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GC and simple smobs??

From: Jean Abou Samra
Subject: GC and simple smobs??
Date: Fri, 17 Jun 2022 23:20:01 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1


Once again, the more I try to understand how GC works at
the C++ level, the more I get lost. Of course, not being
too good at low-level programming doesn't help. Take this
random exported function:

LY_DEFINE (ly_duration_less_p, "ly:duration<?",
           2, 0, 0, (SCM p1, SCM p2),
Is @var{p1} shorter than @var{p2}?
  auto *const a = LY_ASSERT_SMOB (Duration, p1, 1);
  auto *const b = LY_ASSERT_SMOB (Duration, p2, 2);

  if (Duration::compare (*a, *b) < 0)
    return SCM_BOOL_T;
    return SCM_BOOL_F;

This is my understanding:

Duration is a Simple_smob type.

That means a Duration object can be allocated either on the stack
or on the BDWGC heap.

Since p1 and p2 are SCM, they are BDWGC-heap-allocated in this case.

The unsmobbed a and b are thus pointers to data on the BDWGC heap.

Since they are pointers to the part of the SCM that stores the C++
object, and not to the start of the SCM value, they won't cause the
SCM to be marked by their mere existence on the stack.

p1 and p2 are not used after the LY_ASSERT_SMOB calls. The compiler
could drop them before the end of the function.

In that case, nothing protects p1 and p2 between the two LY_ASSERT_SMOB
lines and the Duration::compare. If the ly:duration<? call is a tail
call, Guile itself might not hold references to the duration objects.
If that happens, a crash would ensue.

This pattern looks somewhat frequent, so I hope I am wrong. If so,
where is my mistake?

Thanks in advance,

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