On Mon, Sep 30, 2019 at 3:08 PM Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx> wrote: > > On Mon, Sep 30, 2019 at 11:50 PM Nick Desaulniers > <ndesaulniers@xxxxxxxxxx> wrote: > > > > So __attribute__((always_inline)) doesn't guarantee that code will be > > inlined. [...] inline and __attribute__((always_inline)) > > are a heuristic laden mess and should not be relied upon. > > Small note: in GCC, __attribute__((always_inline)) is documented as > actually guaranteeing to either inline or error otherwise (although > see the remark for indirect calls): > > "Failure to inline such a function is diagnosed as an error. Note Not an error, but a warning at least: https://godbolt.org/z/_V5im1. That's interesting, so it has multiple semantics, because it's also documented to inline even when no optimizations are specified. So when someone uses __attribute__((always_inline)) without a comment, it's not clear whether they mean for there to be a warning when this is not inlined, or for it to be inlined at -O0 (guess not for the kernel), or both. If the kernel wants to enforce the former, why not set `-Werror=attributes`? Maybe that warning is too broad? Seems like a recipe for subtly broken code found at runtime, when we'd rather have stronger compile time guarantees. > that if such a function is called indirectly the compiler may or may > not inline it depending on optimization level and a failure to inline > an indirect call may or may not be diagnosed." > > As for LLVM/Clang, no idea, since it does not say anything about it in > the docs -- but from what you say, it is a weaker guarantee. Filed https://bugs.llvm.org/show_bug.cgi?id=43517 -- Thanks, ~Nick Desaulniers
