Optimizer Assumptions

Global variables, that is, all variables with file scope.

Local variables that have had their addresses taken either explicitly by the & operator, or implicitly by the automatic conversion of array references to pointers.

In general, you do not need to be concerned about this assumption. Standard conformant C programs do not violate this assumption. However, if you have code that does violate this assumption, the optimizer can change the behavior of the program in an undesired way. In particular, you should avoid the following coding practices to ensure correct program execution for optimized code:

Avoid referencing outside the bounds of an array.

Avoid passing incorrect number of arguments to functions.

Avoid accessing an array other than the one being subscripted. For example, the construct a[b-a] where a and b are the same type of array actually references the array b, because it is equivalent to *(a+(b- a)), which is equivalent to *b. Using this construct might yield unexpected optimization results.

Avoid referencing outside the bounds of the objects a pointer is pointing to. All references of the form *(p+i) are assumed to remain within the bounds of the variable or variables that p was assigned to point to.

Avoid using variables that are accessed by external processes. Unless a variable is declared with the volatile attribute, the compiler will assume that a program's data structures are accessed only by that program. Using the volatile attribute may significantly slow down a program.

Avoid using local variables before they are initialized. When you request optimization level 2, 3, or 4, the compiler tries to detect and flag violations of this rule.

Avoid relying on the memory layout scheme when manipulating pointers; incorrect optimizations may result. For example, if p is pointing to the first member of structure , it should not be assumed that p1 points to the second member of the structure. Another example: if p is pointing to the first in a list of declared variables, p1 should not be assumed to be pointing to the second variable in the list.