This commit changes the VCD writer such that for all signals that
have `debug_item.type == VALUE && debug_item.next == nullptr`, it
would only sample the value once.
Commit f2d7a187 added more debug information by including constant
wires, and decreased the performance of VCD writer proportionally
because the constant wires were still repeatedly sampled; this commit
eliminates the performance hit.
Constant wires can represent a significant chunk of the design in
generic designs or after optimization. Emitting them in VCD files
significantly improves usability because gtkwave removes all traces
that are not present in the VCD file after reload, and iterative
development suffers if switching a varying signal to a constant
disrupts the workflow.
This commit changes the VCD writer such that for all signals that
share `debug_item.curr`, it would only emit a single VCD identifier,
and sample the value once.
Commit 9b39c6f7 added redundancy to debug information by including
alias wires, and increased the size of VCD files proportionally; this
commit eliminates the redundancy from VCD files so that their size
is the same as before.
Alias wires can represent a significant chunk of the design in highly
hierarchical designs; in Minerva SRAM, there are 273 member wires and
527 alias wires. Showing them in every hierarchy level significantly
improves usability.
log_signal can result in a string with spaces (when bit selection is
involved), which breaks the rule of IdString not containing whitespace.
Instead, remove the sigspec from the name entirely — given that the
resulting wire will have no users, it will be removed later anyway,
so its name doesn't really matter.
Fixes#2118
Compared to the C++ API, the C API currently has two limitations:
1. Memories cannot be updated in a race-free way.
2. Black boxes cannot be implemented in C.
Debug information describes values, wires, and memories with a simple
C-compatible layout. It can be emitted on demand into a map, which
has no runtime cost when it is unused, and allows late bound designs.
The `hdlname` attribute is used as the lookup key such that original
names, as emitted by the frontend, can be used for debugging and
introspection.
`flatten` cannot derive modules in most cases because that would just
yield processes, and it does not support `-autoproc`; in practice
`flatten` has to be preceded by a call to `hierarchy`, which makes
deriving unnecessary.
whitequark
clairexen
Marcelina Kościelnicka
Peter Crozier
N. Engelhardt
Yehowshua Immanuel
Eddie Hung