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Optimize wide decoder case statements into decoder expressions (#7804)

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Extend the decoder-pattern case optimization to selectors that are too
wide for a full 2^width lookup table. A decoder-pattern case (where
every case item assigns constants to a fixed set of LHSs) is lowered to
a new AstMachMasked expression. AstMachMasked is emitted as a run-time
VL_MATCHMASKEd_* function call. It contains a packed constant pool table,
'matchp', which is a list of '(mask, bits)' pairs. At runtime, the index of the 
first matching entry is returned, and is used to index a value table. This single
(albeit complicated) expression can replace large if-else trees whole, resulting
in much more compact code with fewer static hard to predict branches. It
is worth about 10% speed and 30% code size in some designs.

Example:

```systemverilog
    logic [39:0] sel;
    always_comb
      casez (sel)
        40'b???????????????????????????????????????1: out = 8'h01;
        40'b??????????????????????????????????????1?: out = 8'h02;
        40'b?????????????????????????????????????1??: out = 8'h03;
        default:                                      out = 8'hff;
      endcase
```

is compiled to:

```c++
    out = TABLE_value[VL_MATCHMASKED_Q(sel, CONST_match)];
```

Where 'CONST_match' contains 4 entries, of a 40-bit mask and 40-bit bit
pattern each, and 'TABLE_value' contains 4 entries of the corresponding
8-bit results. (Entries are aligned to word boundaries to avoid runtime
bit swizzling)
This commit is contained in:
Geza Lore
2026-06-19 19:46:13 +01:00
committed by GitHub
parent 59fba72cb6
commit a37e2ee94b
34 changed files with 1114 additions and 87 deletions
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src/V3Const.cpp
+11 -1
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@@ -3019,6 +3019,15 @@ class ConstVisitor final : public VNVisitor {
}
}
void visit(AstClassOrPackageRef* nodep) override { iterateChildren(nodep); }
void visit(AstMatchMasked* nodep) override {
// Do not iterate the tables, they must be constant pool entries
iterate(nodep->lhsp());
if (AstConst* const constp = VN_CAST(nodep->lhsp(), Const)) {
replaceNum(nodep, AstMatchMasked::fold(constp->num(), nodep->matchp()->varp()));
}
}
void visit(AstPin* nodep) override { iterateChildren(nodep); }
void replaceLogEq(AstLogEq* nodep) {
@@ -3281,7 +3290,8 @@ class ConstVisitor final : public VNVisitor {
iterateChildren(nodep);
UASSERT_OBJ(nodep->varp(), nodep, "Not linked");
bool did = false;
if (m_doV && !nodep->varp()->constPoolEntry() && nodep->varp()->valuep() && !m_attrp) {
if (m_doV && (!nodep->varp()->constPoolEntry() || m_selp) && nodep->varp()->valuep()
&& !m_attrp) {
// UINFOTREE(1, valuep, "", "visitvaref");
iterateAndNextNull(nodep->varp()->valuep()); // May change nodep->varp()->valuep()
AstNode* const valuep = nodep->varp()->valuep();