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Re: [avr-gcc-list] AVR C calling convention not matching spec

From: Dylan McKay
Subject: Re: [avr-gcc-list] AVR C calling convention not matching spec
Date: Tue, 20 Dec 2016 21:09:38 +1300

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 <address@hidden> 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.
.global thing
    .type   thing, @function
    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                              ----------------->
    .size   thing, .-thing


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