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

John Vaughters jvaughters04 at yahoo.com
Wed Jul 1 13:34:59 CDT 2020


Jon,

I totally agree, As long as there are no restrictions on code maintenance, I use whatever is comfortable and easy for me. I would use labels in a flash. I still use them on the rare BATCH (MAN I HATE BATCH) scripts I create. In earlier posts, I was just reminiscing on the style I was taught. When programming for a group, structural style matters. When programming for yourself, anything goes to get to the result as quick and painless as possible for me. If you are trying to program for others to see and impress them with your work, just be careful you are using more acceptable styles or they might question your techniques. Or, be ready to give a great explanation on the specific benefit for the unusual style.

2 cents worth half a pence on a good day.

John Vaughters






On Wednesday, July 1, 2020, 2:12:41 PM EDT, Jon Wolfe via TriEmbed <triembed at triembed.org> wrote: 






This is one of those rare circumstances, imho, where goto can reasonably be used, Dijkstra be damned 😊. 
 
Where I have seen it used like this , it’s an optimization, and doesn’t change the semantics of code flow from a traditional switch-case, and it was wrapped in a macro I think so it could be used, if supported, by the compiler. It’s like putting inline assembly in the code: only do it if the benefit is worth the sacrifice in code portability and maintainability. 
 
I’m not a compiler optimization expert, but the code generation optimization of using a jump table would be at the discretion of the optimizer, using a goto construct would be a way to force that. It may also be the case that the way it is implemented at the machine code level is faster using gotos.  
 
 
 
From: Scott Hall
Sent: Wednesday, July 1, 2020 9:13 AM
To: Jon Wolfe
Cc: Rodney Radford; Triangle Embedded Computing Discussion
Subject: Re: [TriEmbed] Hacking a fake vintage radio (with Arduino + Pi 0)
 
NO NO, DON'T USE GOTO !

Seriously though, this has been a behaviour of the Arduino compiler compilation for a while -- its even mentioned in some Arduino books.  The workaround is to have a single function called for each case label and to put the statements desired with the function.  This gets optimized to a jump table anyway, so its just a matter of doing this in practice.

On Wed, Jul 1, 2020 at 12:34 AM Jon Wolfe via TriEmbed <triembed at triembed.org> wrote:
> Yep, GCC has a ton of extensions to C and C++, though they obviously need careful consideration when using, if it’s work the downside of making the code less portable. 
>  
> https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/C-Extensions.html
>  
> One of my favorites is local functions. 
>  
>  
>  
> From: Rodney Radford
> Sent: Tuesday, June 30, 2020 9:04 PM
> To: Jon Wolfe
> Cc: Brian; Triangle Embedded Computing Discussion
> Subject: Re: [TriEmbed] Hacking a fake vintage radio (with Arduino + Pi 0)
>  
> I had never heard of the GCC label variable, so I had to google it... wow, I learned something new tonight!
> 
> https://stackoverflow.com/questions/1777990/is-it-possible-to-store-the-address-of-a-label-in-a-variable-and-use-goto-to-jum
> 
> 
> On Tue, Jun 30, 2020 at 8:57 PM Jon Wolfe via TriEmbed <triembed at triembed.org> 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
>> Sent: Tuesday, June 30, 2020 12:48 PM
>> To: Brian
>> Cc: Triangle Embedded Computing Discussion
>> 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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-- 
Scott G. Hall
Raleigh, NC, USA
scottghall1 at gmail.com
Although kindness is rarely a job, no matter what you do it's always an option.

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