Basically, it is a compiler ABI and varies on different platforms (like Windows and Linux). This is interesting and useful for debugging (at least for understanding how debuggers work...).
For example, a simple piece of code:
1 void f(int arg1, int arg2, int arg3, int arg4, float arg5, int arg6, float arg7,
2 float arg8, int arg9, int arg10, int arg11, int arg12)
3 {
4 printf("%d %d %d %d %f %d %f %f %d %d %d %d\n",
5 arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9,
6 arg10, arg11, arg12);
7 }
8
9 void main()
10 {
11 f(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12);
12 }On Linux i386, above compiles to assembly:
[SID@test]$cc a.c -o a
a.c: In function ‘f’:
a.c:4: warning: incompatible implicit declaration of built-in function ‘printf’
[SID@test]$objdump -d a > a.s
In a.s, we can see the main function calls f function by passing every argument through the stack.
160 0804842e :
161 804842e: 55 push %ebp
162 804842f: 89 e5 mov %esp,%ebp
163 8048431: 83 e4 f0 and $0xfffffff0,%esp
164 8048434: 83 ec 30 sub $0x30,%esp
165 8048437: c7 44 24 2c 0c 00 00 movl $0xc,0x2c(%esp)
166 804843e: 00
167 804843f: c7 44 24 28 0b 00 00 movl $0xb,0x28(%esp)
168 8048446: 00
169 8048447: c7 44 24 24 0a 00 00 movl $0xa,0x24(%esp)
170 804844e: 00
171 804844f: c7 44 24 20 09 00 00 movl $0x9,0x20(%esp)
172 8048456: 00
173 8048457: b8 00 00 00 41 mov $0x41000000,%eax
174 804845c: 89 44 24 1c mov %eax,0x1c(%esp)
175 8048460: b8 00 00 e0 40 mov $0x40e00000,%eax
176 8048465: 89 44 24 18 mov %eax,0x18(%esp)
177 8048469: c7 44 24 14 06 00 00 movl $0x6,0x14(%esp)
178 8048470: 00
179 8048471: b8 00 00 a0 40 mov $0x40a00000,%eax
180 8048476: 89 44 24 10 mov %eax,0x10(%esp)
181 804847a: c7 44 24 0c 04 00 00 movl $0x4,0xc(%esp)
182 8048481: 00
183 8048482: c7 44 24 08 03 00 00 movl $0x3,0x8(%esp)
184 8048489: 00
185 804848a: c7 44 24 04 02 00 00 movl $0x2,0x4(%esp)
186 8048491: 00
187 8048492: c7 04 24 01 00 00 00 movl $0x1,(%esp)
188 8048499: e8 26 ff ff ff call 80483c4
189 804849e: c9 leave
190 804849f: c3 ret Then on X86_64 Linux, the code compiles into following, where parameters are passed to f function through three ways: general purpose registers (di, si, dx, cx, r8d, r9d), xmm registers (xmm0~xmm2), and function stack.
159 000000000040054b :
160 40054b: 55 push %rbp
161 40054c: 48 89 e5 mov %rsp,%rbp
162 40054f: 48 83 ec 20 sub $0x20,%rsp
163 400553: c7 44 24 10 0c 00 00 movl $0xc,0x10(%rsp)
164 40055a: 00
165 40055b: c7 44 24 08 0b 00 00 movl $0xb,0x8(%rsp)
166 400562: 00
167 400563: c7 04 24 0a 00 00 00 movl $0xa,(%rsp)
168 40056a: 41 b9 09 00 00 00 mov $0x9,%r9d
169 400570: f3 0f 10 15 60 01 00 movss 0x160(%rip),%xmm2 # 4006d8 <__dso_handle+0 x30>
170 400577: 00
171 400578: f3 0f 10 0d 5c 01 00 movss 0x15c(%rip),%xmm1 # 4006dc <__dso_handle+0 x34>
172 40057f: 00
173 400580: 41 b8 06 00 00 00 mov $0x6,%r8d
174 400586: f3 0f 10 05 52 01 00 movss 0x152(%rip),%xmm0 # 4006e0 <__dso_handle+0 x38>
175 40058d: 00
176 40058e: b9 04 00 00 00 mov $0x4,%ecx
177 400593: ba 03 00 00 00 mov $0x3,%edx
178 400598: be 02 00 00 00 mov $0x2,%esi
179 40059d: bf 01 00 00 00 mov $0x1,%edi
180 4005a2: e8 1d ff ff ff callq 4004c4
181 4005a7: c9 leaveq
182 4005a8: c3 retq
183 4005a9: 90 nop
184 4005aa: 90 nop
185 4005ab: 90 nop
186 4005ac: 90 nop
187 4005ad: 90 nop
188 4005ae: 90 nop
189 4005af: 90 nop So why the difference? Basically this is part of System V AMD64 ABI convention which GCC and ICC (Intel compiler) implements on Linux, BSD and Mac and which defines that rdi, rsi, rdx, rcx, r8, r9 can be used to pass down integer parameters and xmm0-7 can be used to pass down float point parameters.
This leads to another question, why not other registers? On X86_64, there are 16 general purpose registers that can save integers (rax, rbx, rcx, rdx, rsi, rdi, rbp, rsp r8~r15), and 16 xmm registers that can save float points (xmm0~xmm15). They are divided by compiler ABI into volatile and non-volatile registers. Volatile registers are scratch registers presumed by the caller to be destroyed across a call. Nonvolatile registers are required to retain their values across a function call and must be saved by the callee if used. So volatile registers are naturally suitable for function arguments while there is overhead of using non-volatile registers (must be saved).
The calling conversion ABI is basically about which register is volatile/non-volatile, which is reserved for specially purpose (parameter passing, frame pointer, stack pointer, etc.), what is the order of arguments on stack, who (caller or callee) is responsible for cleaning up the stack, as well as stack layout/alignness.
| Architecture | Calling convention name | Operating system, Compiler | Parameters in registers | Parameter order on stack | Stack cleanup by | Notes |
|---|---|---|---|---|---|---|
| 64bit | Microsoft x64 calling convention | Windows (Microsoft compiler, Intel compiler) | rcx/xmm0, rdx/xmm1, r8/xmm2, r9/xmm3 | RTL (C) | caller | Stack aligned on 16 bytes. 32 bytes shadow space on stack. The specified 8 registers can only be used for parameter number 1,2,3 and 4. |
| System V AMD64 ABI convention | Linux, BSD, Mac (GCC, Intel compiler) | rdi, rsi, rdx, rcx, r8, r9, xmm0-7 | RTL (C) | caller | Stack aligned on 16 bytes. Red zone below stack. |
The above table is only for either user space application or kernel space functions. Likewise, there is always an exception. Here the exception is system calls. System calls trap user space context into kernel space and have specially requirement for parameter passing:
1. User-level applications use as integer registers for passing the sequence %rdi, %rsi, %rdx, %rcx, %r8 and %r9. The kernel interface uses %rdi, %rsi, %rdx, %r10, %r8 and %r9.
2. A system-call is done via the syscall instruction. The kernel destroys registers %rcx and %r11.
3. The number of the syscall has to be passed in register %rax.
4. System-calls are limited to six arguments, no argument is passed directly on
the stack.
5. Returning from the syscall, register %rax contains the result of the system-call. A value in the range between -4095 and -1 indicates an error, it is -errno.
6. Only values of class INTEGER or class MEMORY are passed to the kernel.
