I've figured it out, thanks for all the help guys! On Sat, Sep 2, 2017 at 7:15 PM, Senthil Kumar <senthil.theco...@gmail.com> wrote:
> 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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