使用MSVC 2015比较对bool和int的引用时出现警告 [英] Warning when comparing references to bool and int with MSVC 2015
问题描述
以下代码在MSVC（2015 Update 3）中产生警告-带有 / W4 ：
The following code produces a warning with MSVC (2015 Update 3) - with /W4:
const bool& a = true;
const int& b = 1;
if(a == b)
C4805：'= ='：操作中'const bool'类型和'const int'类型的不安全混合
，但没有引用，它会干净地编译。 / p>
but without the references it compiles cleanly.
const bool a = true;
const int b = 1;
if(a == b)
为什么？
编辑：
仅在不使用const的情况下进行测试
just tested without const as well
bool a = true;
int b = 1;
if(a == b)
并再次出现警告...
and the warning reappeared...
编辑2：
在Debug中进行编译...我确实必须静音 C4127：条件表达式在const noref情况下是常量 ...
Compiling in Debug... I did have to silence C4127: conditional expression is constant in the const noref case though...
编辑3：
以下是3种情况下的拆卸：
here are the disassemblies for the 3 cases:
const ref
const ref
0113BA92 in al,dx
0113BA93 sub esp,24h
0113BA96 mov eax,0CCCCCCCCh
0113BA9B mov dword ptr [ebp-24h],eax
0113BA9E mov dword ptr [ebp-20h],eax
0113BAA1 mov dword ptr [ebp-1Ch],eax
0113BAA4 mov dword ptr [ebp-18h],eax
0113BAA7 mov dword ptr [b],eax
0113BAAA mov dword ptr [ebp-10h],eax
0113BAAD mov dword ptr [ebp-0Ch],eax
0113BAB0 mov dword ptr [ebp-8],eax
0113BAB3 mov dword ptr [a],eax
const bool& a = true;
0113BAB6 mov byte ptr [ebp-9],1
0113BABA lea eax,[ebp-9]
0113BABD mov dword ptr [a],eax
const int& b = 1;
0113BAC0 mov dword ptr [ebp-1Ch],1
0113BAC7 lea ecx,[ebp-1Ch]
0113BACA mov dword ptr [b],ecx
if(a == b)
0113BACD mov edx,dword ptr [a]
0113BAD0 movzx eax,byte ptr [edx]
0113BAD3 mov ecx,dword ptr [b]
0113BAD6 cmp eax,dword ptr [ecx]
0113BAD8 jne DOCTEST_ANON_FUNC_2+5Fh (0113BAEFh)
throw 5;
0113BADA mov dword ptr [ebp-24h],5
0113BAE1 push offset __TI1H (0117318Ch)
0113BAE6 lea edx,[ebp-24h]
0113BAE9 push edx
0113BAEA call __CxxThrowException@8 (01164B04h)
仅const
0137BA92 in al,dx
0137BA93 sub esp,0Ch
0137BA96 mov dword ptr [ebp-0Ch],0CCCCCCCCh
0137BA9D mov dword ptr [b],0CCCCCCCCh
0137BAA4 mov dword ptr [ebp-4],0CCCCCCCCh
const bool a = true;
0137BAAB mov byte ptr [a],1
const int b = 1;
0137BAAF mov dword ptr [b],1
if(a == b)
0137BAB6 mov eax,1
0137BABB test eax,eax
0137BABD je DOCTEST_ANON_FUNC_2+44h (0137BAD4h)
throw 5;
0137BABF mov dword ptr [ebp-0Ch],5
0137BAC6 push offset __TI1H (013B318Ch)
0137BACB lea ecx,[ebp-0Ch]
0137BACE push ecx
0137BACF call __CxxThrowException@8 (013A4B04h)
没有常量没有引用
0012BA92 in al,dx
0012BA93 sub esp,0Ch
0012BA96 mov dword ptr [ebp-0Ch],0CCCCCCCCh
0012BA9D mov dword ptr [b],0CCCCCCCCh
0012BAA4 mov dword ptr [ebp-4],0CCCCCCCCh
bool a = true;
0012BAAB mov byte ptr [a],1
int b = 1;
0012BAAF mov dword ptr [b],1
if(a == b)
0012BAB6 movzx eax,byte ptr [a]
0012BABA cmp eax,dword ptr [b]
0012BABD jne DOCTEST_ANON_FUNC_2+44h (012BAD4h)
throw 5;
0012BABF mov dword ptr [ebp-0Ch],5
0012BAC6 push offset __TI1H (016318Ch)
0012BACB lea ecx,[ebp-0Ch]
0012BACE push ecx
0012BACF call __CxxThrowException@8 (0154B04h)
因此，似乎if语句会有一个跳转在const情况下（调试配置），它没有经过优化并且没有经过警告诊断...
So it seems there is awlays a jump for the if statement so it isn't optimized and not warning-diagnosed in the const case (debug config)...
推荐答案
这是针对以下代码的GCC 6.1会发生的情况：
This is what happens with GCC 6.1 for the following code:
int ref(int num) {
const bool& a = true;
const int& b = 1;
return a == b;
}
int noref(int num) {
const bool a = true;
const int b = 1;
return a == b;
}
int noref_noconst(int num) {
bool a = true;
int b = 1;
return a == b;
}
组装输出：
ref(int):
pushq %rbp
movq %rsp, %rbp
movl %edi, -36(%rbp)
movl $1, %eax
movb %al, -21(%rbp)
leaq -21(%rbp), %rax
movq %rax, -8(%rbp)
movl $1, %eax
movl %eax, -20(%rbp)
leaq -20(%rbp), %rax
movq %rax, -16(%rbp)
movq -8(%rbp), %rax
movzbl (%rax), %eax
movzbl %al, %edx
movq -16(%rbp), %rax
movl (%rax), %eax
cmpl %eax, %edx
sete %al
movzbl %al, %eax
popq %rbp
ret
noref(int):
pushq %rbp
movq %rsp, %rbp
movl %edi, -20(%rbp)
movb $1, -1(%rbp)
movl $1, -8(%rbp)
movl $1, %eax
popq %rbp
ret
noref_noconst(int):
pushq %rbp
movq %rsp, %rbp
movl %edi, -20(%rbp)
movb $1, -1(%rbp)
movl $1, -8(%rbp)
movzbl -1(%rbp), %eax
cmpl -8(%rbp), %eax
sete %al
movzbl %al, %eax
popq %rbp
ret
关于汇编的最简单的注意事项是，在 noref 的情况下，甚至没有跳转。这只是一个简单的 return 1 。
The easiest thing to note here about the assembly is that in the case of noref there isn't even a jump; it's just a simple return 1.
在MSVC中可能发生了同样的事情，然后它就越过了检测范围
The same thing is probably happening in MSVC, which then slips past the detection mechanism for warnings, and you just don't get one.
编辑：
让我们看看您的程序集，尤其是 const 版本：
Let's look at your assembly, and the const only version in particular:
if(a == b)
0137BAB6 mov eax,1
0137BABB test eax,eax
0137BABD je DOCTEST_ANON_FUNC_2+44h (0137BAD4h)
虽然 if 语句（ je ）是正确的在那里，我们可以看到它实际上没有比较变量-这是触发警告的精确操作。只需将1放入寄存器中，然后将寄存器与自身进行比较。
While it is true that the if statement (je) is there, we can see that it doesn't actually compare the variables--which is the precise operation that triggers the warning. It simply puts 1 in a register and then compares the register with itself.
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