Re: Substitute for volatile
- From: Lew Pitcher <Lew.Pitcher@xxxxxx>
- Date: Tue, 10 Jan 2006 08:31:04 -0500
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vijay.rajamanickam@xxxxxxxxx wrote:
> I am a newbie w.r.t. compiler optimizations and I believe these
> optimizations
/which/ optimizations?
> are generally done in function level.
> Can I safely assume that if I use getter & setter functions,
> I can effectively substitute using the qualifier volatile
> for variables in all instances. Thanks in advance
I'm not certain of what you are asking here.
Perhaps I can answer you this way:
"volatile" is a hint to the compiler that the contents of a variable may
change out-of-band, and that it should not make assumptions about the
variable's contents in it's optimizations. It doesn't matter whether the
optimizations are at a function level, or at some other level;
"volatile" says don't make them.
The basic assumption that "volatile" disables is the one that the
compiler can keep track of variable initializations and discard
unnecessary initializations silently. This sort of behaviour occurs when
the variable is set with a value, and the value isn't seen to change
before the next reference to the variable. In a case like that, the
compiler is permitted to discard related operations so long as the
appearance of continuity is preserved.
For instance, given the code fragment...
{
int a = 10; /* A */
a = 10; /* B */
if (a == 10) return; /* C */
}
Since variable "a" is initialized to 10 at line "A", and no intervening
changes are made to the value of "a" between line "A" and line "B", the
compiler may assume that variable "a" has a value of 10 at line "B". It
may then discard the initialization of line "B" completely, because the
end condition of line "B" (a having the value of 10) is already guaranteed.
Similarly, since "a" is still guaranteed to have the value of 10 at line
"C", the compiler may optimize away the test (a == 10) and substitute a
TRUE value for it, and may even further optimize away the entire if
statement (because it is always TRUE) and just generate the code for the
return statement.
With volatile,
{
volatile int a = 10; /* A */
a = 10; /* B */
if (a == 10) return; /* C */
}
the compiler is forbidden to optimize away either line B or line C
Please note that "volatile" is not the default behaviour; if you don't
need "volatile", you don't have to use it.
So, when would you need "volatile"? You'd need it when the variable may
change asynchronously to the flow of the program. Typically, this comes
from the activities of a signal handler, which can run asynchronously to
the main code. There are also per-platform cases where volatile may be
necessary, but these are not explicitly covered by ISO C.
HTH
PS, your reference to "getter" and "setter" functions leads me to
question whether you are programming in C (the topic of comp.lang.c) or
C++. If you have a C++ mindset, then I must warn you that C is a
different language than C++, and while you can write OO code in C, most
do not.
- --
Lew Pitcher, IT Specialist, Enterprise Data Systems
Enterprise Technology Solutions, TD Bank Financial Group
(Opinions expressed here are my own, not my employer's)
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