[llvm] [MSan] Fix va_arg handling on 32-bit machines (PR #211339)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

@thurstond @Egor2001
Random idea, what if we make va_arg - implicit noundef? I believe all acceptable types are suppose to be noundef, we just have no place to put attribute in `...` IR.
They we check at caller, and in callee we assume zero shadow.
Then we we can drop all that TLS magic and significantly simplify the pass.


https://github.com/llvm/llvm-project/pull/211339