Stmtrewrite
Rewrite statements into simpler forms.
This transform simplifies Verilog statements by applying rewrite
rules. The idea is to make later statement processing simpler by reducing the
variety of statements that need to be supported. This is somewhat similar to
how the oprewrite transform eliminates certain operators, leaving us
with fewer operators to support later on. See always-top for how this
fits into our overall handling of always blocks.
Note that, for instance, our rewrites eliminate $display statements.
This is suitable when your goal is to analyze the functionality of the module
from a synthesis/build perspective, e.g., with esim. But it is
obviously not suitable if you want to use the resulting modules with a
Verilog simulator.
Notes:
- This transform should typically be run after unparameterization so
that, e.g., width parameters will have been propagated. For instance, things
like repeat (n) body won't get unrolled unless n has already been
resolved.
- It should typically run before sizing, since we generate unsized
expressions.
Some implemented rewrites include:
- $display(...) → null
- pure-null if/case stmts → null
- eliminate null stmts from blocks
- empty blocks → null
- collapse singleton blocks (i.e., begin stmt end --> stmt)
- flatten compatible sub-blocks
- @(...) null → null (top level only)
- merge nested ifs (without elses)
- wait statements → while loops
- forever statements → while loops
- unroll some repeat statements (up to a limit)
- eliminate always null;
- eliminate initial null;
- eliminate for(...) null;
Note that we don't transform case statements into if statement here, but
that's done in caseelim.
Bozo eventually we should also unroll simple while/for loops.
Subtopics
- Vl-modulelist-stmtrewrite
- (vl-modulelist-stmtrewrite x unroll-limit) maps vl-module-stmtrewrite across a list.
- Vl-blockstmt-rewrite
- Flatten sub-blocks, eliminate unnecessary blocks, and remove any null
statements from a block.
- Vl-repeatstmt-rewrite
- Unroll deterministic repeat statements.
- Vl-flatten-blocks
- Collapse nested begin/end and fork/join blocks.
- Vl-stmt-rewrite-top
- Wrapper for vl-stmt-rewrite that adds additional rewrites that
are valid only for top-level statements.
- Vl-casestmt-rewrite
- Eliminate pure-null case statements.
- Vl-stmt-rewrite
- Core statement rewriter.
- Vl-waitstmt-rewrite
- Convert wait statements into empty while loops.
- Vl-ifstmt-combine-rewrite
- Eliminate pure-null if statements and merge simply nested ifs.
- Vl-forstmt-rewrite
- Eliminate purely null vl-forstmts.
- Vl-initiallist-stmtrewrite
- Vl-alwayslist-stmtrewrite
- Vl-initial-stmtrewrite
- Vl-foreverstmt-rewrite
- Convert forever statements into while loops.
- Vl-always-stmtrewrite
- Vl-module-stmtrewrite
- Vl-design-stmtrewrite
- Top-level stmtrewrite transform.
- Vl-remove-null-statements
- Vl-$vcover-stmt-p
- BOZO Centaur specific.
- Vl-$display-stmt-p
- Recognize a $display statement.
- Vl-caselist-all-null-p
