Hi, The part you're missing is the avr_need_copy_data_p global. It's set to false by default, and is set to true inside hooks that gcc calls whenever it needs to put something in data related sections. The avr_file_end hook function checks the variable and emits a reference to a global "__do_copy_data" only if avr_need_copy_data_p is true. The reference then causes the linker to pull in the implementation of __do_coy_data from libgcc.a(_copy_data.o).
The compiler uses the same trick to decide whether to pull in code to clear bss or not - see avr_need_clear_bss_p. In both cases, the idea is to avoid linking in the copy/clearing code if it's unnecessary (no initialized/zero initialized globals). Hope this helps. Regards Senthil On Thu, Aug 24, 2017 at 7:54 PM, Dylan McKay <m...@dylanmckay.io> wrote: > Hello all, > > I maintain the AVR LLVM backend, for which downstream users typically use > avr-gcc to link, so they get the CRT, startup routines, and libgcc. > > I've noticed that AVR-GCC does not insert the __do_copy_data function for > LLVM generated executables, but it does for GCC generated ones. > > In case reads have not heard of this function, it is one of a few small > routines which copies all variables from the .data, .rodata, and .bss in > program memory into RAM during startup. > > When does AVR-GCC insert __do_copy_data? How do I need to tune the output > of LLVM in order to trigger GCC's insertion of this function? > > I've looked at the GCC source and as far as I can tell, the logic is based > in 'avr_asm_named_section' in 'gcc/config/avr/avr.c'. The code I can see > just looks at the section names, sees if it can find sections prefixed with > '.rodata', '.data', or '.bss'. > > The LLVM backend is correctly placing my global variable into a section > prefixed with '.data', but linking with avr-gcc still does not include > __do_copy_data. > > I have attached the output of 'objdump -t' for my reference LLVM and GCC > program. They are not identical, but they both read from a global variable > named 'FOO'. > > > > _______________________________________________ > AVR-GCC-list mailing list > AVR-GCC-list@nongnu.org > https://lists.nongnu.org/mailman/listinfo/avr-gcc-list > >
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