[arm-gnu] Static template member in assigned section -- section type conflict

[Carlos O'Donell]

On 1/27/2012 12:19 AM, Gene Smith wrote:
>> You will never have *all* of priv in the same place because
>> the `static' clause makes that impossible for>  1 instances
>> of the object (given the current implementation for static).
>>
>> Cheers,
>> Carlos.
> 
> Thanks for the detailed explanation. However, I am not sure 
> I follow it all and will look up the concept of "common" 
> you refer to. While waiting for a response I found a solution 
> by breaking up the code into 3 separate files and I get the 
> result I expect.
> 
> // temp.h
> template<class t1, class t2>
> class outer
> {
> public:
>     t1        a;
>     static t2    b;
> };
> 
> // temp.cpp
> #include "temp.h"
> 
> template<class t1, class t2>
> t2 outer<t1, t2>::b
>         __attribute__((section(".s1")));
> 
> // Explicit instantiation (NEW!)
> template class outer<int,int>;
> 
> //main.c
> #include "temp.h"
> int main (void)
> {
>     static outer<int, int> priv __attribute__((section(".s1")));
> 
>     priv.a = 10;
>     priv.b = 10;
> }
> 
> arm-none-eabi-g++ -c temp.cpp
> arm-none-eabi-g++ -c main.cpp
> arm-none-eabi-g++ main.o temo.o
> Produces a.out with only a warning about lack of _start. An objdump shows all objects/variables in the expected section.
> 
> Why does this work but having it all in a single file doesn't

It doesn't work.

Use -save-temps and -Wl,-Map,linkermap.txt to see where everything goes (I suggest also printing &priv, &priv.a, and &priv.b).

You will see that priv.a goes into .s1 because of the section attribute in main.cpp.

However, because of the explicit instantiation priv.b goes into .bss, it's a specialization, and isn't shared amongst other uses of the template so it's not common data anymore (which is the normal way in which static data is handled).

e.g.

6009d8 &priv
6009d8 &priv.a

.s1             0x00000000006009d8        0x4
 .s1            0x00000000006009d8        0x4 priv2.o

6009f0 &priv.b

.bss            0x00000000006009e0       0x18
 ...
 .bss           0x00000000006009f0        0x0 priv2-temp.o
 .bss._ZN5outerIiiE1bE
                0x00000000006009f0        0x4 priv2-temp.o
                0x00000000006009f0                _ZN5outerIiiE1bE

You are perilously poking under the hood of a complex system :-)

Cheers,
Carlos.
-- 
Carlos O'Donell
Mentor Graphics / CodeSourcery
carlos at codesourcery.com
+1 (613) 963 1026