The builtin function `__builtin_ia32_rdtscp()` is undocumented

[Liu Hao <lh_mouse@xxxxxxx>]

Hello there,

Today I accidentally found the built-in function 
`__builtin_ia32_rdtscp()` on GCC6, but there is no documentation about 
it on <http://gcc.gnu.org/onlinedocs/gcc/x86-Built-in-Functions.html>. 
Is this an oversight?

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Best regards,
LH_Mouse