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.
Introduce a new DfgPrev vertex, representing the value of a variable
before any in-graph assignments. This can be used to break all remaining
cycles in the graph, so all Dfgs become acyclic after V3DfgBreakCycles.
The circular dataflow is still represented, and is taken care of by the
scheduler, it is just the DfgGraph that represents the logic that
becomes acyclic.
This makes V3DfgBreakCycles a mandatory transform, so drop the disabling
-fno-dfg-break-cycles option (still parsed, but has no effect).
Note the effect of this is small, as most cycles can be fixed up by
driver tracing, which is unchanged, but this is required for some
upcoming work.
AstScopeName is an AstNodeExpr, but V3Task stashed the DPI export scoping
marker into the enclosing AstCFunc's statement list (addStmtsp), placing an
expression in a statement position.
Give AstCFunc a dedicated 'scopeNamep' operand to hold the marker instead.
Part of #6280
An elaboration severity system task ($info/$warning/$error/$fatal used
as a module/generate/program/checker item), which is a module item,was
parsed as an AstInitial wrapping an AstElabDisplay. That put
AstElabDisplay (a non-statement) into the procedural statement list of
an AstInitial.
AstElabDisplay is evaluated by V3Width, which then deletes it. The
AstInitial wrapper is historic and has no effect.
Part of #6280
AstConstraintBefore ('solve x before y') is a constraint item that appears
in constraint bodies alongside its siblings AstConstraintExpr,
AstConstraintUnique and AstConstraintIf, all of which are AstNodeStmt.
Part of #6280.
The OrderGraph used during V3Order step deliberately omits some variable
accesses from the dependency graph. E.g.: a read of a variable that is
in the reading block's own hybrid sensitivity list emits no edge, nor
does a read ignored due to a force/release, nor an access to a variable
marked 'ignoreSchedWrite' and friends. For serial mode that is fine, the
logic runs one block at a time. In parallel mode two such blocks can run
concurrently, and if one writes what the other reads, that is a data
race at runtime.
These accesses cannot be recovered from the graph edges. They are now
collected from the AST while the OrderGraph is built, and held by the
OrderLogicVertex performing them.
FixDataHazards is reworked around these access lists stored in
OrderLogicVertex, so it is now aware of all variable accesses the logic
makes, including those not encoded by the dependency graph edges. The
previous heuristic of fixing data hazards by merging same-rank MTasks is
removed. Additional edges are inserted instead to prescribe a fixed
ordering of conflicting MTasks. To insert edges without unduly
increasing the critical path, or introducing cycles, new edges are
added such that they preserve topological ordering, and they are
inserted between vertices sorted by critical path length. See algorithm
details in the code.
Also add a data hazard checker under '--debug-partition', reporting every
unordered accessor pair left in the final MTask graph.
This fixes the race demonstrated by t_sched_hybrid_hazard (#7913),
which is no longer expected to fail.
Under ThreadSanitizer over the vltmt tests: 17 failing before, 3 after,
with no regressions. The 3 remaining are different defects.
This is a large refactor of the MTask graph and coarsening algorithm, in
prep for fixing the bug described in #7913, it can also improve the
resulting multi-threaded schedule.
Two major changes:
OrderMTaskGraph now maintains the critical paths of the MTasks through
mutation. There are 2 ways to mutate the graph, which are done via
methods on the graph itself: adding an edge (used during construction,
and will be used later during fixing data hazards), or merging an MTask
into another (used during contraction). All critical path measures are
automatically updated and propagated on any mutation, so no external
algorithm needs to maintain them explicitly.
The merge candidate scoreboard used during contraction is simplified to
remove deferral of updated scores. This simplifies the code and results
in a greedily more optimal schedule. (The previous tranched rescore was
an optimization to work around the previous std::set based scoreboard,
however since the algorithm now uses an efficient PairingHeap,
verilation time is not impacted by the more accurate scoring, while
yielding better results).
Combining these two into a single patch as the code is highly
interdependent and any one change without the other would be just a
noisy transit point with unclear performance implications. Together it
should be a clear improvement.
Also added a stronger validation step run with '--debug-partition',
which checks all invariants throughout the algorithms.
A '{N{bit}}' mask expands to a bit vector where every word is the same
value, obtained by negating the replicated bit. Recomputing that at each
use is no more expensive than loading it
Artur Bieniek
Pawel Klopotek
Geza Lore
Yilou Wang
Marco Bartoli
Kornel Uriasz
github action
Sunimali Rathnayake
Matthew Ballance
Wilson Snyder
em2machine
Adam Kostrzewski
Demin Han
Sumanth KadiyalaandSumanth Kadiyala
Nikolai Kumar
Todd Strader