Re-associating constants can enable further simplification e.g. when
those constant become all zero/all ones. Run these patterns to fixed
point to capture the simplification, by revisiting the node.
V3Const eagerly deletes nodes (we want that in V3Const instead of using
pushDeletep, as otherwise the transient peak memory use during
verilation can be very high)
The std::map used to cache the predicate checking if a subtree contains
a member access then stale if a new node is allocated in the same address.
Fix by using unique node IDs stored in user4 as cache keys.
This is the bug uncovered by #8147
Related to #6963
Re-associating constants can enable further simplification e.g. when
those constant become all zero/all ones. Run these patterns to fixed
point to capture the simplification, by revisiting the node.
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)
Recognize "decoder" case statements (where every case item only assigns
constants to a fixed set of left-hand sides) and replace them with a
single packed constant lookup table indexed by the case expression.
Small tables are materialized inline in the generated code, and are
always optimized. Larger ones are placed in the constant pool and only
optimized if deemed beneficial over branches.
While this slightly conflicts with V3Table, and is not worth that much
on it's own, there will be a follow up patch that converts more cases of
this form which will be much more valuable. This patch does the
necessary analysis and the simple table conversion when possible.
Split -fcase into -fcase-table (this new conversion) and -fcase-tree (the
existing bitwise branch-tree conversion); -fno-case is now an alias for
both.
Default branches, assignments preceding the case (used as default values),
casez wildcards, multiple and partial left-hand sides, and both blocking and
non-blocking assignments are handled. Cases that cannot be safely tabled (e.g.
non-exhaustive with no default, overlapping writes to one variable, or mixed
blocking/non-blocking assignments) fall back to the existing if/else lowering.
Consequently disabled re-inlining of constant pool variables in V3Const,
and rebuild the constant pool hash in V3Dead (previously we didn't
create constant pool entries early enough for this to matter)
Turn `x = $sformatf(...)` into `$sformat(x, ...)`. The former requires
checking and running a destructor for `x` at the call site, the later
does it in the callee VL_SFORMAT. This reduces the size of the call
site, which can be significant e.g. in the presence of many assertions.
Also added a rewrite of `$sformat(x, "const-string")` back into `x =
"const-string"` for the cases where the `$sformatf` would have been
folded into a constant string.
V3Premit extracts wide sub-expressions via temporaries, which is needed
for emitting wide operations to C++ (calls to `VL_*_W`). The previous
version used to extract both branches of an AstCond unconditionally,
meaning both branches were fully evaluated. Rewriting the AstCond into
an AstIf instead enables evaluating only the required branch. While
this does limit V3Subst, overall the resulting code is ~3% faster,
and contains ~25% fewer branches on a large design.
If the same statements appears in both branches of an 'if', put a single
copy after the 'if', apply recursively. This also has the effect of
getting rid of conditionals with identical branches, but is more widely
applicable.
Introduce new pass that converts impure expressions, or those with
function and method calls into simple assignment statements. Please see
the blurb at the top of the file why this is useful and how it works.
In particular currently it enables more Dfg optimization as functions
will be inlined without AstExprStmt.
Ideally we should enforce this lowering is applied to every procedural
statement (there are still a handful of exceptions). With that, long
term with this pass + #6820, there should be no need to ever use an
AstExprStmt past this new lowering pass, which should enable more easier
optimization down the line.
Also ideally this should be run earlier. Currently it's after V3Tristate
as that calls pinReconnectSimple so we don't have to touch Cell ports.
Currently disabled when code coverage is enabled due to #7119.
Note this might miss some cases where a sub-tree within an And/Or/Xor
tree is optimizeable, but not the whole tree, but in practice this seems
to work better than the alternative of keeping a set of failed nodes and
bail early.
Initial idea was to remodel AssignW as Assign under Alway. Trying that
uncovered some issues, the most difficult of them was that a delay
attached to a continuous assignment behaves differently from a delay
attached to a blocking assignment statement, so we need to keep the
knowledge of which flavour an assignment was until V3Timing.
So instead of removing AstAssignW, we always wrap it in an AstAlways,
with a special `keyword()` type. This makes it into a proper procedural
statement, which is almost equivalent to AstAssign, except for the case
when they contain a delay. We still gain the benefits of #6280 and can
simplify some code. Every AstNodeStmt should now be under an
AstNodeProcedure - which we should rename to AstProcess, or an
AstNodeFTask). As a result, V3Table can now handle AssignW for free.
Also uncovered and fixed a bug in handling intra-assignment delays if
a function is present on the RHS of an AssignW.
There is more work to be done towards #6280, and potentially simplifying
AssignW handing, but this is the minimal change required to tick it off
the TODO list for #6280.
Wilson Snyder
Marco Bartoli
Geza Lore
Nikolai Kumar
Kornel Uriasz
Nick Brereton
Benjamin Collier
Yogish Sekhar
Igor Zaworski
Yilou Wang
github action
Julian Carrier
em2machine
Matthew Ballance
Geza Lore
Jonathan Drolet
Igor Zaworski