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< h1 id = "内联汇编" > < a class = "header" href = "#内联汇编" > 内联汇编< / a > < / h1 >
< blockquote >
< p > 本章内容对于学习 Rust 不是必须的，而且难度很高，大家简单知道有这回事就好，不必非要学会 :D< / p >
< / blockquote >
< p > Rust 提供了 < code > asm!< / code > 宏，可以让大家在 Rust 代码中嵌入汇编代码，对于一些极致高性能或者底层的场景还是非常有用的，例如操作系统内核开发。但通常来说，大家并不应该在自己的项目中使用到该项技术，它为极客而生！< / p >
< p > 本章的例子是基于 < code > x86/x86-64< / code > 汇编的，但是其它架构也是支持的，目前支持的包括：< / p >
< ul >
< li > x86 和 x86-64< / li >
< li > ARM< / li >
< li > AArch64< / li >
< li > RISC-V< / li >
< / ul >
< p > 当使用在不支持的平台上时，编译器会给出报错。< / p >
< h2 id = "基本用法" > < a class = "header" href = "#基本用法" > 基本用法< / a > < / h2 >
< p > 先从一个简单例子开始：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
unsafe {
asm!("nop");
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 注意 < code > unsafe< / code > 语句块依然是必不可少的，因为可能在里面插入危险的指令，最终破坏代码的安全性。< / p >
< p > 上面代码将插入一个 < code > NOP< / code > 指令( 空操作 ) 到编译器生成的汇编代码中，其中指令作为 < code > asm!< / code > 的第一个参数传入。< / p >
< h2 id = "输入和输出" > < a class = "header" href = "#输入和输出" > 输入和输出< / a > < / h2 >
< p > 上面的代码有够无聊的，来点实际的:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let x: u64;
unsafe {
asm!("mov {}, 5", out(reg) x);
}
assert_eq!(x, 5);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码将 < code > 5< / code > 赋给 < code > u64< / code > 类型的变量 < code > x< / code > ，值得注意的是 < code > asm!< / code > 的指令参数实际上是一个格式化字符串。至于传给格式化字符串的参数，看起来还是比较陌生的:< / p >
< ul >
< li > 首先，需要说明目标变量是作为内联汇编的输入还是输出，在本例中，是一个输出 < code > out< / code > < / li >
< li > 最后，要指定变量将要使用的寄存器，本例中使用通用寄存器 < code > reg< / code > ，编译器会自动选择合适的寄存器< / li >
< / ul >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let i: u64 = 3;
let o: u64;
unsafe {
asm!(
"mov {0}, {1}",
"add {0}, 5",
out(reg) o,
in(reg) i,
);
}
assert_eq!(o, 8);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的代码中进一步使用了输入 < code > in< / code > ，将 < code > 5< / code > 加到输入的变量 < code > i< / code > 上，然后将结果写到输出变量 < code > o< / code > 。实际的操作方式是首先将 < code > i< / code > 的值拷贝到输出，然后再加上 < code > 5< / code > 。< / p >
< p > 上例还能看出几点：< / p >
< ul >
< li > < code > asm!< / code > 允许使用多个格式化字符串，每一个作为单独一行汇编代码存在，看起来跟阅读真实的汇编代码类似< / li >
< li > 输入变量通过 < code > in< / code > 来声明< / li >
< li > 和以前见过的格式化字符串一样，可以使用多个参数，通过 {0}, {1} 来指定，这种方式特别有用，毕竟在代码中，变量是经常复用的，而这种参数的指定方式刚好可以复用< / li >
< / ul >
< p > 事实上，还可以进一步优化代码，去掉 < code > mov< / code > 指令:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let mut x: u64 = 3;
unsafe {
asm!("add {0}, 5", inout(reg) x);
}
assert_eq!(x, 8);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 又多出一个 < code > inout< / code > 关键字，但是不难猜，它说明 < code > x< / code > 即是输入又是输出。与之前的分离方式还有一点很大的区别，这种方式可以保证使用同一个寄存器来完成任务。< / p >
< p > 当然，你可以在使用 < code > inout< / code > 的情况下，指定不同的输入和输出:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let x: u64 = 3;
let y: u64;
unsafe {
asm!("add {0}, 5", inout(reg) x => y);
}
assert_eq!(y, 8);
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "延迟输出操作数" > < a class = "header" href = "#延迟输出操作数" > 延迟输出操作数< / a > < / h2 >
< p > Rust 编译器对于操作数分配是较为保守的，它会假设 < code > out< / code > 可以在任何时间被写入，因此 < code > out< / code > 不会跟其它参数共享它的位置。然而为了保证最佳性能，使用尽量少的寄存器是有必要的，这样它们不必在内联汇编的代码块内保存和重加载。< / p >
< p > 为了达成这个目标( 共享位置或者说寄存器，以实现减少寄存器使用的性能优化 )， < code > lateout< / code > 关键字，可以用于任何只在所有输入被消费后才被写入的输出，与之类似的还有一个 < code > inlateout< / code > 。< / p >
< p > 但是 < code > inlateout< / code > 在某些场景中是无法使用的，例如下面的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let mut a: u64 = 4;
let b: u64 = 4;
let c: u64 = 4;
unsafe {
asm!(
"add {0}, {1}",
"add {0}, {2}",
inout(reg) a,
in(reg) b,
in(reg) c,
);
}
assert_eq!(a, 12);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 一旦用了 < code > inlateout< / code > 后，上面的代码就只能运行在 < code > Debug< / code > 模式下，原因是 < code > Debug< / code > 并没有做任何优化，但是 < code > release< / code > 发布不同，为了性能是要做很多编译优化的。< / p >
< p > 在编译优化时，编译器可以很容易的为输入 < code > b< / code > 和 < code > c< / code > 分配同样的是寄存器，因为它知道它们有同样的值。如果这里使用 < code > inlateout< / code > ， < code > a< / code > 和 < code > c< / code > 就可以被分配到相同的寄存器，在这种情况下，第一条指令将覆盖掉 < code > c< / code > 的值，最终导致汇编代码产生错误的结果。< / p >
< p > 因此这里使用 < code > inout< / code > ，那么编译器就会为 < code > a< / code > 分配一个独立的寄存器.< / p >
< p > 但是下面的代码又不同，它是可以使用 < code > inlateout< / code > 的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let mut a: u64 = 4;
let b: u64 = 4;
unsafe {
asm!("add {0}, {1}", inlateout(reg) a, in(reg) b);
}
assert_eq!(a, 8);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 原因在于输出只有在所有寄存器都被读取后，才被修改。因此，即使 < code > a< / code > 和 < code > b< / code > 被分配了同样的寄存器，代码也会正常工作，不存在之前的覆盖问题。< / p >
< h2 id = "显式指定寄存器" > < a class = "header" href = "#显式指定寄存器" > 显式指定寄存器< / a > < / h2 >
< p > 一些指令会要求操作数只能存在特定的寄存器中，因此 Rust 的内联汇编提供了一些限制操作符。< / p >
< p > 大家应该记得之前出现过的 < code > reg< / code > 是适用于任何架构的通用寄存器，意味着编译器可以自己选择合适的寄存器，但是当你需要显式地指定寄存器时，很可能会变成平台相关的代码，适用移植性会差很多。例如 < code > x86< / code > 下的寄存器：< code > eax< / code > , < code > ebx< / code > , < code > ecx< / code > , < code > ebp< / code > , < code > esi< / code > 等等。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
let cmd = 0xd1;
unsafe {
asm!("out 0x64, eax", in("eax") cmd);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面的例子调用 < code > out< / code > 指令将 < code > cmd< / code > 变量的值输出到 < code > 0x64< / code > 内存地址中。由于 < code > out< / code > 指令只接收 < code > eax< / code > 和它的子寄存器，因此我们需要使用 < code > eax< / code > 来指定特定的寄存器。< / p >
< blockquote >
< p > 显式寄存器操作数无法用于格式化字符串中，例如我们之前使用的 {}，只能直接在字符串中使用 < code > eax< / code > 。同时，该操作数只能出现在最后，也就是在其它操作数后面出现< / p >
< / blockquote >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
fn mul(a: u64, b: u64) -> u128 {
let lo: u64;
let hi: u64;
unsafe {
asm!(
// The x86 mul instruction takes rax as an implicit input and writes
// the 128-bit result of the multiplication to rdx:rax.
"mul {}",
in(reg) a,
inlateout("rax") b => lo,
lateout("rdx") hi
);
}
((hi as u128) < < 64) + lo as u128
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这段代码使用了 < code > mul< / code > 指令，将两个 64 位的输入相乘，生成一个 128 位的结果。< / p >
< p > 首先将变量 < code > a< / code > 的值存到寄存器 < code > reg< / code > 中，其次显式使用寄存器 < code > rax< / code > ，它的值来源于变量 < code > b< / code > 。结果的低 64 位存储在 < code > rax< / code > 中，然后赋给变量 < code > lo< / code > ，而结果的高 64 位则存在 < code > rdx< / code > 中，最后赋给 < code > hi< / code > 。< / p >
< h2 id = "clobbered-寄存器" > < a class = "header" href = "#clobbered-寄存器" > Clobbered 寄存器< / a > < / h2 >
< p > 在很多情况下，无需作为输出的寄存器的状态都会被内联汇编修改，这个状态被称之为 “clobbered”。< / p >
< p > 我们需要告诉编译器相关的情况，因为编译器需要在内联汇编语句块的附近存储和恢复这种状态。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::arch::asm;
fn main() {
// three entries of four bytes each
let mut name_buf = [0_u8; 12];
// String is stored as ascii in ebx, edx, ecx in order
// Because ebx is reserved, the asm needs to preserve the value of it.
// So we push and pop it around the main asm.
// (in 64 bit mode for 64 bit processors, 32 bit processors would use ebx)
unsafe {
asm!(
"push rbx",
"cpuid",
"mov [rdi], ebx",
"mov [rdi + 4], edx",
"mov [rdi + 8], ecx",
"pop rbx",
// We use a pointer to an array for storing the values to simplify
// the Rust code at the cost of a couple more asm instructions
// This is more explicit with how the asm works however, as opposed
// to explicit register outputs such as `out("ecx") val`
// The *pointer itself* is only an input even though it's written behind
in("rdi") name_buf.as_mut_ptr(),
// select cpuid 0, also specify eax as clobbered
inout("eax") 0 => _,
// cpuid clobbers these registers too
out("ecx") _,
out("edx") _,
);
}
let name = core::str::from_utf8(& name_buf).unwrap();
println!("CPU Manufacturer ID: {}", name);
}< / code > < / pre >
< p > 例子中，我们使用了< code > cpuid< / code > 指令。这个指令会将CPU的最大cpuid参数写入< code > eax< / code > 中， < code > ebx< / code > 、< code > edx< / code > 和 < code > ecx< / code > 中。< / p >
< p > 即使 < code > eax< / code > 从没有被读取，我们依然需要告知编译器这个寄存器被修改过，这样编译器就可以在执行汇编之前存储寄存器中的值。这个需要通过将输出声明为 < code > _< / code > 而不是一个具体的变量名，代表着该输出值被丢弃。< / p >
< p > 这段代码也会绕过一个限制： < code > ebx< / code > 是一个 LLVM 保留寄存器，意味着 LLVM 会假设它自己拥有该寄存器的全部控制权，并在汇编代码块结束时将寄存器的状态恢复到最开始的状态。由于这个限制，该寄存器无法被用于输入或者输出，除非编译器在分配通用寄存器时恰好选择了它(例如 < code > in(reg)< / code > )。 但这样使用后， < code > reg< / code > 操作数就在使用保留寄存器时变得危险起来，原因是我们可能会无意识的破坏输入或者输出，毕竟它们共享同一个寄存器。< / p >
< p > 为了解决这个问题，我们使用 < code > rdi< / code > 来存储指向输出数组的指针，通过 < code > push< / code > 的方式存储 < code > ebx< / code > ：在汇编代码块的内部读取 < code > ebx< / code > 的值，然后写入到输出数组。后面再可以通过 < code > pop< / code > 的方式来回复 < code > ebx< / code > 到初始的状态。< / p >
< p > < code > push< / code > 和 < code > pop< / code > 使用完成的 64 位 < code > rbx< / code > 寄存器，来确保整个寄存器的内容都被保存。如果是在 32 位机器上，代码将使用 < code > ebx< / code > 替代。< / p >
< p > 还可以在汇编代码内部使用通用寄存器:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::arch::asm;
// Multiply x by 6 using shifts and adds
let mut x: u64 = 4;
unsafe {
asm!(
"mov {tmp}, {x}",
"shl {tmp}, 1",
"shl {x}, 2",
"add {x}, {tmp}",
x = inout(reg) x,
tmp = out(reg) _,
);
}
assert_eq!(x, 4 * 6);
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 由于这块儿内容过于专业，本书毕竟是通用的 Rust 学习书籍，因此关于内联汇编就不再赘述。事实上，如果你要真的写出可用的汇编代码，要学习的还很多…< / p >
< p > 感兴趣的同学可以看看如下英文资料: < a href = "https://doc.rust-lang.org/reference/inline-assembly.html" > Rust Reference< / a > 和 < a href = "https://doc.rust-lang.org/rust-by-example/unsafe/asm.html#clobbered-registers" > Rust By Example< / a > 。< / p >
< / main >
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