Optimization of conditional access to globals: thread-unsafe?

[Andi Kleen]

Tomash Brechko <tomash.brechko@gmail.com> writes:


>   - optimization in question might well turn out to be misoptimization
>     for anything but microbenchmarks (read LKML for cache flush/dirty
>     page issues).

See http://gcc.gnu.org/ml/gcc/2007-10/msg00607.html for a test case
that shows where it can go horrible wrong (optimized code significantly 
slower than unoptimized code) Admittedly it is a constructed
one, but I don't think it is that unrealistic.

>   - there's also a good talk on lawyer-ish vs attached-to-reality
>     approach.  I personally doubt those who continue to advise to use
>     volatile are actually writing such multithreaded programs.  Most
>     argue just for the fun of it.

Also they don't volunteer to audit multi-million LOC code bases to add
volatile everywhere. That has to be always taken into account.  For
the compiler it is a relatively simple localized change and then the
computer does all the work. For the compiled programs
auditing/changing this would be a huge effort done by humans.

-Andi