The may_alias type attribute

You can specify the may_alias type attribute for a type so that lvalues of the type can alias objects of any type, similar to a char type. Types with the may_alias attribute are not subject to type-based aliasing rules.


may_alias type attribute syntax

>>-__attribute__--((--+-may_alias-----+--))--------------------><
                      '-__may_alias__-'

You can specify the may_alias type attribute in the following ways:

struct __attribute__((__may_alias__)) my_struct {} *ps;
typedef long __attribute__((__may_alias__)) t_long;
typedef struct __attribute__((__may_alias__)) my_struct {} t_my_struct;

Instead of specifying -fno-strict-aliasing, you can alternatively specify the may_alias type attribute for a type to violate the ANSI aliasing rules when compiling expressions that contain lvalues of that type. For example:

#define __attribute__(x)     // Invalidates all __attribute__ declarations
typedef float __attribute__((__may_alias__)) t_float;

int main (void){
  int i = 42;
  t_float *pa = (t_float *) &i;
  *pa = 0;
  if (i == 42)
    return 1;
  return 0; 
}

If you compile this code with the -fstrict-aliasing (-qalias=ansi) option at a high optimization level, such as -O3, the executable program returns 1. Because the lvalue *pa is of type float, according to the ANSI aliasing rules, the assignment to lvalue *pa cannot modify the value of i, which is of type int.

If you remove the #define __attribute__(x) statement and compile the code with the same options as before, the executable program returns 0. Because the type of *pa is float __attribute__((__may_alias__)), *pa can alias any other object of any type, and the assignment to lvalue *pa can modify the value of i to 0.

The use of the may_alias type attribute can result in less pessimistic aliasing assumptions by the compiler, leading to more optimization opportunities, compared to usage of the -fno-strict-aliasing (-qalias=noansi) compiler option.

C only begins This attribute is supported at the extc89, extc99, extended, and extc1x language levels. C only ends

C++ only begins This attribute is supported at the extended, extended0x, and extended1y language levels. C++ only ends

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