Re: Optimization of conditional access to globals: thread-unsafe?

[Darryl Miles <darryl-mailinglists at netbauds dot net>]

David Miller wrote:
> From: Darryl Miles <darryl-mailinglists@netbauds.net>
> Date: Mon, 29 Oct 2007 04:53:49 +0000
>
> > What are the issues with "speculative loads" ?
>
> The conditional might be protecting whether the pointer is valid and
> can be dereferenced at all.

This then leads into the question.  Is a pointer allowed to be invalid.

I'm sure I have read a comment on this before, along the line of the 
spec says it must be valid or a certain number of other values (like 
zero or one past being valid).  But I can not cite chapter and verse if 
this is true.

I would agree however (before you say) that 'counter' by itself is just 
a variable, and it is only when execution allows it to be dereferenced 
that the issues about its validity come into play.  This is practical 
common law usage.


> And in another module that GCC can't see when compiling foo():

I agree, any external symbols might not even be in the 'C' language, but 
those symbols do conform to the ABI and the unwritten rules of what the 
value represents is implied when you assign an equivalent type 'int' to 
it in a C variable declaration.


Darryl