Use the DONT_TOUCH attribute in place of KEEP or KEEP_HIERARCHY. The DONT_TOUCH works in the same way as KEEP or KEEP_HIERARCHY attributes however, unlike KEEP and KEEP_HIERARCHY, DONT_TOUCH is forward-annotated to place and route to prevent logic optimization.
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CAUTION! Like KEEP and KEEP_HIERARCHY, be careful when using DONT_TOUCH. In cases where other attributes are in conflict with DONT_TOUCH, the DONT_TOUCH attribute takes precedence.
The values for DONT_TOUCH are TRUE/FALSE or yes/no. You can place this attribute on any signal, module, entity, or component.
Note: The DONT_TOUCH attribute is not supported on the port of a module or entity. If specific ports are needed to be kept, either use the -flatten_hierarchy none setting, or put a DONT_TOUCH on the module/entity itself.
In general, the DONT_TOUCH attribute should be set in RTL only. Signals that need to be kept can often be optimized before the XDC file is read. Therefore, setting this attribute in the RTL ensures that it is used. There is one use case where it is recommended that DONT_TOUCH is set in the XDC file. This would be when DONT_TOUCH is set to yes in the RTL, and it is desired to be taken out without having to change the RTL. In this case, setting DONT_TOUCH to no in XDC when that same signal has DONT_TOUCH set to yes in RTL will effectively remove that attribute without having to change the RTL.
Note: When using the XDC to remove a DONT_TOUCH that is set in RTL, you may end up getting warnings after synthesis when the implementation flow reads the same XDC but the signal in question has been optimized out. These warnings can be ignored. However, you can also bypass them by putting the DONT_TOUCH attributes in an XDC file that is marked as for synthesis only.