Memory leak with implicit result

[Gruruya]

Description

status-im/nim-stew#183

The implicit result causes a memory leak.

func toBytes*(s: string): seq[byte] =
  when nimvm:
    let r = newSeq[byte](4) # arbitrary
    r
  else:
    @(s.toOpenArrayByte(0, s.high))

discard "hello".toBytes

The explicit result does not.

func toBytes*(s: string): seq[byte] =
  when nimvm:
    let r = newSeq[byte](4)
    r
  else:
    result = @(s.toOpenArrayByte(0, s.high))

discard "hello".toBytes

Nim Version

1.9.3
c136ebf

Current Output

nim c --mm:orc -d:useMalloc test
valgrind --leak-check=full ./test

==29138== HEAP SUMMARY:
==29138==     in use at exit: 13 bytes in 1 blocks
==29138==   total heap usage: 7 allocs, 6 frees, 1,161 bytes allocated
==29138== 
==29138== 13 bytes in 1 blocks are definitely lost in loss record 1 of 1
==29138==    at 0x48455EF: calloc (vg_replace_malloc.c:1328)
==29138==    by 0x10AB1A: alloc0Impl__system_1770 (in /home/user/test)
==29138==    by 0x10AB44: allocShared0Impl__system_1783 (in /home/user/test)
==29138==    by 0x1104C2: alignedAlloc0__system_1948 (in /home/user/test)
==29138==    by 0x1107AB: newSeqPayload (in /home/user/test)
==29138==    by 0x110BBF: prepareSeqAdd (in /home/user/test)
==29138==    by 0x111108: setLen__test_12 (in /home/user/test)
==29138==    by 0x11126C: newSeq__test_4 (in /home/user/test)
==29138==    by 0x11130C: at___test_147 (in /home/user/test)
==29138==    by 0x1120C9: toBytes__test_1 (in /home/user/test)
==29138==    by 0x112235: NimMainModule (in /home/user/test)
==29138==    by 0x112154: NimMainInner (in /home/user/test)
==29138== 
==29138== LEAK SUMMARY:
==29138==    definitely lost: 13 bytes in 1 blocks
==29138==    indirectly lost: 0 bytes in 0 blocks
==29138==      possibly lost: 0 bytes in 0 blocks
==29138==    still reachable: 0 bytes in 0 blocks
==29138==         suppressed: 0 bytes in 0 blocks

Expected Output

==29099== HEAP SUMMARY:
==29099==     in use at exit: 0 bytes in 0 blocks
==29099==   total heap usage: 6 allocs, 6 frees, 1,148 bytes allocated
==29099== 
==29099== All heap blocks were freed -- no leaks are possible
==29099== 
==29099== For lists of detected and suppressed errors, rerun with: -s
==29099== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Possible Solution

No response

Additional Information

No response

[Gruruya]

The memleak doesn't occur in vm, to make that clear.

[Gruruya]

Here is a more minimized example

func example*(s: seq[int]): seq[int] =
  when nimvm:
    s
  else:
    @(s.toOpenArray(0, s.high))

discard @[1, 2, 3].example