Hey all, I'm working on the AVR backend for LLVM.
I'm looking into an issue where the current LLVM implementation of the AVR C calling convention is not matching the assembly of what GCC is generating. On top of that, when I run through the argument algorithm described on the Wiki (https://gcc.gnu.org/wiki/avr-gcc), GCC output doesn't seem to match up, whereas LLVM does. The shortest and most inaccurate version of it is: GCC only stores up to 8 bytes of arguments in registers, and every argument after that is located on the stack. The Wiki says that registers between r8-r25 are used, which doesn't seem to match up. Example: Given a function which takes two 64-bit integers (i64 %a, i64 %b) * Start with Rn = 26 * Begin processing %a * Rn -= 8 = 18 * Rn >= 8, therefore this argument will be stored in registers r18-r25 * Begin processing %b * Rn -= 8 = 10 * Rn >= 8, therefore this argument will be stored in registers r10-r17 This indicates that both arguments should be located in registers, but AVR-GCC stores the first argument in registers, and the second argument on the stack. Which is correct? Am I misunderstanding the algorithm? Where in GCC can I find the implementation of this calling convention? I've got an issue on the LLVM bug tracker here ( https://llvm.org/bugs/show_bug.cgi?id=31347). Cheers, Dylan
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