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Re: volatile [was: GCC warnings for unused global variables]
Alexandre Oliva wrote:
On May 3, 2003, Michael Eager <eager@eagercon.com> wrote:
It can also be described in more practical terms:
-- each read or write of a volatile must be exactly as specified by the
abstract machine
-- the ordering and number of reads or writes to a volatile must be
exactly as specified
Which brings an interesting point. GCC won't translate <op>= into a
single machine instruction that loads, operands then stores if the
variable being operated on is volatile. Why is that? Wouldn't it be
just as correct to translate say `i |= 1' into `or i,1' on a machine
that supports such an instruction?
As I said before, volatile semantics are not defined precisely in either
C or C++. The standards do say that "a <op>= b" is equivalent to "a = a <op> b".
IIRC, neither says that it must be expressed exactly this way when a is
volatile, but this is probably a natural interpretation. It does not say
that it cannot be expressed by a single instruction which has the desired
effect. (In many cases, it would seem that a single instruction would be
desirable, but how is the compiler to know for sure?)
If you take the view that "a <op>= b" involves a read and a write, then
translating it into load/operate/store is correct. If you take the view
that this is an atomic operation and that the load and store are implicit
then the single instruction is correct. I believe that the standard is
vague enough to permit either interpretation to be supported.
--
Michael Eager Eager Consulting eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077