Q. When should the register modifier be used? Does it really help?
The register modifier hints to the compiler that the variable will be heavily used and should be kept in the CPU's registers, if possible, so that it can be accessed faster. There are several restrictions on the use of the register modifier.
First, the variable must be of a type that can be held in the CPU's register. This usually means a single value of a size less than or equal to the size of an integer. Some machines have registers that can hold floating-point numbers as well.
Second, because the variable might not be stored in memory, its address cannot be taken with the unary and operator. An attempt to do so is flagged as an error by the compiler. Some additional rules affect how useful the register modifier is. Because the number of registers is limited, and because some registers can hold only certain types of data (such as pointers or floating-point numbers), the number and types of register modifiers that will actually have any effect are dependent on what machine the program will run on. Any additional register modifiers are silently ignored by the compiler.
Also, in some cases, it might actually be slower to keep a variable in a register because that register then becomes unavailable for other purposes or because the variable isn't used enough to justify the overhead of loading and storing it.
So when should the register modifier be used? The answer is never, with most modern compilers. Early C compilers did not keep any variables in registers unless directed to do so, and the register modifier was a valuable addition to the language. C compiler design has advanced to the point, however, where the compiler will usually make better decisions than the programmer about which variables should be stored in registers. In fact, many compilers actually ignore the register modifier, which is perfectly legal, because it is only a hint and not a directive.
In the rare event that a program is too slow, and you know that the problem is due to a variable being stored in memory, you might try adding the register modifier as a last resort, but don't be surprised if this action doesn't change the speed of the program.
Q. When should the volatile modifier be used?
The volatile modifier is a directive to the compiler's optimizer that operations involving this variable should not be optimized in certain ways. There are two special cases in which use of the volatile modifier is desirable. The first case involves memory-mapped hardware (a device such as a graphics adaptor that appears to the computer's hardware as if it were part of the computer's memory), and the second involves shared memory (memory used by two or more programs running simultaneously).
Most computers have a set of registers that can be accessed faster than the computer's main memory. A good compiler will perform a kind of optimization called "redundant load and store removal." The compiler looks for places in the code where it can either remove an instruction to load data from memory because the value is already in a register, or remove an instruction to store data to memory because the value can stay in a register until it is changed again anyway.
If a variable is a pointer to something other than normal memory, such as memory-mapped ports on a peripheral, redundant load and store optimizations might be detrimental. For instance, here's a piece of code that might be used to time some operation:
time_t time_addition(volatile const struct timer *t, int a){ int n; int x; time_t then; x = 0; then = t->value; for (n = 0; n < 1000; n++) { x = x + a; } return t->value - then;}In this code, the variable t->value is actually a hardware counter that is being incremented as time passes. The function adds the value of a to x 1000 times, and it returns the amount the timer was incremented by while the 1000 additions were being performed.
Without the volatile modifier, a clever optimizer might assume that the value of t does not change during the execution of the function, because there is no statement that explicitly changes it. In that case, there's no need to read it from memory a second time and subtract it, because the answer will always be 0. The compiler might therefore "optimize" the function by making it always return 0.
If a variable points to data in shared memory, you also don't want the compiler to perform redundant load and store optimizations. Shared memory is normally used to enable two programs to communicate with each other by having one program store data in the shared portion of memory and the other program read the same portion of memory. If the compiler optimizes away a load or store of shared memory, communication between the two programs will be affected.
Q. Can a variable be both const and volatile?
Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. For instance, the timer structure was accessed through a volatile const pointer. The function itself did not change the value of the timer, so it was declared const. However, the value was changed by hardware on the computer, so it was declared volatile. If a variable is both const andvolatile, the two modifiers can appear in either order.
Q. When should the const modifier be used?
There are several reasons to use const pointers. First, it allows the compiler to catch errors in which code accidentally changes the value of a variable, as in
while (*str = 0) /* programmer meant to write *str != 0 */{ /* some code here */ str++;}in which the = sign is a typographical error. Without the const in the declaration of str, the program would compile but not run properly.
Another reason is efficiency. The compiler might be able to make certain optimizations to the code generated if it knows that a variable will not be changed.
Any function parameter which points to data that is not modified by the function or by any function it calls should declare the pointer a pointer to const. Function parameters that are passed by value (rather than through a pointer) can be declared const if neither the function nor any function it calls modifies the data.
In practice, however, such parameters are usually declared const only if it might be more efficient for the compiler to access the data through a pointer than by copying it.
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