The BSLS_PERFORMANCEHINT_PLACEMENT_NEW_FENCE is intended specifically to work around what appears to be a bug in the xlC compiler.  The bug occurs at least in xlC 11 and probably earlier xlC compilers.  The bug has been reproduced in xlC 11 by IBM and they are currently looking to find the root cause.  The bug does not appear to occur in xlC 12.1, but IBM helpfully mentioned that they will also look to see that this is actually fixed in xlC 12.1, instead of possibly hidden by other improvements.

In the future, the intention is that BSLS_PERFORMANCEHINT_PLACEMENT_NEW_FENCE be #ifdef'd to be a compiler fence only for versions of xlC prior to ??? (TBD).