I find the html output pretty cluttered already. I feel like adding line numbers would only make that worse. I guess I'm not philosophically opposed, but I'd like to see some more arguments for why it is useful. I've never encountered a situation where line numbers might be useful (except maybe for debugging line number generation).

Line numbers may be useful to link SSA instructions with the function code. It gives a better understanding of how the Go code represents in SSA form. Especially, that is actual for people who just start dealing with this.
For example, line numbers in the "genssa" output really help to figure out what asm instructions does when you can see how it relates to your code.

On the other hand, I agree that it'll add more noise into HTML output.
But what if to do something like that:

Could it be switchable? There's a mess of Javascript in there already, this might not be too much.

One sometime use of the SSA output is to copy a column out for processing in an editor (e.g., diffing against another compilation of same code), so a mitigating bell/whistle might be a "copy" button for each column, since those line numbers might be mess up the copy (which sometimes requires post-processing anyhow).

The other solution is to prevent line numbers from being selected and copied with CSS. Without any JS and additional elements.

Just now realized, what happens with line numbers when we inline functions, and how would we want to express that, assuming we haven't made a mess of that information already in the compiler? Color-coded? Tool-tips for the function name?

Note that once we start inlining calls that contain safepoints, we'll need to get this right anyhow for stack traces and debugging.

Yeah, that's a good point.

So, I'd like to add that information into compiler (add CallLine and CallFile values into ssa.Value, ssa.Block and gc.Node types) and add DW_AT_call_file and DW_AT_call_line attributes into DWARF. As you mentioned, we'll need it anyhow.

As for the HTML output, what if to mark inlined instructions with a color and show more info on mouse over. Something like this:

Deferring to @randall77 to decide. Removing proposal label.

Adding this is ok, but please put it behind a flag that is off by default.

/cc @ysmolsky