[release/6.0] JIT: fix bug where a gc struct is not zero initialized #68050
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Port of #67825 to release/6.0.
Customer Impact
Reported by RavenDB (see #65694).
This bug can lead to unexpected crashes during GC.
Details
An optimization added in .NET 5 can in some cases lead to the jit reporting an uninitialized struct field as a gc reference.
The attached test case shows a simple app that exhibits this problem. The key components are:
S
with one or more ref-class fieldss
of typeS
,s
viaDictionary.TryGetValue
using the.?
operator for the dictionary reference,s
ifTryGetValue
returns trueTesting
Verified with a local RavenDB repro and new test case.
Risk
Low. The IL pattern that leads to this situation is not common. Fix had very minor diffs in SPMI.