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Maybe that's applied to the TU that defines it? I don't see it in the glibc sources.

It's indeed used:

Definition of macro: https://sourceware.org/git/?p=glibc.git;a=blob;f=include/lib...

Use: https://sourceware.org/git/?p=glibc.git;a=blob;f=string/memm...

For a bunch of other places -fno-builtin-* seems to be used.

My guess is that the attribute interacts weirdly with optimizations spanning function calls, like inlining. It might be safe for TUs that define a single function, like with memmove above. Having said that applying the option to the TU itself would be equivalent, and not discouraged by GCC docs.

Because these optimizations are not supposed to be a part of the user interface. They're internal passes and may one day go away or be merged or subsumed etc.

As far as I can tell it's due to some bugs