Different register allocation on three-parameter function

[blackgeorge-boom]

Minimal example:

void rec(int a, int b, int c)
{
	if (a == 0)
		return;
	else {
		rec(a - 1, b, c);
		return;
	}
}

int main()
{
	rec(1, 2, 3);
}

 [STACKMAPS CHECK] Checking stackmaps for main_aarch64_aligned.out main_x86_64_aligned.out
make[1]: warning: jobserver unavailable: using -j1.  Add '+' to parent make rule.
make[1]: Entering directory '/home/blackgeorge/Documents/phd/unified_abi/stack-metadata'
make[1]: Nothing to be done for 'all'.
make[1]: Leaving directory '/home/blackgeorge/Documents/phd/unified_abi/stack-metadata'
WARNING: rec: callsite 0, value locations 0/0 have different location type (3 vs. 1)
WARNING: rec: callsite 0, value locations 0/0 have different location offset or  different constant (-40 vs. 0)
WARNING: rec: callsite 0, value locations 1/1 have different location type (1 vs. 3)
WARNING: rec: callsite 0, value locations 1/1 have different location offset or  different constant (0 vs. -40)
ERROR: stackmaps in 'main_aarch64_aligned.out' & 'main_x86_64_aligned.out' differ - different stack layout!
make: *** [../../common/common.mk:193: stackmaps-check] Error 1

[blackgeorge-boom]

It seems that AArch64 spills the first argument, inside the recursive call to rec, whereas X86 spills the second.

AArch64:

0000000000501020 rec:
  501020: ff 03 01 d1                  	sub	sp, sp, #0x40
  501024: f4 4f 02 a9                  	stp	x20, x19, [sp, #0x20]
  501028: fd 7b 03 a9                  	stp	x29, x30, [sp, #0x30]
  50102c: fd c3 00 91                  	add	x29, sp, #0x30
  501030: e1 03 02 29                  	stp	w1, w0, [sp, #0x10]
  501034: e2 0f 00 b9                  	str	w2, [sp, #0xc]
  501038: 00 01 00 34                  	cbz	w0, #0x20 <rec+0x38>
  50103c: f4 23 42 29                  	ldp	w20, w8, [sp, #0x10]
  501040: f3 0f 40 b9                  	ldr	w19, [sp, #0xc]
  501044: 00 05 00 51                  	sub	w0, w8, #0x1
  501048: e1 03 14 2a                  	mov	w1, w20
  50104c: e2 03 13 2a                  	mov	w2, w19
  501050: e0 0b 00 b9                  	str	w0, [sp, #0x8]    <---
  501054: f3 ff ff 97                  	bl	#-0x34 <rec>
...

X86:

0000000000501020 <rec>:
  501020:	55                   	push   rbp
  501021:	48 89 e5             	mov    rbp,rsp
  501024:	53                   	push   rbx
  501025:	41 57                	push   r15
  501027:	48 83 ec 20          	sub    rsp,0x20
  50102b:	89 7d e4             	mov    DWORD PTR [rbp-0x1c],edi
  50102e:	89 75 e0             	mov    DWORD PTR [rbp-0x20],esi
  501031:	89 55 dc             	mov    DWORD PTR [rbp-0x24],edx
  501034:	85 ff                	test   edi,edi
  501036:	0f 84 1c 00 00 00    	je     501058 <rec+0x38>
  50103c:	8b 5d e4             	mov    ebx,DWORD PTR [rbp-0x1c]
  50103f:	ff cb                	dec    ebx
  501041:	8b 75 e0             	mov    esi,DWORD PTR [rbp-0x20]
  501044:	89 75 d8             	mov    DWORD PTR [rbp-0x28],esi   <---
  501047:	44 8b 7d dc          	mov    r15d,DWORD PTR [rbp-0x24]
  50104b:	89 df                	mov    edi,ebx
  50104d:	44 89 fa             	mov    edx,r15d
  501050:	0f 1f 00             	nop    DWORD PTR [rax]
  501053:	e8 c8 ff ff ff       	call   501020 <rec>
...