> On May 12, 2016, at 1:12 AM, 军其 胡 <junq...@me.com> wrote: > > The Xcode (or Apple Clang) gives the results with both 3.000000. I have > tried to compile the same code by GCC on Fedora 22 and obtain the opposite > results with both -3.000000. It's ridiculous since abs and std::abs are lying > in different namespaces.
I believe fabs is one of those magic standard-library functions that the compiler is allowed to replace with inline assembly, since that’s more efficient than a function call. In that case it would just ignore your definition. (This is probably defined somewhere in the C spec, but I’m not about to dive in and look for it.) (For example, I know this happens with memcpy — when invoked with a small constant size, both GCC and Clang will frequently generate a series of load/store instructions rather than call the actual memcpy function.) These seem like very artificial examples; is there something realistic you’re trying to do that’s giving you trouble with Clang? —Jens
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