Performance implications of long double. Why does C choose 64-bits instead of the hardware's 80-bit for its default?

floating-point, long-double, performance, visual-c++, x86

Solution

The answer, according to Mysticial, is that Microsoft uses SSE2 for its `double` data-type. The Floating point unit (FPU) x87 is seen as outdated and slow in comparison to modern CPU extensions. SSE2 does not support 80-bit, hence the compiler's choice of 64-bit precision.

On 32-bit x86 architecture, since all CPUs don't have SSE2 yet, Microsoft still uses the floating point unit (FPU) x87 unless the compiler switch `/arch:SSE2` is given. Which makes the code incompatible with those older? CPUs.

Problem

For specifics I am talking about x87 PC architecture and the C compiler. I am writing my own interpreter and the reasoning behind the `double` datatype confuses me. Especially where efficiency is concerned. Could someone explain WHY C has decided on a 64-bit `double` and not the hardware native 80-bit `double`? And why has the hardware settled on an 80-bit `double`, since that is not aligned? What are the performance implications of each? I would like to use an 80-bit `double` for my default numeric type. But the choices of the compiler developers make me concerned that this is not the best choice. - `double` on x86 is only 2 bytes shorter, why doesn't the compiler use the 10 byte `long double` by default? - Can I get an example of the extra precision gotten by 80-bit `long double` vs `double`? - Why does Microsoft disable `long double` by default? - In terms of magnitude, how much worse / slower is `long double` on typical x86/x64 PC hardware?

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