I think you're correct. When I use the on-chip test tool, linking LLVM generated code to GCC generated code, everything works fine, even when I have multiple 64-bit arguments being passed.
Thanks for the help Pitchumani! On Tue, Dec 20, 2016 at 5:54 AM, Pitchumani Sivanupandi < pitchumani.sivanupa...@microchip.com> wrote: > On Sunday 11 December 2016 07:13 AM, Dylan McKay wrote: > >> 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). >> > > I guess you misunderstood the assembly generated. > (snip) > .global thing > .type thing, @function > thing: > push r10 <-------------- > push r11 > push r12 > push r13 save call-used registers > push r14 > push r15 > push r16 > push r17 --------------> > /* prologue: function */ > /* frame size = 0 */ > /* stack size = 8 */ > .L__stack_usage = 8 > ldi r30,lo8(4) ; > ldi r31,0 ; load address 4 to Z register > st Z,r10 <----- > std Z+1,r11 > std Z+2,r12 > std Z+3,r13 > std Z+4,r14 store argument b (which is in r10-r17) > std Z+5,r15 into memory starts at 4 (4-11) > std Z+6,r16 > std Z+7,r17 -----> > /* epilogue start */ > pop r17 <---------------- > pop r16 > pop r15 > pop r14 > pop r13 restore call-used registers > pop r12 > pop r11 > pop r10 -----------------> > ret > .size thing, .-thing > (snip) > > Regards, > Pitchumani >
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