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There are two elements involved:
1) Apply the relevant full_case and/or parallel_case attribute(s) to
the generated AST_CASE node(s), so that the existing AST frontend and
subsequent passes will generate RTLIL with appropriate behaviour.
(This is handled in the parser "if_attr" non-terminal.)
2) Rearrange the AST_CASE structure when necessary. For "priority if"
(i.e., full_case), this requires only ensuring that directly nested
"else if" branches also inherit the full_case attribute. For
"unique if" and "unique0 if" (i.e., parallel_case+full_case and
parallel_case alone), there are two steps:
a) Flatten the AST_CASE structure such that any direct "else if"
branches are mapped to additional AST_CONDs in the parent;
b) Reverse the "direction" of the test: the constant 1 (true)
is provided in the AST_CASE node, and the expression(s) in the
if statement(s) are given in each AST_COND. This is necessary
because the constant 1, being the common factor, must occupy the
shared AST_CASE position.
(This is handled in the parser "TOK_IF" expansion of behavioral_stmt.)
Observe that:
* The generated AST has not been changed for bare "if"s (those
without unique/priority). This should minimise the risk of
unexpected regressions.
* It is possible that the flattening described in 2) a) above might
affect the behaviour of expressions with side effects in "unique if"
statements (consider "unique if( a ) ...; else if( b++ ) ...": if
a is true, is b incremented?). While it might be possible to provide
precise semantics here, IEEE 1800-2012 12.4.2 seems to be deliberately
vague ("In unique-if and unique0-if, the conditions may be evaluated
and compared in any order[...] The presence of side effects in
conditions may cause nondeterministic results.") and so it seems
doubtful that there is benefit in Yosys providing stronger promises
on the interpretation of questionable code.
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