[clang] [analyzer] Ignoring `T v=v; ` idiom for uninitialized variable checker and dead store checker (PR #187530)

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I don't think there is codegen fixes to be done here. Self-assign like this is UB, and it would be an unnecessary amount of work to try to make this a 'no-op' instead.
> 
> Is it UB, though? I think, at least in C, it's not clear-cut.

Its an uninitialized read, which further look seems to be 'indeterminate value'.  I think C++ is 'erroneous' now for this.  Either way, our implementation is conforming here, self-init for non-constructor-types is implemented as effectively a no-op instead of 'no ops'.

> > That said, as 'UB', self-assign's 'behavior' through LLVM-IR is effectively no effect, even if it is wasted instructions (that is, `mov eax, eax` won't change anything at all except perhaps invalidating speculation/etc).
> > IMO the clang behavior is completely making sense: we diagnose when this COULD be problematic (such as causing copy constructor from a not-yet-life-constructed self).
> > I'm a bit concerned about: `The self-assignment initialization Type var = var; is an idiom in C code.` from the commit message. Is this true? Is this REALLY that common? To what end?
> 
> I've seen it in C code but don't have a good idea of prevalence. The uses I've seen were for silencing "unused variable" warnings in C89 code but I'm not certain how crucial they are to support.
> 
> Clang and GCC have some difference in behavior for how we handle `-Winit-self` and `-Wuninitialized`, too. It's not clear whether those differences are intentional or not.

My understanding is that Clang's warnings is at least reasonable, so I'd probably need to see some justification to change that?  Carving out this exception in the static-analyzer is a 'neutral' for me, though I've done SOME googling and see some suggestions to do self-init to suppress `-Wuninitialized` on google.  The carve-out of `-Winit-self` being separate seems to be supporting evidence for this... so I guess I'm slightly-above-neutral? 

https://github.com/llvm/llvm-project/pull/187530