regression(0.20.0 => devel): var params assignment gives silently wrong results in VM

[timotheecour]

regression(0.20.0 => devel): in VM, a copy of a var param is affected when that param is later re-assigned.

Example

when true:
  type Foo = object
    f0: int

  proc fn(a: var Foo) =
    var s: Foo
    a = Foo(f0: 2)
    s = a
    doAssert s.f0 == 2
    a = Foo(f0: 3)
    echo s.f0

  proc main()=
    var a = Foo(f0: 1)
    fn(a)

  static: main()
  main()

Current Output

CT:
3
RT:
2

Expected Output

CT:
2
RT:
2

Additional Information

regression:

• fails with: devel 1.5.1 c39fa0d, and 1.0.2
• works with: nim 0.20.0
• 1.0.0 gives: nfIsRef in regs[ra].node.flags [AssertionError]
• likewise in other cases where you'd have a copy, eg: var s: seq[Foo] and s.add a
• root cause for RT vs CT flyx/NimYAML#91 (nimyaml gives wrong results at CT), after a painful, painful reduction

git bisect output

162d74db91412d98a4a67b28c1e1c9c1d79a9873 is the first bad commit
commit 162d74db91412d98a4a67b28c1e1c9c1d79a9873
Author: Andreas Rumpf <[email protected]>
Date:   Thu Sep 19 14:34:56 2019 +0200

    fixes #10981; fixes #7261 (#12217)

 compiler/vmgen.nim    |  5 ++-
 tests/vm/tmisc_vm.nim | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 103 insertions(+), 1 deletion(-)
bisect run success

=> points to #12217 /cc @Araq

[jangko]

same first bad commit with #12551 and similar symptom involving var param.