Add __sync_fetch_and_nand (again)


Add __sync_fetch_and_nand (again)

Prior to GCC 4.4, __sync_fetch_and_nand was implemented as:

{ tmp = *ptr; *ptr = ~tmp & value; return tmp; }

but this was changed in GCC 4.4 to be:

{ tmp = *ptr; *ptr = ~(tmp & value); return tmp; }

in response to this change, support for sync_fetch_and_nand (and
sync_nand_and_fetch) was removed in r99522 in order to avoid miscompiling code
depending on the old semantics. However, at this point:

  1. Many years have passed, and the amount of code relying on the old semantics is likely smaller.
  2. Through the work of many contributors, all LLVM backends have been updated such that "atomicrmw nand" provides the newer GCC 4.4+ semantics (this process was complete July of 2014 (added to the release notes in r212635).
  3. The lack of this intrinsic is now a needless impediment to porting codes from GCC to Clang (I've now seen several examples of this).

It is true, however, that we still set GNUC_MINOR to 2 (corresponding to GCC
4.2). To compensate for this, and to address the original concern regarding
code relying on the old semantics, I've added a warning that specifically
details the fact that the semantics have changed and that we provide the newer

Fixes PR8842.


hfinkelOct 2 2014, 1:53 PM
rL218904: Limit test to Darwin