__asm__(...);必须作为函数体吗？

nswcfd

不是吧？

1. echo 'asm("nop");' | gcc -c -x c -
   

复制代码

可以正常编译的？

nswcfd

好像是的，换成asm( "nop" : )就会报错。（使用扩展汇编语法）

nswcfd

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).

nswcfd

注，naked属性，好像x86没有用

Use this attribute on the ARM, AVR, IP2K, RX and SPU ports to indicate that the specified function does not need prologue/epilogue sequences generated by the compiler

http://stackoverflow.com/questio ... or-functions-in-gcc

nswcfd

回复 8# captivated

y = x + 3；

嗯，从这个角度考虑就更容易理解了。

PS，extend asm可以跟c的操作数相关联，涉及到input/output寄存器的分配和选择，这在file scope里是没有意义的。