(Re)move some GC pushes to be defensive about #35708 #38355
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JeffBezanson
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Nov 9, 2020
| for (size_t i = 0; i < na; i++) { | ||
| set_nth_field(type, (void*)jv, i, args[i]); | ||
| } | ||
| init_struct_tail(type, jv, na); |
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Isn't this still needed?
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Yes, sorry this got dropped in rebase. I have it fixed locally, but didn't push the version yet.
Somebody sent me a (Julia 1.4) trace that turned out to have the same root cause as #35708. I took a look around just to be sure that there was no other instance of this pattern and while I didn't see any, I did see a useless push/pull pair as well as a GC_PUSH of an unitialized struct. While neither are a problem by themselves, they will prevent the GC analyzer from giving an error if any of the function in between ever become safepoints (since the GC analyzer doesn't track initilized-ness). I think it as a rule of thumb, we should never push uninitialized values into a GC frame. Doing so assumes that there are no safepoints before the value is fully initialized, but if that is the case, the GC_PUSH may also be delayed until after initialization and if the assumption ever changes, at least the GC analyzer will catch it.
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@JeffBezanson good to go? |
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Somebody sent me a (Julia 1.4) trace that turned out to have the
same root cause as #35708. I took a look around just to be sure
that there was no other instance of this pattern and while I
didn't see any, I did see a useless push/pull pair as well
as a GC_PUSH of an uninitialized struct. While neither are a
problem by themselves, they will prevent the GC analyzer from
giving an error if any of the function in between ever become
safepoints (since the GC analyzer doesn't track initilized-ness).
I think it as a rule of thumb, we should never push uninitialized
values into a GC frame. Doing so assumes that there are no safepoints
before the value is fully initialized, but if that is the case,
the GC_PUSH may also be delayed until after initialization and if
the assumption ever changes, at least the GC analyzer will catch it.