[Bug c/26369] New: value expectation attribute

["acahalan at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Something used roughly like this:

unsigned foo __attribute__((
  min(5),  // value will never be below this
  max(0x007fff04),  // value will never be above this
  set(0x00000004),  // these bits will always be set
  clear(0xff800002),  // these bits will never be set
  likely_min(100),  // the value is rarely below this
  likely_max(12345),  // the value is rarely above this
  likely_set(0x00000604),  // these bits are usually set
  likely_clear(0xfffff032)  // these bits are usually clear
)) = 100;

The idea is that I tell gcc that my variable (or type) is certain or likely to
have various values. This lets me avoid marking up lots and lots of code with
__builtin_expect. I can just mark the variable or type.

A bit of error checking can be done too. It would be invalid to pass a value
that never exceeds 1 to a function whose argument is marked as always having
bit 4 set. The compiler could catch this and halt with an error.


-- 
           Summary: value expectation attribute
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: acahalan at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26369