Is RDTSC pure?

[IchorDev]

Recently I wanted to write a pure function that returns an 
unpredictable number, so I decided to use RDTSC (and any 
equivalent instruction for other CPU architectures) to do this, 
since the compiler allows RDTSC to be marked as `pure`.
However, in the end I discarded this idea because I figured that 
a `pure` function should never return a different value with the 
same input; and doing so would surely break any applicable 
memoisation. Inline assembly isn't checked by the compiler, so I 
was essentially doing the same thing as misusing `@trusted`…

Or so I thought. Today I remembered that `pureMalloc` exists, 
which surely doesn't follow these rules and would definitely not 
work when memoised. So how come it's still allowed to be `pure` 
just by resetting ERRNO? It can return a different value with the 
same input, so does that mean that using RDTSC is also `pure`?