Avoid removing an if (false) statement and its corresponding branch

Avoid removing an if (false) statement and its corresponding branch

[Julian Waters]

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Hi all,

Is there a way to stop gcc from nuking an if (false) statement and the
corresponding branch from the compiled code for debugging purposes?
Even turning off all optimizations doesn't achieve this

best regards,
Julian

Re: Avoid removing an if (false) statement and its corresponding branch

[Gabriel Ravier]

On 7/23/23 14:45, Julian Waters via Gcc-help wrote:
> Hi all,
>
> Is there a way to stop gcc from nuking an if (false) statement and the
> corresponding branch from the compiled code for debugging purposes?
> Even turning off all optimizations doesn't achieve this
>
> best regards,
> Julian

IMO the simplest way would be to define something like `static const 
volatile bool unoptimizable_false = false;` somewhere and use it in 
place of `false`, when you want to avoid the if statement being 
optimized out.

Re: Avoid removing an if (false) statement and its corresponding branch

[Julian Waters]

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I just tried it and it does work, even on O3, however I would like to avoid
allocating a variable if possible. It's a shame that gnu::used doesn't
seem to work with if statements (If anyone knows where to look in the
source code do tell me). But thanks for the suggestion nonetheless!

best regards,
Julian

On Sun, Jul 23, 2023 at 10:14 PM Gabriel Ravier <gabravier@gmail.com> wrote:

> On 7/23/23 14:45, Julian Waters via Gcc-help wrote:
> > Hi all,
> >
> > Is there a way to stop gcc from nuking an if (false) statement and the
> > corresponding branch from the compiled code for debugging purposes?
> > Even turning off all optimizations doesn't achieve this
> >
> > best regards,
> > Julian
>
> IMO the simplest way would be to define something like `static const
> volatile bool unoptimizable_false = false;` somewhere and use it in
> place of `false`, when you want to avoid the if statement being
> optimized out.
>
>

Re: Avoid removing an if (false) statement and its corresponding branch

[David Brown]

On 23/07/2023 16:27, Julian Waters via Gcc-help wrote:
> I just tried it and it does work, even on O3, however I would like to avoid
> allocating a variable if possible. It's a shame that gnu::used doesn't
> seem to work with if statements (If anyone knows where to look in the
> source code do tell me). But thanks for the suggestion nonetheless!
> 

How about:

static inline bool False(void) {
     bool b = false;
     asm volatile("" : "+r" (b));
     return b;
}

Then use "if (False()) ..." instead of "if (false) ...".

The generated overhead is going to be minimal, and no volatile variables 
are created.  It's also quite cool (IMHO) to have 100% portable inline 
assembly!

mvh.,

David



> best regards,
> Julian
> 
> On Sun, Jul 23, 2023 at 10:14 PM Gabriel Ravier <gabravier@gmail.com> wrote:
> 
>> On 7/23/23 14:45, Julian Waters via Gcc-help wrote:
>>> Hi all,
>>>
>>> Is there a way to stop gcc from nuking an if (false) statement and the
>>> corresponding branch from the compiled code for debugging purposes?
>>> Even turning off all optimizations doesn't achieve this
>>>
>>> best regards,
>>> Julian
>>
>> IMO the simplest way would be to define something like `static const
>> volatile bool unoptimizable_false = false;` somewhere and use it in
>> place of `false`, when you want to avoid the if statement being
>> optimized out.
>>
>>
> 

Re: Avoid removing an if (false) statement and its corresponding branch

[Julian Waters]

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Thanks for the reply, but the solution was right there all along...

if (volatile bool _ = false)

On Mon, Jul 24, 2023 at 12:53 AM David Brown <david.brown@hesbynett.no>
wrote:

> On 23/07/2023 16:27, Julian Waters via Gcc-help wrote:
> > I just tried it and it does work, even on O3, however I would like to
> avoid
> > allocating a variable if possible. It's a shame that gnu::used doesn't
> > seem to work with if statements (If anyone knows where to look in the
> > source code do tell me). But thanks for the suggestion nonetheless!
> >
>
> How about:
>
> static inline bool False(void) {
>      bool b = false;
>      asm volatile("" : "+r" (b));
>      return b;
> }
>
> Then use "if (False()) ..." instead of "if (false) ...".
>
> The generated overhead is going to be minimal, and no volatile variables
> are created.  It's also quite cool (IMHO) to have 100% portable inline
> assembly!
>
> mvh.,
>
> David
>
>
>
> > best regards,
> > Julian
> >
> > On Sun, Jul 23, 2023 at 10:14 PM Gabriel Ravier <gabravier@gmail.com>
> wrote:
> >
> >> On 7/23/23 14:45, Julian Waters via Gcc-help wrote:
> >>> Hi all,
> >>>
> >>> Is there a way to stop gcc from nuking an if (false) statement and the
> >>> corresponding branch from the compiled code for debugging purposes?
> >>> Even turning off all optimizations doesn't achieve this
> >>>
> >>> best regards,
> >>> Julian
> >>
> >> IMO the simplest way would be to define something like `static const
> >> volatile bool unoptimizable_false = false;` somewhere and use it in
> >> place of `false`, when you want to avoid the if statement being
> >> optimized out.
> >>
> >>
> >
>
>