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</o:shapelayout></xml><![endif]--></head><body lang=EN-IE link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>While getting our implementation of the compiler ready for v3.8 we came across a significant problem in the new implementation of LibC++’s handling of the ISO C Standard headers that needs to be fixed before the v3.8 release.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>We discovered the problem when running some tests that were derived from real-world code. The initial example involved a C++ program including ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ and using ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ but the issue applies equally to the other headers, for example:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include <math.h><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>...<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>__fp16 y; // We use FP16 a lot<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs(y); // really -> fabs((double)y);<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>In C there is only one ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ with the signature:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>double fabs(double)<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>With the new implementation of the headers for LibC++, there is now a file named ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>c++/math.h</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ that includes the C version and then supplements it for C++. When I use this, the above use of ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ above results in an overload ambiguity error.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>It’s easy to say “It’s C++, use ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><cmath></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’”, but in the real-world C++ programmers very commonly use the C headers whether they should or not. In any case, ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ does not belong to C++ and the C++ implementation should not interfere with the interface expressed by a foreign header when that header is included directly.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>After looking further into this I realised that there is an inadvertent bug in the new implementation rather than an intent to interfere with the C interface. It also breaks C++’s ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><cmath></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ interface requirements. The supplementary elements used to be in ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><cmath></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ itself, but since they were extracted they are in the global namespace and not in ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>namespace std</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ where they are intended to be, and they are consequently introducing overloads into the C usage. I will describe this for ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ but the pattern appears to be in all of the new ISO C wrapper headers. In ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>c++/math.h</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ we have (abbreviated for illustration):<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#ifndef _LIBCPP_MATH_H<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#define _LIBCPP_MATH_H<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include <__config><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include_next <math.h><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#ifdef __cplusplus<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif;background:yellow;mso-highlight:yellow'>// Missing: _LIBCPP_BEGIN_NAMESPACE_STD</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#if !(defined(_LIBCPP_MSVCRT) || defined(_AIX))<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>inline _LIBCPP_INLINE_VISIBILITY float fabs(float __lcpp_x) _NOEXCEPT {return fabsf(__lcpp_x);}<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>inline _LIBCPP_INLINE_VISIBILITY long double fabs(long double __lcpp_x) _NOEXCEPT {return fabsl(__lcpp_x);}<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#endif<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>template <class _A1><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>inline _LIBCPP_INLINE_VISIBILITY<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>typename std::enable_if<std::is_integral<_A1>::value, double>::type<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs(_A1 __lcpp_x) _NOEXCEPT {return fabs((double)__lcpp_x);}<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif;background:yellow;mso-highlight:yellow'>// Missing: _LIBCPP_END_NAMESPACE_STD</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#endif // __cplusplus<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#endif // _LIBCPP_MATH_H<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>but because this is no longer between ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>_LIBCPP_BEGIN_NAMESPACE_STD</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ and ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>_LIBCPP_END_NAMESPACE_STD</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’, the new declarations introduce overloads at the global namespace and not in the ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>std</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ namespace.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>Now when ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ is included explicitly you get ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ as follows:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>double ::fabs(double) // From ‘include/math.h’<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>float ::fabs(float) // From ‘include/c++/math.h’<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>long double ::fabs(long double) // Also from ‘include/c++/math.h’<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>With the namespace correction explicitly including ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ would produce:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>double ::fabs(double) // From ‘include/math.h’<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>float std::fabs(float) // From ‘include/c++/math.h’<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>long double std::fabs(long double) // Also from ‘include/c++/math.h’<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>Which will not break the C compatible use case.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>However, I think that using the ISO C math header explicitly like this should not be supplemented with partial ISO C++math header requirements, it’s neither C’s ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ nor C++’s ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><cmath></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ and has odd incongruities such as:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include <math.h><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>float (*pf)(float) = std::fabs; // Okay, we have this<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>long double (*pld)(long double) = std::fabs; // Okay, we have this too<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>double (*pd)(double) = std::fabs; // Oops, no such function<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>This is not as serious as the namespace bug, but it is strangely inconsistent and shouldn’t really be there. What I propose for this is an equally simple change. In ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><cmath></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ replace:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include <math.h><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>with:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#define _LIBCPP_INCLUDING_STDC_HEADER<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#include <math.h><o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#undef _LIBCPP_INCLUDING_STDC_HEADER<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>and then in ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>c++/math.h</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’, replace:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#ifdef __cplusplus<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>with:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:36.0pt'><span style='font-size:12.0pt;font-family:"Courier New",serif'>#ifdef _LIBCPP_INCLUDING_STDC_HEADER<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>I’ve only referred to ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>fabs</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ and ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'><math.h></span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’, but of course it is all the overloaded “from C” functions in the ‘</span><span style='font-size:12.0pt;font-family:"Courier New",serif'>std::</span><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>’ namespace that are affected, and the same problem is present in the other ISO C wrapper headers.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>I will work on this over the weekend and hope to be able to submit a tested patch file on Monday or Tuesday that will correct this. I’m at SVN revision #258500 for the v3.8 branch at the moment, but I will update and merge to the most recent version before I submit a patch file for your consideration.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'>Thanks,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'> MartinO<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:12.0pt;font-family:"Book Antiqua",serif'><o:p> </o:p></span></p></div></body></html>