Re: Can I tell Nim to NOT use *reference* for a var parameter?

[monster]

@Araq Here is a minimal example. This is compiled with "vcc". It compiles to C, 
but not to C++.


type
  VolatilePtr*[T] = distinct ptr T

proc toVolatilePtr*[T](t: var T): VolatilePtr[T] =
  cast[VolatilePtr[T]](addr t)
  # Pretend we're actually checking it's volatile, which is apparently not 
possible to do.

var my_vbyte {.volatile.}: byte = 42'u8
var my_vbyte_ptr = toVolatilePtr[byte](my_vbyte)

type
  AtomType* = SomeNumber|pointer|ptr|char|bool
when defined(cpp):
  proc interlockedOr8(p: pointer; value: int8): int8
{.importcpp: "_InterlockedOr8(static_cast(#), #)", 
header: "".}
else:
  proc interlockedOr8(p: pointer; value: int8): int8
{.importc: "_InterlockedOr8", header: "".}

proc atomicLoadFull*[T: AtomType](p: VolatilePtr[T]): T {.inline.} =
  let pp = cast[pointer](p)
  when sizeof(T) == 1:
cast[T](interlockedOr8(pp, 0'i8))
  else: # TODO, sizeof(T) == (2|4|8)
static: assert(false, "invalid parameter size: " & $sizeof(T))

assert(atomicLoadFull(my_vbyte_ptr) == 42'u8)



src\nimcache\abc_volatile2.cpp(306): error C2664: 'NU8 
*toVolatilePtr_EP8VL8ilqAQ1Zz9bJAA4rwQ(NU8 &)':
 cannot convert argument 1 from 'volatile NU8' to 'NU8 &'
src\nimcache\abc_volatile2.cpp(306): note: Conversion loses qualifiers


I've had the issue that the error doesn't always come, if I don't delete the 
nimcache, but I think it's caused by VS Code compiling automatically as well, 
using a different configuration.

Re: Can I tell Nim to NOT use *reference* for a var parameter?

[Araq]

Please give me more details.

Re: Can I tell Nim to NOT use *reference* for a var parameter?

[monster]

@Araq I have read it, several times. But I do not see how this helps me, if I 
don't want to directly read or write a volatile, but rather I have to pass a 
pointer to a volatile location to an OS method that expects one.

Re: Can I tell Nim to NOT use *reference* for a var parameter?

[Araq]

`volatile.nim` is a stdlib module, you can read its code to see how it works. 
Don't use the type system to model `volatile`, that does not work well.

Can I tell Nim to NOT use *reference* for a var parameter?

[monster]

I'm (still) trying to put the "missing bits" of atomic methods together into a 
cross-platform/cross-backend module. I've now got to the point that the VCC C 
version compiles and run (I only test with one thread, so idk if it really 
gives the expected atomic behaviour yet). When I try to build and run the VCC 
C++ version, I have an issue with C++ references. It seems "(t: var T)" is 
translated to a pointer in C, and a reference in C++. And this doesn't seem to 
play well with volatiles.

How to I make this work in C++?


type
  VolatilePtr*[T] = distinct ptr T

proc toVolatilePtr*[T](t: var T): VolatilePtr[T] =
  cast[VolatilePtr[T]](addr t)

var my_vbyte {.volatile.}: byte = 0'u8
var my_vbyte_ptr: VolatilePtr[byte] = toVolatilePtr[byte](my_vbyte)

echo(cast[pointer](my_vbyte_ptr) == nil)


When trying to use this, I get this kind of error:


error C2664: 'NU8 *toVolatilePtr_WWtk4tL3VWMmQk62xcO0Ew(NU8 &)': cannot 
convert argument 1 from 'volatile NU8' to 'NU8 &'

Re: var parameter

[h42]

Thank you Araq.

var parameter

[h42]

Can I assume that a var parameter will always be a pointer where I can use 
(addr varParm) as copymem destination. It seems to work but I can copy to 
tempory local variable and use normal assignment if there is no guarentee it 
will always work.