luke/verilator
1
0
Fork 0
mirror of https://github.com/verilator/verilator.git synced 2026-09-04 16:40:12 +02:00
Code Issues Packages Projects Releases Wiki Activity

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

Browse Source 
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
Download Patch File Download Diff File Expand all files Collapse all files
src/V3Options.cpp
+3
View File
@@ -1449,9 +1449,11 @@ void V3Options::parseOptsList(FileLine* fl, const string& optdir, int argc,
DECL_OPTION("-facyc-simp", FOnOff, &m_fAcycSimp);
DECL_OPTION("-fassemble", FOnOff, &m_fAssemble);
DECL_OPTION("-fcase", CbFOnOff, [this](bool flag) {
m_fCaseDecoder = flag;
m_fCaseTable = flag;
m_fCaseTree = flag;
});
DECL_OPTION("-fcase-decoder", FOnOff, &m_fCaseDecoder);
DECL_OPTION("-fcase-table", FOnOff, &m_fCaseTable);
DECL_OPTION("-fcase-tree", FOnOff, &m_fCaseTree);
DECL_OPTION("-fcombine", FOnOff, &m_fCombine);
@@ -2356,6 +2358,7 @@ void V3Options::optimize(int level) {
const bool flag = level > 0;
m_fAcycSimp = flag;
m_fAssemble = flag;
m_fCaseDecoder = flag;
m_fCaseTable = flag;
m_fCaseTree = flag;
m_fCombine = flag;