Safe reading of CPUID

[AntoinePrv]

Not implemented previously in xsimd, CPUID requires to first check which leaves are available before calling it, otherwise the value returned may be invalid.

• Before reading leaf < 0x80000000, check leaf 0 EAX
• Before reading leaf > 0x80000000, check leaf 0x80000000 EAX
• Before reading leaf 7 subleaf 1, check leaf 7 EAX

[AntoinePrv]

@serge-sans-paille this one is ready too

[serge-sans-paille]

I'm puzzled by the fact that this functions takes a non-const ref but returns a const-ref on the same storage. leaf has an integral type, so maybe pass everything by value?

[serge-sans-paille]

And rename leaf has default_value or something

[AntoinePrv]

I'm puzzled by the fact that this functions takes a non-const ref but returns a const-ref on the same storage.

Well that where the caching happens. This function is a utility to factor code internally.
The input must be a non-cons reference. It is passed the class member, that may or may not be initialized and returns something that should not modify the object itself.
It need to be given the class member leaf because the point it to factor the code between different leaves, so we need to know which one and its type.

leaf has an integral type, so maybe pass everything by value?

Leaf is 4 32 bit integers. Not sure if better to return by value or const ref, but probably does not make a difference after the compiler has done its pass.

And rename leaf has default_value or something

It's not really the default value though, it's the "storage" that we write to if it was uninitialized (in the C++ sens everything is initialized, just not with the value we care about).