[clang] [clang][analyzer] Stable order for SymbolRef-keyed containers (PR #121551)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> That being said, do we know how exactly the unstable addresses result in unstable results?

I cannot pinpoint the responsible code fragment, but in general this is a typical property of associative data structures. A typical associative map doesn't preserve the order of insertion, and instead stores the key-value pairs in some arbitrary order that facilitates quick and effective access (for example, many implementations sort them according to the keys -- or the hash values of the keys). When the program iterates over a data structure of this kind, it will see the key-value pairs in this arbitrary order, which may depend on pointer values, whose order relationships are undefined and may change if the program is reran with the same input. If the calculations performed by the loop are non-commutative (e.g. "sum all the values" is commutative, "return the first value which is larger than 10" is not), then this unstable iteration order produces unstable output.

By the way, the Python language (where performance is not prioritized) rules out these subtle bugs by recording the order of insertion within its `dict` (associative map) datatype and using that as the iteration order.

https://github.com/llvm/llvm-project/pull/121551