Wrap the tableswitch default target in a (default ...) node

[AndrewScheidecker]

Wrap the tableswitch default target in a (default ...) node so that something like (default (case x)) stands out from its (case ...) siblings that define targets rather than references to targets.

Before:

(tableswitch (i32.const 1)
    (table (case $0))
    (case $default)
    (case $default ...)
    (case $0 ...)
)

After:

(tableswitch (i32.const 1)
    (table (case $0))
    (default (case $default))
    (case $default ...)
    (case $0 ...)
)

[kg]

It would be much better if this were a part of the case node instead of a new imaginary (i.e. not actually a functional part of the AST or encoded into binary) s-expression node that adds extra nesting.

We have something like align=N for loads and stores, so how about (case default $label) vs the regular (case $label)? Alternately, you introduce a (default-case $label) opcode.

[AndrewScheidecker]

I went with (default ...) to match the (table ...) node preceding it in the tableswitch syntax.

Alternately, you introduce a (default-case $label) opcode.

Then you'd also need default-br, since tableswitch supports a non-case label as the default, distinguished by (default (br ...)) (using this commit's syntax). Maybe finding a way to avoid that distinction would be best, though.

[rossberg]

I agree the syntax of the default target can be slightly irritating, but is it really a problem serious enough to justify adding extra syntactic baggage?

If we think so, then I'd rather revive the earlier suggestion of simply including the default in the table, and define dispatch by clamping the index value. See #163 (comment) and subsequent discussion. At least that would make the syntax simpler, not more complicated, though I also understand Dan's reservations.

Another option would be to use a different keyword for the case in the table/default, but I don't have a good suggestion.

[sunfishcode]

Another alternative is to remove the default from tableswitch altogether. A proposal to do that is now at WebAssembly/design#482 .

[rossberg]

Obsolete with br_table, closing.