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fix reversed bit order in PriorityEncoder #640

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fix reversed bit order in PriorityEncoder #640

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hansfbaier
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@hansfbaier hansfbaier commented Oct 9, 2021 •
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After the change it works as expected:
image
Slicing already has reversed bit numbering in comparison to verilog,
so the reversed function was not necessary.
Or, rather, because the last m.d.comb wins, that did the reversing.

@hansfbaier
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with reversed(..) it looks like this:
image

@whitequark
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This PR breaks the testsuite.

@hansfbaier
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Ah now I see what the module and the test is doing. It gets the bit number of the least significant nonzero bit. I was expecting to get the most significant nonzero bit according to https://en.m.wikipedia.org/wiki/Priority_encoder
Is that intentional?

@whitequark
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whitequark commented Oct 9, 2021 •
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Yes. PriorityEncoder has an invariant:

Assert(enc.n | enc.i.bit_select(enc.o, 1))

@hansfbaier
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Ok,. Thanks

@hansfbaier hansfbaier closed this Oct 9, 2021
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@hansfbaier @whitequark