[Bug c/57612] New: add builtin to assert that expression does not have side effects

[tromey at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57612

            Bug ID: 57612
           Summary: add builtin to assert that expression does not have
                    side effects
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tromey at gcc dot gnu.org

It would sometimes be useful to be able to assert
that an expression does not have side effects.
For example, this would be very nice to have for
macros which expand their arguments more than once:

#define multi_expand(x)  __builtin_assert_no_side_effects(x) + (x)

Specifically, this builtin would check its argument and
give an error if the argument had an assignment or a call
to a function which is neither "const" nor "pure".
The builtin would evaluate to the value of its argument.