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找到“理论基础”了
https://gcc.gnu.org/onlinedocs/gcc/Basic-Asm.html#Basic-Asm
Using extended asm (see Extended Asm) typically produces smaller, safer, and more efficient code, and in most cases it is a better solution than basic asm. However, there are two situations where only basic asm can be used:
* Extended asm statements have to be inside a C function, so to write inline assembly language at file scope (“top-level”), outside of C functions, you must use basic asm. You can use this technique to emit assembler directives, define assembly language macros that can be invoked elsewhere in the file, or write entire functions in assembly language.
* Functions declared with the naked attribute also require basic asm (see Function Attributes).
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