Run-Time Addresses for Names
Introduction: The storage-allocation strategy and the layout of local data in an activation record for a procedure determine how the storage for names is accessed. In Code-generation, we assumed that a name in a three-address statement is really a pointer to a symbol-table entry for that name.
This approach has a significant advantage; it makes the compiler more portable, since the front end need not be changed even when the compiler is moved to a different machine where a different run-time organization is needed. On the other hand, generating the specific sequence of access steps while generating intermediate code can be of significant advantage in an optimizing compiler, since it lets the optimizer take advantage of details it would not see in the simple three-address statement.
In either case, names must eventually be replaced by code to access storage locations. We thus consider some elaborations of the simple three-address copy statement x = 0. After the declarations in a procedure are processed, suppose the symbol-table entry for x contains a relative address 12 for x. For consider the case in which x is in a statically allocated area beginning at address static. Then the actual run-time address of x is static 12. Although the compiler can eventually determine the value of static 12 at compile time, the position of the static area may not be known when intermediate code to access the name is generated. In that case, it makes sense to generate three-address code to "compute" static 12, with the understanding that this computation will be carried out during the code generation phase, or possibly by the loader, before the program runs. The assignment x = 0 then translates into
static = 0
If the static area starts at address 100, the target code for this statement is
LD 112, #0