如何在没有运行时库的情况下使用 VC++ 内在函数
我参与了其中一项挑战，即您尝试生成尽可能小的二进制文件，因此我正在构建我的程序， 没有 C 或 C++ 运行时库 (RTL).我没有链接到 DLL 版本或静态版本.我什至没有 #include 头文件.我有这个工作正常.
I'm involved in one of those challenges where you try to produce the smallest possible binary, so I'm building my program without the C or C++ run-time libraries (RTL). I don't link to the DLL version or the static version. I don't even #include the header files. I have this working fine.
某些 RTL 函数，例如 memset()，可能很有用，因此我尝试添加自己的实现.它在调试版本中运行良好(即使在编译器生成对 memset() 的隐式调用的那些地方).但是在 Release 版本中，我收到一条错误消息，指出我无法定义内在函数.您会看到，在 Release 版本中，启用了内部函数，并且 memset() 是一个内部函数.
Some RTL functions, like memset(), can be useful, so I tried adding my own implementation. It works fine in Debug builds (even for those places where the compiler generates an implicit call to memset()). But in Release builds, I get an error saying that I cannot define an intrinsic function. You see, in Release builds, intrinsic functions are enabled, and memset() is an intrinsic.
我希望在我的发布版本中使用 memset() 的内在函数，因为它可能是内联的，并且比我的实现更小、更快.但我似乎是第 22 名.如果我没有定义 memset()，链接器会抱怨它是未定义的.如果我确实定义了它，编译器会抱怨我无法定义内在函数.
I would love to use the intrinsic for memset() in my release builds, since it's probably inlined and smaller and faster than my implementation. But I seem to be a in catch-22. If I don't define memset(), the linker complains that it's undefined. If I do define it, the compiler complains that I cannot define an intrinsic function.
有谁知道定义、声明、#pragma 以及编译器和链接器标志的正确组合，以在不引入 RTL 开销的情况下获得内在函数?
Does anyone know the right combination of definition, declaration, #pragma, and compiler and linker flags to get an intrinsic function without pulling in RTL overhead?
Visual Studio 2008、x86、Windows XP+.
Visual Studio 2008, x86, Windows XP+.
为了让问题更具体一点:
To make the problem a little more concrete:
extern "C" void * __cdecl memset(void *, int, size_t);
#ifdef IMPLEMENT_MEMSET
void * __cdecl memset(void *pTarget, int value, size_t cbTarget) {
char *p = reinterpret_cast<char *>(pTarget);
while (cbTarget > 0) {
*p++ = static_cast<char>(value);
--cbTarget;
}
return pTarget;
}
#endif
struct MyStruct {
int foo[10];
int bar;
};
int main() {
MyStruct blah;
memset(&blah, 0, sizeof(blah));
return blah.bar;
}
我是这样构建的:
cl /c /W4 /WX /GL /Ob2 /Oi /Oy /Gs- /GF /Gy intrinsic.cpp
link /SUBSYSTEM:CONSOLE /LTCG /DEBUG /NODEFAULTLIB /ENTRY:main intrinsic.obj
如果我使用 memset() 的实现进行编译，我会收到一个编译器错误:
If I compile with my implementation of memset(), I get a compiler error:
error C2169: 'memset' : intrinsic function, cannot be defined
如果我在没有实现 memset() 的情况下编译它，我会收到一个链接器错误:
If I compile this without my implementation of memset(), I get a linker error:
error LNK2001: unresolved external symbol _memset
推荐答案
我想我终于找到了解决方案:
I think I finally found a solution:
首先，在头文件中，使用编译指示声明 memset()，如下所示:
First, in a header file, declare memset() with a pragma, like so:
extern "C" void * __cdecl memset(void *, int, size_t);
#pragma intrinsic(memset)
这允许您的代码调用 memset().在大多数情况下，编译器会内联内部版本.
That allows your code to call memset(). In most cases, the compiler will inline the intrinsic version.
其次，在单独的实现文件中，提供一个实现.防止编译器抱怨重新定义一个内在函数的技巧是首先使用另一个 pragma.像这样:
Second, in a separate implementation file, provide an implementation. The trick to preventing the compiler from complaining about re-defining an intrinsic function is to use another pragma first. Like this:
#pragma function(memset)
void * __cdecl memset(void *pTarget, int value, size_t cbTarget) {
unsigned char *p = static_cast<unsigned char *>(pTarget);
while (cbTarget-- > 0) {
*p++ = static_cast<unsigned char>(value);
}
return pTarget;
}
这为优化器决定不使用内部版本的情况提供了一种实现.
This provides an implementation for those cases where the optimizer decides not to use the intrinsic version.
突出的缺点是您必须禁用整个程序优化(/GL 和/LTCG).我不知道为什么.如果有人找到了一种无需禁用全局优化的方法，请加入.
The outstanding drawback is that you have to disable whole-program optimization (/GL and /LTCG). I'm not sure why. If someone finds a way to do this without disabling global optimization, please chime in.
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