strformat(or pairs) will free local var memory with refc

[slangmgh]

Description

The following code give wrong result: compile with --mm:refc and -d:danger I don't know it is the strformat or pairs problem, because change them will make the program run fine.

import strutils, strformat

proc get_all_codes(): seq[string] =
   for i in 1..5000:
      result.add align($i, 5, '0')

proc get_names(codes: seq[string]): seq[string] =
   for code in codes:
      result.add "abc" & code

proc check_codes() =
   var lines: seq[string]

   let codes = get_all_codes()
   let names = get_names(codes)

   # using for code in codes: , program run fine
   # var i = 0
   for i, code in codes:
      var name = names[i]
      #inc(i)

      if code.len != 5:
         # this should not be happened
         echo "oops ", i, " ", code
         quit()

      # this line maybe the culprit
      # change to:
      #   lines.add code & ":" & name
      # seems fine

      lines.add &"{code}:{name}"

   echo "ok"

when isMainModule:
   check_codes()

Nim Version

latest devel

Current Output

oops 245 abc00198

Expected Output

ok

Possible Solution

No response

Additional Information

No response

[slangmgh]

Only compile with msvc (any version) will generate the problem, use gcc doesn't.

[Varriount]

@slangmgh What version of mvcc are you using?

[slangmgh]

msvc 2005 2008 2010 2013 2017 2019 32bit/64bit

[slangmgh]

add line GC_Ref(codes) before loop will prevent the bug happen. Or add reference like echo codes.len before the proce return will prevent the bug happen too.

[slangmgh]

And the debug version is fine.

[slangmgh]

Most likely bug of msvc c optmizer. Possible generate the same code with for code in codes, which use pointer rather than object, and dont know it will fix the problem, but simple test run fine.

[slangmgh]

This is the gc bug with msvc compiler optimizer. When the msvc optimizer the code, the variable codes is not on the stack (maybe in register, I doesn't check the assembler code yet), so the mark & sweep process think it is not referenced by any variable and free the memory.

[slangmgh]

The code in gc_common does check the register to find object reference, so I don't know why there is no reference to the object codes in any stack and register(In the msvc generated code, it store codes pointer in rsi).