On Mon, Sep 14, 2015 at 7:07 AM, Marshall Clow <mclow.li...@gmail.com> wrote:
> mclow.lists added a comment. > > I have two concerns about this patch (w/o commenting on the actual code). > > 1. Until very recently, I was under the impression that C libraries > _either_ defined a macro, or had a function. I was quite surprised to find > that glibc did both. Yes, this is required by the C standard. C11 7.1.4/1 says: "Any function declared in a header may be additionally implemented as a function-like macro defined in the header [...]. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro. [Footnote: This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function.]" Have you checked other C libraries (Apple, FreeBSD, Android, Windows) to > see if they also define both? No, but libstdc++ does the same #undef thing, so any platform it supports must have a non-broken C standard library. > 2. This adds a lot of header files. Each header file slows down > compilation, and standard library header files get included *a lot*. We may > not be able to avoid this, but we should think about the costs here. I created a .cpp file that includes all of the <*.h> headers and does nothing else (which should maximize the performance difference), and built it with and without this change. I could not measure any difference (the average compile time with this change was slightly lower, but that is almost certainly noise).
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