Compiler crash in LambdaRewriter #21506
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FYI @dotnet/roslyn-infrastructure |
agocke
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Aug 15, 2017
In the introduction of the new 'this' optimization routine, one of the things which was changed was to treat 'this' more like a formal parameter of the method, as opposed to a variable living in an implicit, higher scope. This has some advantages in simplicity for analysis, but created a problem when it came to proxies. The current analysis builds the proxy list by walking the tree and finding all captured variables and adding them to the proxy dictionary keyed by the original variable symbol. For instance, if a local variable is captured to a field, during rewriting it will be added to the proxy list as (original symbol, hoisted field). Since most symbols are only ever captured to a single replacement field, this usually works fine -- all proxies can exist side-by-side in the proxy list since there is no intersection. However, this is not true for captured environment pointers. When a new environment is introduced, a local will be created to point to that environment. That local may itself be captured by nested variables, creating a linked list from nested scopes to parent scope. Most notably, *multiple* nested environments may capture the *same* environment pointer in *different* hoisted fields. This means that the proxies dictionary cannot hold all mappings at once, since the mapping for a given captured environment pointer will depend on the current scope. The current code actually accounts for this already by adding a captured environment pointer to the proxy list on a nested scope's introduction, and removing it upon leaving that scope. By changing 'this' to be treated like a formal parameter, I circumvented this logic, introducing a bug. When two scopes tried to capture the 'this' pointer, the compiler crashed due to trying to add two mappings to the same key. This change fixes this problem by treating the 'this' parameter like an environment pointer for the purposes of capturing and hoisting. It's possible that we want to treat it like a formal parameter, but if so it's probably better to treat all captured environment pointers the same way and introduce a scope-aware notion of proxies, rather than having a global dictionary. Fixes #21506
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VS insertion to vsuml failed due to a bug in the compiler. The insertion is blocked until this issue is fixed.
Stack trace:
Crash dump: %internal_share%\public\tomat\Bugs\LambdaCrash
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