AR# 2758: M1.3/M1.4 MAP: unclear messages about FDCEs being "covered by optimization" in the MAP .mrp file when the -os option is specified
M1.3/M1.4 MAP: unclear messages about FDCEs being "covered by optimization" in the MAP .mrp file when the -os option is specified
Keyword: map, optimization, cover
Description: When running map with the -os speed option in M1.3.7, one may get messages of the type, "FF (FDCE) was covered by optimization" in the map.mrp report:
"c0/c1/inst_slotsequencer/inst_dispatch/addr1_reg<10>" (FDCE) was covered by optimization. "c0/c1/inst_slotsequencer/inst_dispatch/addr1_reg<11>" (FDCE) was covered by optimization.
The meaning of such messages may not be clear.
Reference #: 100637
This message usually appears when the user uses the -os option in Map. The -os option causes Map to invoke the OPTX optimizer. One workaround is to not specify the option when running Map.
The message does not mean that the components have been trimmed out. It means that the FFs specified are located in CLBs whose logic has been optimized.
When OPTX changes a CLB, it places it in a mapper structure called a GROUP. Any other logical elements that get combined with the logical elements of a given GROUP also become members of that GROUP.
When Map reports on logic in a GROUP, it reports on ALL elements in the GROUP. So when Map issues the message, "FFs (FDCE) are covered by optimization", it is reporting that the flip-flops are part of that group, possibly having been merged into the GROUP in the process of optimizing the design.
If the design has already been optimized by a synthesis tools such as Synopsys, you should not be using the -os optimization switch.