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On x86-64, clang -O3 compiles the following function:
PyCArgObject *
PyCArgObject_new(void)
{
 PyCArgObject *p;
 p = PyObject_New(PyCArgObject, &PyCArg_Type);
 if (p == NULL)
 return NULL;
 p->pffi_type = NULL;
 p->tag = '0円';
 p->obj = NULL;
 memset(&p->value, 0, sizeof(p->value));
 return p;
}
like that:
 0x00007fffe9c6acb0 <+0>:	push rax
 0x00007fffe9c6acb1 <+1>:	mov rdi,QWORD PTR [rip+0xe308] # 0x7fffe9c78fc0
 0x00007fffe9c6acb8 <+8>:	call 0x7fffe9c5e8a0 <_PyObject_New@plt>
 0x00007fffe9c6acbd <+13>:	test rax,rax
 0x00007fffe9c6acc0 <+16>:	je 0x7fffe9c6acdf <PyCArgObject_new+47>
 0x00007fffe9c6acc2 <+18>:	mov QWORD PTR [rax+0x20],0x0
 0x00007fffe9c6acca <+26>:	mov BYTE PTR [rax+0x28],0x0
 0x00007fffe9c6acce <+30>:	xorps xmm0,xmm0
 0x00007fffe9c6acd1 <+33>:	movaps XMMWORD PTR [rax+0x30],xmm0
 0x00007fffe9c6acd5 <+37>:	mov QWORD PTR [rax+0x40],0x0
 0x00007fffe9c6acdd <+45>:	pop rcx
 0x00007fffe9c6acde <+46>:	ret 
 0x00007fffe9c6acdf <+47>:	xor eax,eax
 0x00007fffe9c6ace1 <+49>:	pop rcx
 0x00007fffe9c6ace2 <+50>:	ret 
The problem is that movaps requires the memory address to be aligned on 16 bytes, whereas PyObject_New() uses pymalloc allocator (the requested size is 80 bytes, pymalloc supports allocations up to 512 bytes) and pymalloc only provides alignment on 8 bytes.
If PyObject_New() returns an address not aligned on 16 bytes, PyCArgObject_new() crash immediately with a segmentation fault (SIGSEGV).
CPython must be compiled using -fmax-type-align=8 to avoid such alignment crash. Using this compiler flag, clag emits expected machine code:
 0x00007fffe9caacb0 <+0>:	push rax
 0x00007fffe9caacb1 <+1>:	mov rdi,QWORD PTR [rip+0xe308] # 0x7fffe9cb8fc0
 0x00007fffe9caacb8 <+8>:	call 0x7fffe9c9e8a0 <_PyObject_New@plt>
 0x00007fffe9caacbd <+13>:	test rax,rax
 0x00007fffe9caacc0 <+16>:	je 0x7fffe9caacdf <PyCArgObject_new+47>
 0x00007fffe9caacc2 <+18>:	mov QWORD PTR [rax+0x20],0x0
 0x00007fffe9caacca <+26>:	mov BYTE PTR [rax+0x28],0x0
 0x00007fffe9caacce <+30>:	xorps xmm0,xmm0
 0x00007fffe9caacd1 <+33>:	movups XMMWORD PTR [rax+0x30],xmm0
 0x00007fffe9caacd5 <+37>:	mov QWORD PTR [rax+0x40],0x0
 0x00007fffe9caacdd <+45>:	pop rcx
 0x00007fffe9caacde <+46>:	ret 
 0x00007fffe9caacdf <+47>:	xor eax,eax
 0x00007fffe9caace1 <+49>:	pop rcx
 0x00007fffe9caace2 <+50>:	ret 
"movaps" instruction becomes "movups" instruction: "a" stands for "aligned" in movaps, whereas "u" stands for "unaligned" in movups.