Implement the IEEE 1364-2005 Clause 18 extended VCD task family
($dumpports / $dumpportsall / $dumpportsoff / $dumpportson /
$dumpportsflush / $dumpportslimit), which were previously registered as
"not implemented" stubs.
The new vpi/sys_evcd.c reaches each port's value-bearing net through the
port's vpiLowConn relationship and reads per-bit drive strength with
vpiStrengthVal, emitting $var port declarations and p-records (per-bit
state character plus strength0/strength1 components). Scalar and vector
ports are supported; the initial checkpoint is deferred to the read-only
synch so it records settled values. The encoding is byte-compatible with
the GHDL --evcd writer so one waveform reader works across Verilog and
VHDL.
To resolve a port to its net, the VPI is completed: vpiLowConn and
vpiHighConn are now implemented on vpiPortInfo. tgt-vvp emits the port's
low (formal) and high (actual) net symbols in the .port_info directive,
the vvp assembler parses them (compile_port_info), and vpiPortInfo
resolves each to the __vpiSignal sharing that net. New vpi_user.h
constants vpiHighConn (135) and vpiLowConn (136) use iverilog's private
numbering to avoid colliding with vpiArgument/vpiBit.
Tests: ivtest evcd_basic (scalar in/out/inout), evcd_bus (vector ports)
and evcd_onoff ($dumpportsoff/on/all), each gold-diffed.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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