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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


Keyword: map, optimization, cover

Urgency: Standard

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.

Please refer to (Xilinx Solution 2680)
for guidance on when the -os switch should be used.
AR# 2758
Date 04/03/2000
Status Archive
Type General Article