Fetching a thread-local is relatively expensive. Random initializing
wides used to do it once or twice per word, and on short runs for
large designs can be noticeably expensive, so fetch once per variable
instead. Also remove unused VL_RAND_RESET_{Q,W} functions.
Recognize the common single-bit scan loop idioms in V3Unroll (before it
unrolls) and lower them to bit-reduction primitives, replacing a literal
W-iteration loop with one intrinsic-backed expression:
target=0; for (i=0;i<W;i++) if (vec[i]) target = i + 1; -> $mostsetbitp1(vec)
target=0; for (i=0;i<W;i++) if (vec[i]) target = target + 1; -> $countones(vec)
The leading-one form lowers to a new AstMostSetBitP1 node, emitted as
VL_MOSTSETBITP1_{I,Q,W}; those runtime helpers now use __builtin_clz where
available (same pattern as VL_REDXOR's __builtin_parity), with the existing
bit scan as fallback. The count-ones form reuses AstCountOnes ($countones,
popcount); as the DFG requires a 32-bit countones result it is built at 32
bits and narrowed to the accumulator width with a select.
Matching is structural to stay sound: the index must start at 0, increment
by exactly 1, and scan all W==width(vec) bits via a single 1-bit select of a
distinct vector, with the target pre-zeroed and no else branch. The loop
bound is accepted as a strict ascending 'idx < W' written either way and
signed or unsigned (Gt/GtS/Lt/LtS). Gated by -fbit-scan-loops (on at -O).
Adds t_bit_scan_loops (I/Q/W, count-ones and unsigned-index positives;
step-2, start-1, idx*2+1, vec[idx+1], target=idx and W!=width negatives, all
self-checked and asserted via --stats not to lower) plus t_bit_scan_loops_off
for the disable flag.
Motivated by a transformer inference design whose 80-bit leading-one detector
ran every cycle (~37% of runtime); the lowering is worth ~39% there.
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)
Add an overload that takes `const char*` format string. This avoids
having to construct/destruct a temporary `std::string` at the call site
when calling these functions with a string literal, which is fairly
common. This is worth 25% code size on some assertion heavy design.
Also use `std::string::clear()` instead of assigning an empty string
which is more efficient.
This was a fudge to work around using VlWide in `if` conditions without
a `_ != 0` check. That check is actually inserted by V3Width (or an
equivalent reduction), so the offending code was only generated
internally. Hopefully fixed the single instance where this really
happened. (If not, C++ will fail to compile with "cannot convert VlWide
to bool in 'if (__HERE__)'" errors, still better than the old version
which used to silently not do the right test due to incorrect implicit
conversions.)
Yilou Wang
BRDR LIFE
Artur Bieniek
github action
24bit-xjkp
Bartosz Skorowski
Kornel Uriasz
Nick Brereton
Jeffrey Song
Adam Kostrzewski
Jakub Michalski
Dragon-Git
Pawel Kojma
Geza Lore
Joshua Leahy
Wilson Snyder
Matthew Ballance
Thomas Santerre
Nikolai Kumar
pawelktk
Paul Swirhun
Tracy Narine