synth: load the slang plugin when read_slang is not built in

[oharboe]

b18e110c1 ("drop bundled yosys-slang, use Yosys 0.67 built-in read_slang") removed the explicit plugin load from synth_preamble.tcl, on the strength of Yosys 0.67 vendoring the slang frontend under frontends/slang. That is correct for 0.67+ — I confirmed frontends/slang is absent in v0.65/v0.66 and present in v0.67/v0.68.

It leaves Yosys 0.66 and older with no way to use SYNTH_HDL_FRONTEND=slang:

ERROR: No such command: read_slang (type 'help' for a command overview)

even when the same code is available as the out-of-tree plugin (povik/sv-elab, formerly yosys-slang) and is already on YOSYS_PLUGIN_PATH — because nothing loads it any more.

This is not hypothetical: the Bazel Central Registry yosys module is still on 0.64. Yosys ships no Bazel files upstream, so that module is a hand-written overlay and nobody has carried it past 0.64. Every ORFS consumer building through BCR yosys is on 0.64 and cannot use the slang frontend today.

The change

if { [info commands read_slang] eq "" } {
  plugin -i slang
}

plugin -i slang resolves through YOSYS_PLUGIN_PATH. The guard makes it a strict no-op on a Yosys that has the frontend built in, so ORFS works against either without the caller having to declare which it has — and 0.67+ behaviour is unchanged.

Testing

Exercised through bazel-orfs, whose //slang:test and //slang:blackbox_synth targets are regression coverage for exactly this path (slang frontend, and SYNTH_BLACKBOXES + --empty-blackboxes through it). Both had been failing since b18e110c1; both build green with this patch against BCR yosys 0.64.

[gemini-code-assist]

Code Review

This pull request updates the synthesis preamble script (flow/scripts/synth_preamble.tcl) to conditionally load the slang plugin if the read_slang command is not already available. This ensures compatibility with older Yosys versions that do not have the frontend built-in, while remaining a no-op for newer versions. There are no review comments, so I have no feedback to provide.

[oharboe]

let lie for now