在组装中线程本地存储 [英] thread local storage in assembly

问题描述

我想在汇编中增加TLS变量，但在汇编代码中给出了分段错误.我不想让编译器更改任何其他寄存器或内存.有没有一种方法可以不使用gcc输入和输出语法?

I want to increment a TLS variable in assembly but is gives a segmentation fault in the assembly code. I don't want to let compiler change any other register or memory. Is there a way to do this without using gcc input and output syntax?

__thread unsigned val;
int main() {
  val = 0;
  asm("incl %gs:val");
  return 0;
}

推荐答案

如果由于某些原因确实确实需要执行此操作，则应通过将其地址预加载到C中来从汇编语言访问线程局部变量，像这样:

If you really really need to be able to do this for some reason, you should access a thread-local variable from assembly language by preloading its address in C, like this:

__thread unsigned val;
void incval(void)
{
  unsigned *vp = &val;
  asm ("incl\t%0" : "+m" (*vp));
}

这是因为访问线程局部变量所需的代码序列对于GCC支持的几乎每个操作系统和CPU组合都是不同的，并且如果您要为共享库而不是可执行文件(例如，使用-fPIC).上面的构造允许编译器为您发出正确的代码序列.如果可以在没有任何额外指令的情况下访问线程局部变量，则地址生成将被折叠到汇编操作中.举例说明，这是用于x86/Linux的gcc 4.7如何以几种不同的方式进行编译(为清楚起见，我在所有情况下都去除了一堆汇编程序指令)...

This is because the code sequence required to access a thread-local variable is different for just about every OS and CPU combination supported by GCC, and also varies if you're compiling for a shared library rather than an executable (i.e. with -fPIC). The above construct allows the compiler to emit the correct code sequence for you. In cases where it is possible to access the thread-local variable without any extra instructions, the address generation will be folded into the assembly operation. By way of illustration, here is how gcc 4.7 for x86/Linux compiles the above in several different possible modes (I've stripped out a bunch of assembler directives in all cases, for clarity)...

# -S -O2 -m32 -fomit-frame-pointer
incval:
        incl    %gs:val@ntpoff
        ret

# -S -O2 -m64
incval:
        incl    %fs:val@tpoff
        ret

# -S -O2 -m32 -fomit-frame-pointer -fpic
incval:
        pushl   %ebx
        call    __x86.get_pc_thunk.bx
        addl    $_GLOBAL_OFFSET_TABLE_, %ebx
        leal    val@tlsgd(,%ebx,1), %eax
        call    ___tls_get_addr@PLT
        incl    (%eax)
        popl    %ebx
        ret

# -S -O2 -m64 -fpic
incval:
        .byte   0x66
        leaq    val@tlsgd(%rip), %rdi
        .value  0x6666
        rex64
        call    __tls_get_addr@PLT
        incl    (%rax)
        ret

要意识到，如果我针对x86/OSX进行编译，所有四个示例将有所不同，而对于x86/Windows，则再次有所不同.

Do realize that all four examples would be different if I'd compiled for x86/OSX, and different yet again for x86/Windows.

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