[5.3] [Runtime] Zero out the entire witness table during instantiation. #32370
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During witness table instantiation, the witness table is constructed several sources: the pattern, the resilient witnesses, the private data, and default implementations. The private data area is the only one that was being zeroed out; the rest we rely on always filling in the data from the conformance descriptor and provided info. However, witness table instantiation uses the presence of a NULL pointer for a particular witness in the resulting table to indicate that no witness fulfilled that requirement, so that it can fill in the default witnesss. Except that, without zeroing that part of the table beforehand, we aren't guaranteed to have a NULL pointer for witness entries that the client (protocol conformance) did not know about at the time it was compiled. Zero out the entire witness table so default implementations can be filled in appropriately. Fixes rdar://problem/64295849.
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devincoughlin
approved these changes
Jun 15, 2020
AnthonyLatsis
added
swift 5.3
🍒 release cherry pick
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During witness table instantiation, the witness table is constructed
several sources: the pattern, the resilient witnesses, the private
data, and default implementations. The private data area is the only
one that was being zeroed out; the rest we rely on always filling in
the data from the conformance descriptor and provided info.
However, witness table instantiation uses the presence of a NULL
pointer for a particular witness in the resulting table to indicate
that no witness fulfilled that requirement, so that it can fill in the
default witnesss. Except that, without zeroing that part of the table
beforehand, we aren't guaranteed to have a NULL pointer for witness
entries that the client (protocol conformance) did not know about at
the time it was compiled.
Zero out the entire witness table so default implementations can be
filled in appropriately.
Fixes rdar://problem/64295849.