[ridiculous_fish]

"strlen gets marked as pure by the compiler if there is no non-pure definition that overrides it"

That's what I meant when I wrote that "the compiler recognizes strlen, and optimizes it specially." It was not my intent to say that only builtins or all builtins may be hoisted in this way, although now I see how someone could interpret that. That was bad wording on my part.

The point I was trying to communicate is that gcc can do special optimizations on functions that it recognizes as builtins - for example, replacing printf() with fputs(). One illustration is how strlen is treated as pure, even if it is not marked as pure. As someone with far more gcc expertise than me, would you agree with that point?

"Basically, the compiler defines a function named "strlen" that is pure and nothrow behind your back, but you can override it by providing your own definition. This is unrelated to whether it is a builtin"

Well, the optimization is defeated by -fno-builtin, so I assumed that the underlying mechanism is that a call to strlen is replaced by a call to the builtin. Was this wrong?

[DannyBee]

It is wrong, but only because of the weirdness of how this works. The call to strlen is not replaced with builtin_strlen, it defines both a builtin_strlen and a strlen. Both are marked pure and nothrow.

Due to some wonderful oddness around freestanding environments, if you use -fno-builtin, it will define neither.