https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52792
--- Comment #5 from Kai Tietz <ktietz at gcc dot gnu.org> ---
For the c++ sample:
typedef struct agg { long a; long b; long c; } agg;
class abc {
long m;
public:
agg foo(long a, long b, long c);
agg boo(void);
};
agg abc::foo(long a, long b, long c)
{
agg r = { a, b, c};
m = a + b +c;
return r;
}
MS' compiler produces for foo() method:
?foo@abc@@QAE?AUagg@@JJJ@Z:
push ebp
mov ebp, esp
mov eax, [ebp+8]
mov edx, [ebp+10h]
push esi
mov esi, [ebp+14h]
push edi
mov edi, [ebp+0Ch]
mov [eax+4], edx
add edx, edi
mov [eax], edi
add edx, esi
pop edi
mov [eax+8], esi
mov [ecx], edx
pop esi
pop ebp
retn 10h
g++ produces on x86 (with -O2 -fno-omit-frame-pointer):
__ZN3abc3fooElll:
pushl %ebp
movl %ecx, %eax
movl %esp, %ebp
pushl %ebx
movl 16(%ebp), %ecx
movl 12(%ebp), %ebx
movl 20(%ebp), %edx
movl %ecx, 4(%eax)
addl %ebx, %ecx
movl %ebx, (%eax)
movl %edx, 8(%eax)
addl %ecx, %edx
movl 8(%ebp), %ecx
movl %edx, (%ecx)
popl %ebx
popl %ebp
ret $16
As we can see are for the ms compiler the argument 'this' passed in ecx, and
the aggregate pointer as first stack argument. For g++ 'this' is passed on
stack, and the aggregate uses the 'ecx' register.
For x64 ms' compiler produces for foo() member:
?foo@abc@@QEAA?AUagg@@JJJ@Z: ; DATA XREF: .rdata:off_140002748↓o
mov r10d, [rsp+28h]
lea eax, [r8+r9]
add eax, r10d
mov [rdx], r8d
mov [rcx], eax
mov rax, rdx
mov [rdx+4], r9d
mov [rdx+8], r10d
retn
g++ (with -O2) produces for x86_64 msabi:
_ZN3abc3fooElll:
.LFB0:
.seh_endprologue
movq %rcx, %rax
movl 40(%rsp), %ecx
movl %r9d, 4(%rax)
addl %r8d, %r9d
movl %r8d, (%rax)
movl %ecx, 8(%rax)
addl %r9d, %ecx
movl %ecx, (%rdx)
ret
For ms' compiler it passes 'this' in rcx, and the aggregate pointer in 'rdx'.
For g++ it passes 'this' in rdx, and the aggregate pointer in 'rcx'.
