I have set the -mshort-calls
flag which is documented to replace the 4 byte long CALL
instruction with the 2 byte long RCALL
instruction when possible to save flash space and and cycles…
-mshort-calls
Assume that RJMP and RCALL can target the whole program memory.
This option is used internally for multilib selection. It is not an
optimization option, and you don’t need to set it by hand.
https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html#index-mshort-calls
…and it set in Atmel Studio here…
Unfortunately the compiler is still using full CALL
instructions even for a call that is only 2 bytes away…
...
35f6: 0e 94 fd 1a call 0x35fa ; 0x35fa
35fa: f8 94 cli
...
What am I doing wrong?
Best Answer
This option was removed in gcc 4.8 but not all the documentation was updated. Atmel Studio also still uses the obsolete flag.
The new option is called
-mrelax
on the compiler command line or--relax
on the linker command line...https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/AVR-Options.html
After adding that flag, the above code compiled to...
If you are using Atmel Studio, the new flag is misleadingly called "Relax Branches" and is located here...
In my case, enabling relative jumps and calls resulted in significant flash savings. If you are here because you also need to save flash space and/or cycles, I'd also recommend looking into enabling Link Time Optimization which also offers great savings at no cost.