[regression] var param corruption and variable mutation when run the code with VM

[jangko]

Very hard to shorten the example code, sorry if it's a bit intimidating.
It can be a single bug or two bugs.

Example

type
  StUint = object
    lo: uint64

func `+=`(x: var Stuint, y: Stuint) =
  x.lo += y.lo

func `-`(x, y: Stuint): Stuint =
  result.lo = x.lo - y.lo

func `+`(x, y: Stuint): Stuint =
  result.lo = x.lo + y.lo

func `-=`(x: var Stuint, y: Stuint) =
  x = x - y

func `<`(x, y: Stuint): bool=
  x.lo < y.lo

func `==`(x, y: Stuint): bool =
  x.lo == y.lo

func `<=`(x, y: Stuint): bool =
  x.lo <= y.lo

proc div3n2n(r: var Stuint, b: Stuint) =
  var d: Stuint
  r = d
  r += b

func div2n1n(r: var Stuint, b: Stuint) =
  div3n2n(r, b)

func divmodBZ(x, y: Stuint, r: var Stuint)=
  div2n1n(r, y)
  r.lo = 3

func `mod`(x, y: Stuint): Stuint =
  divmodBZ(x, y, result)

func doublemod_internal(a, m: Stuint): Stuint =
  result = a
  if a >= m - a:
    result -= m
  result += a

func mulmod_internal(a, b, m: Stuint): Stuint =
  var (a, b) = (a, b)
  swap(a, b)
  debugEcho "x1 a: ", a
  a = doublemod_internal(a, m)
  debugEcho "x2 a: ", a
  a = doublemod_internal(a, m)
  debugEcho "x3 a: ", a

func powmod_internal(a, m: Stuint): Stuint =
  var a = a
  debugEcho "z1 m: ", m
  debugEcho "z2 a: ", a
  result = mulmod_internal(result, a, m)
  debugEcho "z3 a: ", a
  a = mulmod_internal(a, a, m)

func powmod*(a, m: Stuint) =
  discard powmod_internal(a mod m, m)

static:
  var x = Stuint(lo: high(uint64))
  var y = Stuint(lo: 12)

  powmod(x, y)

Current Output

z1 m: (lo: 12)
z2 a: (lo: 3)
x1 a: (lo: 3)
x2 a: (lo: 6)
x3 a: (lo: 18446744073709551604) # -> corruption, should be 0
z3 a: (lo: 18446744073709551604) # -> mutation, should stay the same as z2
x1 a: (lo: 18446744073709551604)
x2 a: (lo: 18446744073709551568)
x3 a: (lo: 18446744073709551496)

Expected Output

z1 m: (lo: 12)
z2 a: (lo: 3)
x1 a: (lo: 3)
x2 a: (lo: 6)
x3 a: (lo: 0)
z3 a: (lo: 3)
x1 a: (lo: 3)
x2 a: (lo: 6)
x3 a: (lo: 0)

Additional Information

• It was working in version 0.19.6
• compile time issue with nim v1.0.2 status-im/nim-stint#98

$ nim -v
Nim Compiler Version 1.0.2 [Windows: amd64]
Compiled at 2019-10-29
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: de5f6a07c239ccf5ca3d41d70350a37112025bcf
active boot switches: -d:release

[jangko]

bad commit 162d74d

[timotheecour]

@jangko can you try to minimize, and does that reduce to #15974 ?

[jangko]

iirc my example is very weird, insertion and removal of simple echo or other stmt could hide the bug.
I've try to minimize it at that time but could not reproduce. but because it has the same root and similarity with #15974, it could be the very same bug, but I could not prove it at this time.