Compiler-generated implicit symbols and --gc-sections

[Timo Sintonen]

On Friday, 3 January 2014 at 18:14:58 UTC, Mike wrote:
> I ran into a problem recently that resulted in a segmentation 
> fault in my program whenever I called a member function of one 
> of my classes.  Sometimes it occurred and sometimes it didn't 
> depending on the order of certain things in my code.
>
>
> I eventually tracked it down to the fact that I was compiling 
> with -ffunction-sections and -fdata-sections and linking with 
> --gc-sections and symbols like...
>
> .data._D38TypeInfo_E14TypeInfo_Class10ClassFlags6__initZ
> .data._D40TypeInfo_E15TypeInfo_Struct11StructFlags6__initZ
>
> ... were being discarded.  I'm assuming this is the mangled 
> .init values of these types, yes?
>
>
>
> My linker script contained...
>
> .data : AT (__data_rom_begin)
>     {
> 	. = ALIGN(4);
> 	__data_ram_begin = .;
> 	
> 	. = ALIGN(4);
> 	*(.data)
> 	*(.data*)
>
> 	. = ALIGN(4);
> 	__data_ram_end = .;
>     } >SRAM
>
> ... so I was forced to conclude that the reason they were being 
> discarded was because it couldn't find any code that was 
> reaching these symbols.
>
>
>
> After adding...
>
> KEEP(*(.data.*init*))
>
> ... to my linker script, the problem was resolved.
>
> I'm guessing these are generated implicitly by the GDC 
> compiler, but it does appear that my code never reaches these 
> symbols, so discarding them should be OK.  However, it seems 
> discarding them causes dislocation in memory.
>
> I'm still a novice with GCC-based toolchains, so forgive the 
> ignorance of this question, but is this to be expected, or is 
> this an indication of a problem with the compiler?
>
> Mike
>
> Compiler:
> Latest GDC 4.8 compiled for arm-none-eabi (ARM Cortex-M)

Again, I am guessing a little, but...

In dmd and ides it is common to compile and link everything at 
once. the compiler has all information available and may remove 
unused code and data.

The gcc system is made for separate compilation. When compiling a 
file, the compiler has no idea how other files call functions and 
objects in this file. So there has to be at least the default set 
of resources. If they are used is known only at linking phase. I 
do not know if the linker is able to remove unused code or data 
and what flags are needed.

Because tha data is referenced from other files, there has to be 
a common naming system. Maybe it would be possible to use named 
variables but for some reason they have decided to name a 
separate section for every piece of info. Every class, struct etc 
will have its own sections and there will be lots of them. I have 
just included all of them without thinking.

It may also be possible that the code or data is in use. In asm 
file there is a table of data after each funtion. The code may 
get a word from the table. This may be a pointer to another table 
in another function in another module. There may be an offset to 
another place in the table and there may be a pointer to this 
strange section. Without looking the whole program in debugger it 
is impossible to say whether the code and data are actually used 
or not.