frontend: don't connect a const net to ports connected to x
#1447
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prjunnamed normalizes ports that are not present in the primitive to be all-x. On iCE40, this can cause a false placement conflict between `SB_IO` cells where one's clock input is `x` and another's is some other net.
whitequark
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Jan 30, 2025
Roundtrip tested with the Glasgow `uart` applet, with these nextpnr PRs required: - YosysHQ/nextpnr#1447 - YosysHQ/nextpnr#1448 Co-authored-by: Catherine <[email protected]>
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prjunnamed normalizes ports that are not present in the primitive to be all-x. On iCE40, this can cause a false placement conflict between
SB_IOcells where one's clock input isxand another's is some other net.I haven't tested every other architecture (nor do I have the means to) so I can't know for sure this change won't have some other fallout, but it seems pretty weird to treat an absent port differently from an all-x port so arguably that would be an architecture bug.
Alternatively an iCE40-specific workaround in
arch_place.cccould be used.