`.comment' format mismatch

[rlutz]

The syntax of the .comment statement expected by IceStorm does not appear to match the syntax output by arachne-pnr.

icepack expects comments to be in the following syntax:

.comment
First line
Second line

Everything following .comment on the first line is ignored. Each subsequent line is treated as a comment until the comment block is terminated by a line starting with .. This line is treated normally; it is not ignored. If multiple comment blocks are specified, all except the last one are ignored.

icepack includes the comments in the following form at the beginning of the .bin output:

ff 00 <First line> 00 <Second line> 00 00 ff

However, arachne-pnr outputs a comment of the following form:

.comment arachne-pnr <version> (git sha1 <commit>, <compiler version and options>)

This is understood by icepack as an empty comment block, so it adds the pattern

ff 00 00 ff

at the beginning of every .bin file generated from arachne-pnr output.

I stumbled across this when I realized that icepack actually interprets .comment statement. I'm not sure what the correct behavior is supposed to be, but I guess it's one of the following:

• .comment blocks are intended as a mechanism to include arbitrary strings in a .bin file, but arachne-pnr got the format wrong, so instead of the version string, an empty comment block is included in the output
• .comment blocks are intended as a mechanism to include arbitrary strings in a .bin file, but arachne-pnr mistook the syntax for a “real” comment and inadvertently inserts an empty comment block in every file
• .comment statements are intended as a way to be able to add comments to an .asc file; icepack is not supposed to include their content in the output

[cliffordwolf]

As far as I am concerned this is a bug in arachne-pnr. @cseed ?

[swillner]

Just confirming nextpnr also puts its comment on the same line

    out << ".comment from next-pnr" << std::endl;

https://github.com/YosysHQ/nextpnr/blob/master/ice40/bitstream.cc#L300

By the logic of the other statements, maybe this could be the expected behaviour?

[byteit101]

I Just noticed this when trying to create 4-byte aligned output via comments. I can create a patch to support trailing/one line comments if people are ok with that?