This patch adds DfgLogic, which is a vertex that represents a whole,
arbitrarily complex combinational AstAlways or AstAssignW in the
DfgGraph.
Implementing this requires computing the variables live at entry to the
AstAlways (variables read by the block), so there is a new
ControlFlowGraph data structure and a classical data-flow analysis based
live variable analysis to do that at the variable level (as opposed to
bit/element level).
The actual CFG construction and live variable analysis is best effort,
and might fail for currently unhandled constructs or data types. This
can be extended later.
V3DfgAstToDfg is changed to convert the Ast into an initial DfgGraph
containing only DfgLogic, DfgVertexSplice and DfgVertexVar vertices.
The DfgLogic are then subsequently synthesized into primitive operations
by the new V3DfgSynthesize pass, which is a combination of the old
V3DfgAstToDfg conversion and new code to handle AstAlways blocks with
complex flow control.
V3DfgSynthesize by default will synthesize roughly the same constructs
as V3DfgAstToDfg used to handle before, plus any logic that is part of a
combinational cycle within the DfgGraph. This enables breaking up these
cycles, for which there are extensions to V3DfgBreakCycles in this patch
as well. V3DfgSynthesize will then delete all non synthesized or non
synthesizable DfgLogic vertices and the rest of the Dfg pipeline is
identical, with minor changes to adjust for the changed representation.
Because with this change we can now eliminate many more UNOPTFLAT, DFG
has been disabled in all the tests that specifically target testing the
scheduling and reporting of circular combinational logic.
Added an algorithm that can break some combinational cycles in DFG, by
attempting to trace driver logic until we escape the cycle. This can
eliminate a decent portion of UNOPTFLAT warnings. E.g. due to this code:
```systemverilog
assign a[0] = .....;
assign a[1] = ~a[0];
```
The manual for the BLKANDNBLK warning describes that it is safe to
disable that error if the updated ranges are non-overlapping. This
however was not true (see the added t_nba_mixed_update* tests).
In this patch we change V3Delayed to use a new ShadowVarMasked
scheme for variables that have mixed blocking and non-blocking
updates (or the FlagUnique scheme for unpacked variables), which
is in fact safe to use when the updated parts are non-overlapping.
Furthermore, mixed assignments are safe as far as scheduling is
concerned if either:
- They are to independent parts (bits/members/etc) (with this patch)
- Or if the blocking assignment is in clocked (or suspendable) logic.
The risk in scheduling is a race between the Post scheduled NBA
commit, and blocking assignments in combinational logic, which might
order incorrectly.
The second point highlights that we can handle stuff like this safely,
which is sometimes used in testbenches:
```systemverilog
always @(posedge clk) begin
if ($time == 0) a = 0;
end
always @(posedge clk) begin
if ($time > 0) a <= 2;
end
````
The only dangerous case is:
```systemverilog
always @(posedge clk) foo[idx] <= val;
assign foo[0] = bar;
```
Whit this patch, this will still resolve fine at run-time if 'idx' is
never zero, but might resolve incorrectly if 'idx' is zero.
With the above in mind, the BLKANDNBLK warning is now only issued if:
- We can't prove that the assignments are to non-overlapping bits
- And the blocking assignment is in combinational logic
These are the cases that genuinely require user attention to resolve.
With this patch, there are no more BLKANDNBLK warnings in the RTLMeter
designs.
Fixes#6122.
Jakub Wasilewski
Wilson Snyder
Lan Zongwei
Congcong Cai
Geza Lore
Sergey Fedorov
Michael Bedford Taylor
Rodrigo Batista de Moraes
George Polack
Danny Oler
Max Wipfli
Ibrahim Burak Yorulmaz
Artur Bieniek
Igor Zaworski