[clang] [TBAA] Emit "omnipotent char" for intrinsics with type cast (PR #107793)

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huhu233 wrote:

> I'll need to look to see if the TBAA metadata we're adding matches what we initially envisaged but my gut feeling matches @arsenm.
> 
> I'm assuming the original code emitted an error, something along the lines of "no version of svst1 available for long long*", which you've "fixed" by introducing a cast that breaks strict aliasing rules.
> 
> I recall warnings relating to strict aliasing violations being hit and miss? but perhaps we can do better.

Hi, @paulwalker-arm, ACLE allows users to do instruction-level development, but mixing intrinsic and regular C code may break some of the rules set by the compiler. For example, in this case, the explict cast causes `res2` to have different tbaa in different contexts. The compiler needs to detect these behaviors and provide some hints, or work around them to avoid a series of wrong optimizations that follow.

https://github.com/llvm/llvm-project/pull/107793