What can _not_ be marked pure?

[Daniel Kozak via Digitalmars-d-learn]

Dne 9.3.2016 v 11:26 Guillaume Piolat via Digitalmars-d-learn napsal(a):
> On Wednesday, 9 March 2016 at 10:08:33 UTC, ag0aep6g wrote:
>> On 09.03.2016 10:56, Guillaume Piolat wrote:
>>> If I understand purity correctly
>>> (http://klickverbot.at/blog/2012/05/purity-in-d/), every function out
>>> there can be marked pure as long as it doesn't modify globals, shared
>>> variables or do I/O?
>>
>> Pure functions also can't *read* mutable globals. But then that's it, 
>> I think.
>
> Thanks. Now I'm excited to spam it over my codebase and see the kind 
> of speedup one can achieve.

I am not sure, but my guess is there will be no speed up :-).