HP C Programmer's Guide (92434-90009)

Chapter 4 105
Optimizing HP C Programs
Aliasing Options
Aliasing Options
To be conservative, the optimizer assumes that a pointer can point to any object in the
entire application. Instead, if the optimizer can be educated on the application's pointer
usage, then the optimizer can generate more efficient code, due to the elimination of some
false assumptions. Such behavior can be communicated to the optimizer by using the
following options:
• +O[no]ptrs_strongly_typed
• +O[no]ptrs_to_globals[=list]
• +O[no]global_ptrs_unique[=list]
• +O[no]ptrs_ansi
where list is a comma-separated list of global variable names.
Here are the type-inferred aliasing rules:
• Type-aliasing optimizations are based on the assumption that pointer dereferences
obey their declared types.
• A C variable is considered
address-exposed if and only if the address of that variable
is assigned to another variable or passed to a function as an actual parameter. In
general, address-exposed objects are collected into a separate group based on their
declared type. Global variables and static variables are considered address-exposed by
default. Local variables and actual parameters are considered address-exposed only if
their address has been computed using the address operator .
• Dereferences of pointers to a certain type will be assumed to only alias with the
corresponding equivalent group. An equivalent group includes all the address exposed
objects of the same type. The dereferences of pointers are also assumed to alias with
other pointer dereferences associated with the same equivalent group.
In the example
int *p, *q;
*p and *q are assumed to alias with any objects of type int. Also *p and *q are
assumed to alias with each other.
• Signed/Unsigned type distinctions are ignored in grouping objects into an equivalent
group. Likewise, long and int types are considered to map to the same equivalent
group. However, the volatile type qualifier is considered significant in grouping
objects into equivalent groups (e.g., a pointer to int will not be considered to alias with
a volatile int object).
• If two type names reduce to the same type, they are considered synonymous.
In the following example, both types type_old and type_new will reduce to the same
type, struct foo.
typedef struct foo_st type_old;
typedef type_old type_new;