[Bug c/70967] New: doc: Please document rotate idiom

["hpa at zytor dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70967

            Bug ID: 70967
           Summary: doc: Please document rotate idiom
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hpa at zytor dot com
  Target Milestone: ---

The idiom that gcc has recognized since time immemorial (gcc 2-something) to
generate a rotate:

    ((foo >> n) + (foo << (sizeof(foo) * CHAR_BIT - n))

    /* | instead of + has been recognized for almost but not quite as long */

... is undefined behavior under the C standard when n == 0.  There is a
technically portable idiom (bug 57157) which has been recognized since gcc
4.6.3, but that is not portable to older versions of gcc without generating
suboptimal code.

It would be great if the gcc manual could be augmented to document this idiom
for rotate as a GNU C extension so that people are less concerned about its
validity, and compilers that mimic the GNU C dialect (icc, clang, etc.) know to
implement it correctly.