VS2010 优化器的全局指针和全局引用之间的最大区别是什么？为什么参考没有解决？
typedef unsigned char byte_t;
typedef unsigned short word_t;
struct byte_reg_t
{
byte_t low;
byte_t high;
};
union word_reg_t
{
word_t value;
byte_reg_t part;
};
word_reg_t r16;
byte_t& low_ref = r16.part.low;
byte_t* const low_ptr = &r16.part.low;
#define SPLIT() _asm nop;
int main()
{
low_ref = 4;
SPLIT()
byte_t a = r16.part.low;
SPLIT()
byte_t b = low_ref;
SPLIT()
byte_t c = *low_ptr;
SPLIT()
return a+b+c;
}
使用汇编输出在发布模式下编译产生这个结果
;byte_t a = r16.part.low;
mov cl, BYTE PTR ?r16@@3Tword_reg_t@@A
;byte_t b = low_ref;
mov edx, DWORD PTR ?low_ref@@3AAEA ; low_ref
mov dl, BYTE PTR [edx]
;byte_t c = *low_ptr;
mov al, BYTE PTR ?r16@@3Tword_reg_t@@A
未经修改的拆卸
.text:00401000 _main proc near ; CODE XREF: __tmainCRTStartup+11D
.text:00401000 mov eax, ?low_ref@@3AAEA ; uchar & low_ref
.text:00401005 mov byte ptr [eax], 4
.text:00401008 nop
.text:00401009 mov cl, ?r16@@3Tword_reg_t@@A ; word_reg_t r16
.text:0040100F nop
.text:00401010 mov edx, ?low_ref@@3AAEA ; uchar & low_ref
.text:00401016 mov dl, [edx]
.text:00401018 nop
.text:00401019 mov al, ?r16@@3Tword_reg_t@@A ; word_reg_t r16
.text:0040101E nop
.text:0040101F movzx eax, al
.text:00401022 movzx edx, dl
.text:00401025 movzx ecx, cl
.text:00401028 add eax, edx
.text:0040102A add eax, ecx
.text:0040102C retn
.text:0040102C _main endp
.data:00403374 ?r16@@3Tword_reg_t@@A db ? ; DATA XREF: _main+9
.data:00403374 ; _main+19
.data:00403375 align 4
.data:00403018 ; unsigned char & low_ref
.data:00403018 ?low_ref@@3AAEA dd offset ?r16@@3Tword_reg_t@@A ; DATA XREF: _main
.data:00403018 ; _main+10
.data:00403018 ; word_reg_t r16
我测试了几个变体（从函数返回等） - 如果 low_ref 正在使用则无法解决
- 优化器傻吗？
- 一个不常见的优化案例？
- 一些 c/c++ 标准限制？
更新
这似乎是一个不常见的优化案例 - thx Michael Burr
如果引用在函数范围内 - 或者在函数范围内实例化的类或结构内，它就可以工作（但优化器解析 ptr const 但不解析引用仍然很奇怪 - 它们是 100% 相同的）
更新 2
更奇怪的是-如果您从 byte_t 切换到 int 两种解析都有效- const ptr 和 reference
- 全局 ptr const 到全局 byte_t var：已解决
- 全局 ptr const 到全局 int var：已解决
- 对全局 byte_t var 的全局引用：未解决
- 对全局 int var 的全局引用：已解决
- 对本地 byte_t var 的全局引用：已解决
- 对本地 int var 的全局引用：已解决
所以 ptr const 和引用的优化器、引用范围......和引用类型......有时有一点区别：）
更新 3
更简单的测试代码 - 使用 VS2010 和 clang 3.1 检查
typedef unsigned char byte_t;
typedef unsigned int dword_t;
//for msvc
#define SPLIT() _asm nop _asm nop;
//for clang
//#define SPLIT() asm("nop"); asm("nop");
byte_t byte;
dword_t dword;
byte_t& global_ref_byte = byte;
dword_t& global_ref_dword = dword;
byte_t* const global_ptrc_byte = &byte;
dword_t* const global_ptrc_dword = &dword;
int main(int argc, char** argv)
{
byte_t& local_ref_byte = byte;
dword_t& local_ref_dword = dword;
dword_t random = (dword_t)argv;
byte = (byte_t)random;
dword = (dword_t)random;
SPLIT()
byte_t a = global_ref_byte;
SPLIT()
dword_t b = global_ref_dword;
SPLIT()
byte_t c = *global_ptrc_byte;
SPLIT()
dword_t d = *global_ptrc_dword;
SPLIT()
byte_t e = local_ref_byte;
SPLIT()
dword_t f = local_ref_dword;
SPLIT()
dword_t result = a+b+c+d+e+f;
return result;
}
VS2010拆解
.text:00401000 ; int __cdecl main(int argc, const char **argv, const char **envp)
.text:00401000 _main proc near ; CODE XREF: ___tmainCRTStartup+11D
.text:00401000
.text:00401000 argc = dword ptr 8
.text:00401000 argv = dword ptr 0Ch
.text:00401000 envp = dword ptr 10h
.text:00401000
.text:00401000 push ebp
.text:00401001 mov ebp, esp
.text:00401003 mov eax, [ebp+argv]
.text:00401006 push ebx
.text:00401007 push esi
.text:00401008 push edi
.text:00401009 mov byte_403374, al
.text:0040100E mov dword_403378, eax
.text:00401013 nop
.text:00401014 nop
.text:00401015 mov eax, off_40301C
.text:0040101A mov al, [eax]
.text:0040101C nop
.text:0040101D nop
.text:0040101E mov ecx, dword_403378
.text:00401024 nop
.text:00401025 nop
.text:00401026 mov dl, byte_403374
.text:0040102C nop
.text:0040102D nop
.text:0040102E mov esi, dword_403378
.text:00401034 nop
.text:00401035 nop
.text:00401036 mov bl, byte_403374
.text:0040103C nop
.text:0040103D nop
.text:0040103E mov edi, dword_403378
.text:00401044 nop
.text:00401045 nop
.text:00401046 movzx edx, dl
.text:00401049 movzx ebx, bl
.text:0040104C add edx, edi
.text:0040104E movzx eax, al
.text:00401051 add edx, ebx
.text:00401053 add eax, edx
.text:00401055 pop edi
.text:00401056 add eax, esi
.text:00401058 pop esi
.text:00401059 add eax, ecx
.text:0040105B pop ebx
.text:0040105C pop ebp
.text:0040105D retn
.text:0040105D _main endp
铿锵3.1反汇编
.text:004012E0 sub_4012E0 proc near ; CODE XREF: sub_401020+91
.text:004012E0
.text:004012E0 arg_4 = dword ptr 0Ch
.text:004012E0
.text:004012E0 push ebp
.text:004012E1 mov ebp, esp
.text:004012E3 call sub_4014F0
.text:004012E8 mov eax, [ebp+arg_4]
.text:004012EB mov byte_402000, al
.text:004012F0 mov dword_402004, eax
.text:004012F5 nop
.text:004012F6 nop
.text:004012F7 movzx eax, byte_402000
.text:004012FE nop
.text:004012FF nop
.text:00401300 add eax, dword_402004
.text:00401306 nop
.text:00401307 nop
.text:00401308 movzx ecx, byte_402000
.text:0040130F add ecx, eax
.text:00401311 nop
.text:00401312 nop
.text:00401313 add ecx, dword_402004
.text:00401319 nop
.text:0040131A nop
.text:0040131B movzx eax, byte_402000
.text:00401322 add eax, ecx
.text:00401324 nop
.text:00401325 nop
.text:00401326 add eax, dword_402004
.text:0040132C nop
.text:0040132D nop
.text:0040132E pop ebp
.text:0040132F retn
.text:0040132F sub_4012E0 endp
没有 nops，两个优化器都可以生成更好的代码 - 但 clang 仍然更好
VS2010（更多代码，因为未解析的字节引用）
.text:00401003 mov eax, [ebp+argv]
.text:00401006 movzx ecx, al
.text:00401009 lea edx, [eax+eax*2]
.text:0040100C mov byte_403374, al
.text:00401011 mov dword_403378, eax
.text:00401016 lea eax, [edx+ecx*2]
.text:00401019 mov ecx, off_40301C
.text:0040101F movzx edx, byte ptr [ecx]
.text:00401022 add eax, edx
铿锵3.1：
.text:004012E8 mov eax, [ebp+arg_4]
.text:004012EB mov byte_402000, al
.text:004012F0 mov dword_402004, eax
.text:004012F5 movzx ecx, al
.text:004012F8 add ecx, eax
.text:004012FA lea eax, [ecx+ecx*2]