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这些是内嵌函数，可以在c6x.h中找到对该函数的声明，找不到具体定义的地方的。内联函数就是对应着汇编指令，因此就是一条机器语言，没有具体的实现函数。请参考下面C6000编译器手册Table 8-4. C6000 C/C++ Intrinsics Support by Device https://www.ti.com/lit/ug/sprui04b/sprui04b.pdf
直接调用就可以了，另外要include c6x.h头文件，这样编译器会识别的。在上面user guide第8.6.6 Using Intrinsics to Access Assembly Language Statements有说明。
The C6000 compiler recognizes a number of intrinsic operators. Intrinsics allow you to express the meaning of certain assembly statements that would otherwise be *** or inexpressible in C/C++. Intrinsics are used like functions; you can use C/C++ variables with these intrinsics, just as you would with any normal function. The intrinsics are specified with a leading underscore, and are accessed by calling them as you do a function. For example:
比如这里的_ftod， _itoll函数去哪里可以找到函数体？有没有什么通用的方法找到？
手册里只是介绍了这个函数的大概作用，但是没说具体怎么用两个float生成一个double