These warnings occur with the new XST parser for Virtex-6, Spartan-6, and later architectures.
The reason is that the attribute "syn_preserve" is being set to 1 as outlined in the Synplify user documentation.
/* synthesis syn_preserve=1 */;
To workaround the warning and to correctly apply the constraint, use the following:
/* synthesis syn_preserve=TRUE */;
This issue is fixed in the 14.1 release.
The following message is generated instead:
WARNING:Xst - Value "1" of property "syn_preserve" is not applicable. List of valid values is "true, false, yes, no"