With "=r", amd64:

ffffffff809a08a0 <stack_save>:
ffffffff809a08a0:       55                      push   %rbp
ffffffff809a08a1:       48 89 e5                mov    %rsp,%rbp
ffffffff809a08a4:       41 57                   push   %r15
ffffffff809a08a6:       41 56                   push   %r14
ffffffff809a08a8:       41 55                   push   %r13
ffffffff809a08aa:       41 54                   push   %r12
ffffffff809a08ac:       53                      push   %rbx
ffffffff809a08ad:       50                      push   %rax
ffffffff809a08ae:       48 89 7d d0             mov    %rdi,-0x30(%rbp)
ffffffff809a08b2:       49 89 ef                mov    %rbp,%r15
ffffffff809a08b5:       65 48 8b 1c 25 00 00    mov    %gs:0x0,%rbx
ffffffff809a08bc:       00 00 
ffffffff809a08be:       e8 bd 00 d4 ff          callq  ffffffff806e0980 <stack_zero>

with "=g", amd64:

ffffffff809a08a0 <stack_save>:
ffffffff809a08a0:       55                      push   %rbp
ffffffff809a08a1:       48 89 e5                mov    %rsp,%rbp
ffffffff809a08a4:       41 57                   push   %r15
ffffffff809a08a6:       41 56                   push   %r14
ffffffff809a08a8:       41 55                   push   %r13
ffffffff809a08aa:       41 54                   push   %r12
ffffffff809a08ac:       53                      push   %rbx
ffffffff809a08ad:       50                      push   %rax
ffffffff809a08ae:       48 89 7d d0             mov    %rdi,-0x30(%rbp)
ffffffff809a08b2:       49 89 ef                mov    %rbp,%r15
ffffffff809a08b5:       65 48 8b 1c 25 00 00    mov    %gs:0x0,%rbx
ffffffff809a08bc:       00 00 
ffffffff809a08be:       e8 bd 00 d4 ff          callq  ffffffff806e0980 <stack_zero>

with "=r", i386:

c09ae000 <stack_save>:
c09ae000:       55                      push   %ebp
c09ae001:       89 e5                   mov    %esp,%ebp
c09ae003:       53                      push   %ebx
c09ae004:       57                      push   %edi
c09ae005:       56                      push   %esi
c09ae006:       83 ec 08                sub    $0x8,%esp
c09ae009:       8b 75 08                mov    0x8(%ebp),%esi
c09ae00c:       89 eb                   mov    %ebp,%ebx
c09ae00e:       64 8b 3d 00 00 00 00    mov    %fs:0x0,%edi
c09ae015:       89 34 24                mov    %esi,(%esp)
c09ae018:       e8 d3 77 d5 ff          call   c07057f0 <stack_zero>

with "=g", i386:

c09ae000 <stack_save>:
c09ae000:       55                      push   %ebp
c09ae001:       89 e5                   mov    %esp,%ebp
c09ae003:       53                      push   %ebx
c09ae004:       57                      push   %edi
c09ae005:       56                      push   %esi
c09ae006:       83 ec 08                sub    $0x8,%esp
c09ae009:       8b 75 08                mov    0x8(%ebp),%esi
c09ae00c:       89 eb                   mov    %ebp,%ebx
c09ae00e:       64 8b 3d 00 00 00 00    mov    %fs:0x0,%edi
c09ae015:       89 34 24                mov    %esi,(%esp)
c09ae018:       e8 d3 77 d5 ff          call   c07057f0 <stack_zero>