Saleem Abdulrasool 23ccb02c59 CodeGen: add a missing check for bit-slice overlap in CV (#75504) ...

Type dereferenced fragments are specified by offset and length in bits.
The representation in CodeView is defined in terms of byte offsets. If
the bit slice overlaps at a byte that is included, we would create
invalid definition ranges.

Consider the following scenario:

~~~
01234567   01234567
---------+---------
 ====  ======
~~~

Here bits 1-4 are marked as defined as well as bits 7-9. The byte range
for the second portion overlaps and so we would say that bytes 1 and 2
are valid though there is potentially a hole. There is no way to
represent this in the defined range for the local variable in CodeView.
We simply can drop the fragment definition in such a scenario with the
variables are "optimized out".

Thanks to @rnk and @hjyamauchi for the discussion around this.

2023-12-14 13:59:15 -08:00

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