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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