The Shape of Code

The contents of some storage locations, used by a program, might be modified outside of the control of that program, e.g., a real-time clock or the input port of a communications device. In some cases writing to particular storage locations has an external effect, e.g., a sequence of bits is sent down a communications channel. This kind of behavior commonly occurs in embedded systems

C and C++ support the existence of variables that have been mapped to such storage locations through the use of the volatile type qualifier.

volatile long flag;
volatile time_t timer;
 
struct {
                    int f1 : 2;
          volatile int f2 : 2;
                    int f3 : 3;
         } x;

When a variable is declared using volatile compilers must assume that its value can change in ways unknown to the compiler and that storing values into such a variable can have external effects. Consequently almost all optimizations involving such variables are off limits. Idioms such as timer = timer; are used to reset or refresh timers and are not dead code.

Volatile is a bit of an inconvenience for writers of code optimizers, requiring them to add checks to make sure the expression or statement they want to attempt to optimize does not contain a volatile qualified variable. In many environments the semantics of volatile are not applicable, which means that bugs in optimizers have a much smaller chance of being detected than faults in other language constructs.

There have been a number of research projects investigating the use of the const qualifier, but as far as I know only one that has investigated volatile. The ‘volatile’ project found that all of the compilers tested generated incorrect code for some of the tests. License agreements prevented the researchers giving details for some compilers. One interesting observation for gcc was that the number of volatile related faults increased with successive compiler releases; it looks like additional optimizations were being implemented and these were not checking for variables being volatile qualified.

One practical output from the project was a compiler stress tester targeting volatile qualified variables. The code generated by stress testers often causes compilers to crash and hang and the researchers reported the same experience with this tool.

There is one sentence in the C Standard whose overly broad wording is sometimes a source of uncertainty: What constitutes an access to an object that has volatile-qualified type is implementation-defined. This sentence applies in two cases: datatypes containing lots of bytes and bit-fields.

To save space/money/time/power some processors access storage a byte, or half-word, at a time. This means that, for instance, an access to a 32-bit storage location may occur as two separate 16-bit operations; from the volatile perspective does that constitute two accesses?

Most processors do not contain instructions capable of loading/storing a specified sequence of bits from a byte. This means that accesses to bit-fields involve one or more bytes being loaded and the appropriate bits extracted. In the definition of x above an access to field f1 is likely to result in field f2 (and f3) being accessed; from the volatile perspective does that constitute two accesses?

Unfortunately it is very difficult to obtain large quantities of source code for programs aimed at the embedded systems market. This means that it is not possible to obtain reliable information on common usage patterns of volatile variables. If anybody knows of any such code base please let me know.