On Thu, Nov 18, 2010 at 8:20 PM, Neal Becker <ndbecker2@xxxxxxxxx> wrote:
> cmath has patterns like this (taking cos as an example):
>
> using ::cos;
>
> inline float
> cos(float __x)
> { return __builtin_cosf(__x); }
>
> inline long double
> cos(long double __x)
> { return __builtin_cosl(__x); }
>
> template<typename _Tp>
> inline typename __gnu_cxx::__enable_if<__is_integer<_Tp>::__value,
> double>::__type
> cos(_Tp __x)
> { return __builtin_cos(__x); }
>
> Do I understand correctly, that this means float, long double will use
> __builtin_cos(xxx), but plain double will not use __builtin_cos???
>
> I've tried tracing the generated code, and that's the way it looks. It
> appears to just call the cos function in glibc.
<cmath> is required to be compatible with <math.h>
Most <math.h> come from the C compiler/library.
So, plain cos() (for double) is whatever your C library provides.
That is what the top using-declaration is all about.
If you are using glibc, I believe it will ultimately use __builtin_cos
unless you tell the compiler not to.
>
> Is this correct and if so, would not __builtin_cos be faster?
>
>
