)

• option 2: if you need to branch depending on whether nim was compiled with that vmops, you can add in condsyms.nim something like: defineSymbol("nimHasCopySign") (nimHas is typically a property of nim binary, not of stdlib); and then use it in code via: when defined(nimHasCopySign)

note

pending nim-lang#14648 (which was close for now...), nimVersionCT would simplify this, avoiding having to add yet another symbol in condsyms (instead relying on increasing versions, which is also more self-descriptive and easier to cleanup). We'd bump NimPatch by 2, eg: (1,5,1) => (1,5,3), and use when nimVersionCT >= (1,5,3):

links

compiler/vmops.nim
tests/vm/tvmops.nim