[TriEmbed] Hacking a fake vintage radio (with Arduino + Pi 0)

Pete Soper pete at soper.us
Tue Jun 30 20:33:46 CDT 2020 

What's ironic about this is that for switch statements that are "dense 
enough" (consecutive case values) gcc will create exactly this kind of 
"computed goto" for you if the right optimization level is selected (but 
probably limited to some set of target machines).

-Pete

On 6/30/20 8:57 PM, Jon Wolfe via TriEmbed wrote:
>
> There is a trick you can use with gcc that is a non-standard C 
> construct where you can use ‘goto’ and give it a variable containing 
> the address of a label. You then create an array of label address and 
> you can then dynamically index that array to jump to various 
> locations.  I’ve seen it used as an optimization technique, and you 
> can also have more control over the program flow, though it is using 
> the infamous keyword, Essentially though it end up looking pretty much 
> like a switch-case.
>
> That is really odd about that gcc bug.  It’s not like I’ve never seen 
> them, but 99.9% of the time when I thought I had found a compiler bug 
> in C/C++, it turns out to be something else. (hafl the time one of 
> those things that disappears with a “clean/rebuild all”)  I remember 
> the Arduino /AVR/gcc linker used to have a bug related to 8-bit AVR 
> chips that had more than 64KB of flash memory, such as the ATMega 
> 1284. Those chips address by 16 bit words not bytes, so 128kb of flash 
> is accessible without trick likes far pointers, but the linker would 
> mess up the address calculations sometimes I think for interrupt 
> handlers or functions called by interrupt handlers that crossed the 
> 64kb boundary.
>
> *From: *Huan Truong via TriEmbed <mailto:triembed at triembed.org>
> *Sent: *Tuesday, June 30, 2020 12:48 PM
> *To: *Brian <mailto:triembed at undecidedgames.net>
> *Cc: *Triangle Embedded Computing Discussion 
> <mailto:triembed at triembed.org>
> *Subject: *Re: [TriEmbed] Hacking a fake vintage radio (with Arduino + 
> Pi 0)
>
> Oh yeah, that explains my issue. I definitely ran into that issue
> where I have checked and had no reason to believe I was doing
> something wrong, yet, when I evacuated each switch to a function, the
> switch worked correctly. But neither scoping with an anonymous scope
> nor renaming the variables work.
>
> The reason I used the switch was that I read on stackoverflow at one
> point and someone said that we should use switches instead of elseifs
> when we have a lot of cases. Then, using switches, the compiler will
> be able to (at some point) create a lookup table for you so it's
> faster. I doubt that was what happening at least on the Arduino case.
> You'll need a giant lookup table which the uCs don't have memory for.
> I suspect that in a lot of cases, using switches is probably just as
> slow as using elseifs. Now as I see that it is so buggy, I probably
> will not use switches, at least on Arduino.
>
> Cheers,
> - Huan.
>
> On Tue, Jun 30, 2020 at 9:23 AM Brian via TriEmbed
> <triembed at triembed.org> wrote:
> >
> > Side note:
> >
> > The arduino compiler has bugs in how it handles switch statements. I've
> > run into situations lately where the order of the case statements matter
> > (which it never should); cases are completely ignored, etc.
> >
> > I believe it may be tied to the use of local scoping within a case, 
> e.g.:
> >
> > switch(thing) {
> > case 1:
> > {
> > // stuff with case-local scope
> > }
> > break;
> > }
> >
> > Syntactically- and semantically-correct code has proven to generate
> > incorrect runtime results.
> >
> > I haven't had time/motivation to submit a bug report, but I should do
> > that. At any rate, a potential workaround is to reorder your cases.
> >
> > -B
> >
> > On 6/24/20 9:51 PM, Huan Truong via TriEmbed wrote:
> > > Thanks Pete!
> > >
> > > I feel like there was something really mysterious about the switch 
> statement. Even if I pasted the whole blocks of code of each function 
> I would have called to the {} inside a case, the code still wouldn’t 
> work. That baffled me by a mile.
> > >
> > > But yeah, I spent way too much time on the project that I’m 
> comfortable with the idea of not understanding some of it now. The 
> watchdog timer code was baffling too.
> > >
> > > Please excuse my typos, sent from phone.
> > >
> > > On Jun 24, 2020, at 10:14 AM, Pete Soper via TriEmbed 
> <triembed at triembed.org> wrote:
> > >
> > > What a beautifully presented adventure. Loved reading it. And 
> when you say a problem "could be bad" you make your point. :-) (meant 
> as a "find Waldo" exercise for alert readers)
> > >
> > > Hadn't heard of "kev" or any other Arduino emulator for that 
> matter. That aspect was interesting too.
> > >
> > > The other issue with redeclaration of the vars local to the switch 
> statement is that they literally don't exist outside it, so 
> communicating their values outside the block would be difficult. :-) 
> In general, every {} defines a local scope in C/C++ and you can 
> declare variables inside that scope but they cease to be defined 
> outside the scope. The scope outside any {} (aka "global") or vars 
> declared "static" can avoid this issue but not the redefine issue.
> > >
> > > Thanks for sharing this!
> > >
> > > Pete
> > >
> > >
> > >> On 6/24/20 12:43 PM, Huan Truong via TriEmbed wrote:
> > >> This has taken me way more time than I thought, but finishing this
> > >> retrofit is a big achievement for me. It's really silly and serves
> > >> exactly no purpose other than RE'ing something no one cares about. So
> > >> I just want to share for some shits and giggles.
> > >>
> > >> 
> http://www.tnhh.net/posts/adventures-hacking-fake-vivitar-vintage-radio.html
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
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>
> -- 
>
> Huan Truong
> www tnhh.net / twitter @huant
>
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