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Thanks for pointing to this. I'm surprised the specification verbiage differs so much between the two standards. However I think the language for C is aspirational and there are cases where compiler optimizations will fail this.

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There are a few rare cases where compiler optimization will fail this in GCC. There are also cases where the C spec has defined behavior that optimizers break. Where I know about them, I file bugs. It would be up for users to insist that those get fixed. This includes optimization issues that affect Rust as well.

Some of those cases have been fixed. For example, Clang used to reorder UB around volatile loads, but the latest version no longer does so.

This is good news! Clang representatives were very basically the only ones against this clarification. Do you have a pointer?

https://llvm.org/docs/LangRef.html#volatile-memory-accesses:

> Volatile operations are permitted to trap. The compiler may not assume that execution will continue after a volatile operation.

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