Add "falls through" comments

[zsx]

GCC 7 will have -Wimplicit-fallthrough turned on by -Wextra. There are a
number of ways to inform the compiler that falling through is intended.
This commit just uses the comment to do that, which is the most
conservative (least destructive) way: if the compiler doesn't recognize
it, nothing's gonna be broken.

[hostilefork]

Sure. Note I've been adding spaces whenever I can to:

switch (x) {
case A: {
     blah blah
     break; }
 case B: {
     blah blah
     break; }
  ...
  }

To produce:

switch (x) {
case A: {
     blah blah
     break; }

 case B: {
     blah blah
     break; }
  ...
  }

And using no spaces between cases for fallthrough only. That's been the standard I've been going for. It's tough to know exactly how to indent or how to write code, but I have come to like this pattern... because it's more or less readable and keeps rightward indentation from going out of control.

I don't have a problem with putting a fail or panic in the default case, if appropriate. Technically this creates slightly more overhead for the switch, but I'd have to see hard proof of it being a problem in the codegen of a particular instance before complaining too much.

(To me, part of "the project" is to keep the C code readable on 80 column terminals, it's all part of the "Amish programming" aesthetic... I want a surprisingly powerful engine to come from code that could have been understood circa the K&R C book...)

[zsx]

The style is good to me, however, GCC doesn't recognize it, and GCC 7 fails to compile with RIGOROUS=yes.

[hostilefork]

Hm? Does it have a problem with the style I suggest or just if you leave out the // falls through comments? I am fine with the comments.

[zsx]

Sorry, I meant that it didn't recognize the style as a "falls through" indicator, so it didn't compile without the comments.

[hostilefork]

Right. No problem, I like it. Just trying to elaborate on what my preferred style has become for switch statements--seems to be readable enough and keeps one from indenting too much e.g.

switch (x) {
    case A:
         {
             some code indented way too much...