With this change, it is easier to see which signals carry state (only
wire<>s appear as `reg` in VCD files) and to construct a minimal
checkpoint (CXXRTL_WIRE debug items represent the canonical smallest
set of state required to fully reconstruct the simulation).
Although logically two separate steps, these were treated as one for
historic reasons. Splitting the two makes it possible to have designs
that are only 2× slower than fastest possible (and are without extra
delta cycles) that allow probing all public wires.
Historically, elision was implemented before localization, so levels
with elision are lower than corresponding levels with localization.
This is unfortunate for two reasons:
1. Elision is a logical subset of localization, since it equals to
not giving a name to a temporary.
2. "Localize" currently actually means "unbuffer and localize",
and it would be useful to split those steps (at least for
public wires) for improved design visibility.
Although these options can be thought of as optimizations, they are
essentially orthogonal to the core of -O, which is managing signal
buffering and scope. Going from -O4 to -O2 means going from limited
to complete design visibility, yet in both cases proc and flatten
are desirable.
Before this commit, Verilog expressions like `x && 1` would result in
references to `logic_and_us` in generated CXXRTL code, which would
not compile. After this commit, since cells like that actually behave
the same regardless of signedness attributes, the signedness is
ignored, which also reduces the template instantiation pressure.
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.
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.
The $div and $mod cells use truncating division semantics (rounding
towards 0), as defined by e.g. Verilog. Another rounding mode, flooring
(rounding towards negative infinity), can be used in e.g. VHDL. The
new $divfloor cell provides this flooring division.
This commit also fixes the handling of $div in opt_expr, which was
previously optimized as if it was $divfloor.
The $div and $mod cells use truncating division semantics (rounding
towards 0), as defined by e.g. Verilog. Another rounding mode, flooring
(rounding towards negative infinity), can be used in e.g. VHDL. The
new $modfloor cell provides this flooring modulo (also known as "remainder"
in several languages, but this name is ambiguous).
This commit also fixes the handling of $mod in opt_expr, which was
previously optimized as if it was $modfloor.
whitequark
N. Engelhardt
Yehowshua Immanuel
Eddie Hung
Claire Wolf
Alberto Gonzalez
Xiretza