marcov wrote on Fri, 18 Dec 2015:
In our previous episode, Jonas Maebe said:
The reason the need for volatile on embedded systems is far less in FPC,
is the existence of the "absolute" keyword. In C, you have to declare
[snip]
(you also need it for variables you access from interrupt routines, as they
might change while a procedure is running)
If they're global variables, which they presumably are, that's no more
necessary than in the multi-threading case.
In FPC, every statement is basically considered to be a compiler-level
memory barrier for accesses to global variables. So within a single
statement multiple accesses can be optimised, but that cannot happen not
across statements. It's not perfect since it inhibits some optimisation,
but on the flip side it's very simple and easy to reason about.
That's what I remember from earlier "volatile" discussions. But what about
LLVM? If you want to squeeze performance out of it, won't that need
volatile sooner or later?
It will help, but optimising accesses to global variables is not the
only thing LLVM can do :) Right now, I'll indeed should mark all loads
from and stores to global variables as volatile (which I don't do yet
and therefore tests/test/tmt1.pp gets into an endless loop right now
when compiled with LLVM).
Jonas
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