将标签地址获取到 ARM 上的寄存器?

问题描述

如何编写将标签引用的地址放入寄存器的指令?

How do I write an instruction which will have the address a label is referencing put into a register?

推荐答案

有四种方法，三种在 Sourceware 的 Gnu 汇编器手册.我猜标签是这样的，

There are four ways, three are documented at Sourceware's Gnu Assembler manual. I guess the label is something like,

 target:
     .long 0xfeadbeef

1. adr r0,target
2. adrl r0,target
3. ldr r0,=target
4. sub r0,pc,#(.+8-target)

前两个非常相似，生成sub r0,pc,#offset.3 将 long 放入 literal pool 并通过 ldr r0,[pc,#offset2] 加载它，或者它可以使用 mov 如果汇编器发现它可以(通常是对齐的标签，如 0x8000).最后一个版本是手动计算的.

The first two are very similar and generate sub r0,pc,#offset. The 3 puts a long in a literal pool and loads this via ldr r0,[pc,#offset2] or it may use a mov if the assembler finds it can (usually an aligned label, like at 0x8000). The last version is to manually calculated it.

adr 和 adrl 的区别在于直接操作数.它们以 2 的倍数旋转 8 位.因此，如果地址很远，您可能需要执行两条指令，这通常比 3 ldr 变体更快，后者通过 数据缓存或内存.

The difference between adr and adrl comes from immediate operands. They are 8bits rotated by a multiple of two. So if the address is far, you may need to perform two instructions, which will usually be faster than the 3 ldr variant which get a full 32-bits via the data cache or memory.

另请参阅:汇编器中的重定位

Thumb2 添加了 movw 和 movt 的组合.例如，

Thumb2 adds the combination movw and movt. For example,

label:
 ; data
...
movw    r0, :lower16:label - .
movt    r0, :upper16:label - .

这会将偏移量放在 r0 中.它对 PC 相对值没那么有用，但对绝对值或直接加载常量很有用.

This will put the offset in r0. It is not as useful for PC relative but useful for absolutes or direct loads of constants.

参见:关于常量的 ARM 博客

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