使用GCC的内联汇编直接C函数调用 [英] Direct C function call using GCC's inline assembly

问题描述

如果要从内联汇编程序调用C / C ++函数，可以执行以下操作：

If you want to call a C/C++ function from inline assembly, you can do something like this:

void callee() {}
void caller()
{
    asm("call *%0" : : "r"(callee));
}

GCC随后将发出如下代码：

GCC will then emit code which looks like this:

movl $callee, %eax
call *%eax

这可能是有问题的，因为间接调用将破坏较旧CPU上的管道。

This can be problematic since the indirect call will destroy the pipeline on older CPUs.

由于地址 callee 最终是一个常量，可以想象可以使用 i 约束。从GCC在线报价 docs ：

Since the address of callee is eventually a constant, one can imagine that it would be possible to use the i constraint. Quoting from the GCC online docs:

`i'

`i'

允许使用立即数整数（一个常量）。该
包含符号常量，其
的值仅在汇编
或更晚的时间才知道。

An immediate integer operand (one with constant value) is allowed. This includes symbolic constants whose values will be known only at assembly time or later.

如果我尝试这样使用它：

If I try to use it like this:

asm("call %0" : : "i"(callee));

我从汇编器中收到以下错误：

I get the following error from the assembler:

错误：后缀或操作数对'call'无效

Error: suffix or operands invalid for `call'

这是因为GCC发出了代码

This is because GCC emits the code

call $callee

而不是

call callee

所以我的问题是，是否有可能使GCC输出正确的 call 。

So my question is whether it is possible to make GCC output the correct call.

推荐答案

我得到了 answer 从GCC的邮件列表中：

I got the answer from GCC's mailing list:

asm("call %P0" : : "i"(callee));

现在我只需要找出％P0 实际上意味着因为它似乎是一个未记录的功能...

Now I just need to find out what %P0 actually means because it seems to be an undocumented feature...

编辑：在查看了GCC源代码之后，它并不完全是清除约束前面的代码 P 的含义。但是，除其他外，它阻止了GCC将 $ 放在常量值前面。在这种情况下，正是我所需要的。

Edit: After looking at the GCC source code, it's not exactly clear what the code P in front of a constraint means. But, among other things, it prevents GCC from putting a $ in front of constant values. Which is exactly what I need in this case.

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