Re: Cortex M0 Floating Point Library

[Daniel Engel <libgcc at danielengel dot com>]

Hi Richard, 

I've only used custom linker scripts with my embedded work, so I don't know much about the GCC default.  

Presently, every library function is already in its own section to facilitate --gc-sections optimization.  However, I #include every file together to ensure that all of the sections are still built into a single object file.  So far the linker has done what I've expected (i.e., placed all of the used functions/sections in a contiguous block of memory and discarded the unused functions).  

Is the linker aware of section hierarchy, such that using a common section prefix (e.g. ".text.m0fp.*") would gather the appropriate sections together from multiple object files?  I didn't think it was so aware; rather, that such prefixes were just a convention to help organize custom linker scripts.  Adding such rules to the default linker script wouldn't be ideal, as everyone using a custom script might then have library breakage unless they knew to add equivalent rules.  

If the consensus is to split the library, it might help to add a set of intermediate branches (trampolines?) in the libm portion.  This would add execution cycles, but not require as many extra bytes.  

Regards,
Daniel


On Thu, Nov 8, 2018, at 11:19 PM, Richard Henderson wrote:
> On 11/7/18 6:10 PM, Daniel Engel wrote:
> > Also, loss of control of linking order would require all short branches in the libm section to be replaced with long branches.  This particularly impacts the exception handling in almost every function.
> 
> You could partially remedy this by placing all the code into a unique section,
> e.g. ".text.m0fp".  The default linker script would place all instances of this
> section together.  Additional tricks can be played if we're willing to modify
> the linker scripts further.
> 
> 
> r~