GCC loses CONST data

Chris Williams Guest 2010-04-30 16:42 #1694275

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using arm-elf-g++ to compile a simple file with a single int in it:

int fred = 10;

If I do arm-elf-objdump I can see fred.

NOW, if I make fred a const:

const int fred = 10;

it is no longer visible in the object file. In fact, the object file is 
smaller and I think the data is not there! (I have done this with a 
const array and you can see its not in the object file)

Note that this is NOT a startup problem, it is actually missing from the 
object file directly after compiling.

Can anybody explain where its gone and what I should do about it?

Thanks for any help.

Regads,

Chris.

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Jörg W. (dl8dtl) Moderator 2010-04-30 17:38 #1694351

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I guess, by making it "const", the compiler can figure out you
are never going to use the data (depeding on the compilation
options used), so it can safely optimize them away.

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Chris Williams Guest 2010-04-30 17:44 #1694353

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OK. But although I don't use it in this file, I want to link to it from 
a second source file! In a separate file I have:

extern const int fred;

If I take the const out of both files this would work. With them in it 
does not.

I have taken out all the compiler options to make sure they are not 
having any effect.

Are there any options the would FORCE the output I want?

Thanks for taking time to think about this.

Regards,

Chris.

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Jörg W. (dl8dtl) Moderator 2010-04-30 17:59 #1694378

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Errm, only now I notice you're using C++.  I've been thinking of C.

In C++, const declares a real constant, rather than an unmodifiable
object.  AFAICT (I'm not a C++ expert), a const declaration must have an
initializer, and the compiler will only use it when referenced within
the compilation unit.  IOW, you cannot have "external constants",
as there is no actual object associated to it.

Someone more knowledgable about C++ might correct me if I'm wrong
here.

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Chris Williams Guest 2010-04-30 18:31 #1694406

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That's an interesting idea!

I have just tried compiling the same under Linux. If you compile it as a 
'C' file it works. If you make it a C++ file it fails!

So, there are 2 questions:

1. Under Gnu-Arm even if I make it a 'C' file it still does not work. 
Looks like it is compiling as C++ regardless. How do I force a 'C' 
compile?

2. If its C++, how do I handle such a situation? Actually what I have is 
a bit map as a C array that I have generated. I simply want to compile 
it and access it from another file. Of course I want to fore it into 
flash memory, NOT into RAM.

Thanks for some great help.

Regards,

Chris.

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(prx) A. K. (prx) 2010-04-30 18:54 #1694424

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This true "const"ness in C++ only holds for some scalar datatypes, not 
arrays.

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Andreas F. (aferber) 2010-04-30 19:00 #1694432

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Chris Williams wrote:
> 1. Under Gnu-Arm even if I make it a 'C' file it still does not work.
> Looks like it is compiling as C++ regardless. How do I force a 'C'
> compile?

Is your source file named with .cpp, .cc or .C (capital C) extension? 
Especially the capital C case can be a bugger on Windows since Windows 
filesystems are case insensitive, however GCC treats the filename case 
sensitively AFAIK.

If so, gcc will compile it as C++ even if called via "gcc". You can 
force it to use C via the "-x c" parameter. The parameter has to come 
before the first source file on the command line, since it only affects 
language selection for files following it.

> 2. If its C++, how do I handle such a situation? Actually what I have is
> a bit map as a C array that I have generated.

Define it as "extern const int fred = 10;", this will put it into the 
object file (as constant data, just like for example string literals) 
even when compiling as C++.

Andreas

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Chris Williams Guest 2010-05-04 14:07 #1699037

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Yes. That does the trick. I have not used C++ in the embedded 
environment before so I have a bit to learn!

Thanks to all who replied.

Regards,

Chris.

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Rolf Magnus Guest 2010-05-04 19:47 #1699554

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> In C++, const declares a real constant, rather than an unmodifiable
> object.

It's both. You can use it as a real constant, e.g. for array sizes or 
template arguments, but you can still e.g. take its address.

> AFAICT (I'm not a C++ expert), a const declaration

definition

> must have an initializer,

Yes, unless it's of a class type with a default constructor.

> and the compiler will only use it when referenced within
> the compilation unit.

Even with const, it's still a regular variable. The difference is that 
has internal linkage by default, so it's as if you had made it static.

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