切换执行；手臂;汇编器aarch64;臂64 [英] switch implementation; arm; assembler; aarch64; arm64

问题描述

我对在aarch64汇编器上实现"switch"运算符的方式很感兴趣.在arm32平台上，我使用了类似的

I'm interesting in the ways to implement 'switch' operator on aarch64 assembler. On arm32 platforms I used something like

    ldr         pc,         [pc, ta, LSL#2]
    nop                                         // alignment
    .int        .L.case1
    .int        .L.case2
    ...
    .int        .L.caseN

但是，由于64位版本对"pc"寄存器的使用有很多限制，因此这种实现不再起作用.

But since 64bit version has a lot of restriction on 'pc' register usage, such implementation doesn't work any more.

似乎最简单的方法是使用一对比较和分支操作，例如

It seems that the easiest way is to use pair of compare and branch operations, like

cmp ta, #1
b.eq .L.case1
cmp ta, #2
b.eq .L.case2
...

但是有时最多有十几个案件，这将导致在到达最后一个案件"之前出现严重的延迟.

But sometimes there are up to dozen cases and it would lead to a significant delay before reaching the last 'case'.

请您分享一下如何在aarch64上实现快速切换的想法.

Could you share your ideas how to implement fast switch on aarch64, please.

谢谢:)

推荐答案

我没有64位ARM汇编程序可以对此进行测试，但是我相信您可以执行以下操作来实现跳转表:

I don't have an 64-bit ARM assembler to test this with but I believe you would do something like the following to implement a jump table:

    adr  x0, jmp_table
    ldr  x0, [x0, x1, LSL#3]
    br   x0

jmp_table:
    .quad .L.case1
    .quad .L.case2
    .quad .L.case3

第一条说明 ADR ，将标签的地址加载到寄存器中.最后一条指令 BR 跳转到地址存储在寄存器中.

The first instruction, ADR, loads the address of a label into a register. The last instruction, BR, jumps to the address stored in the register.

如果您要创建共享库或与位置无关的可执行文件，则可以尝试以下操作:

If you're creating a shared library or a position independent executable you can try something like following:

    adr  x0, jmp_table
    add  x0, x0, x1, LSL#2
    br   x0

jmp_table:
    b .L.case1
    b .L.case2
    b .L.case3

其他PIC示例

    adr  x0, jmp_table
    ldr  w1, [x0, x1, LSL#2]
    add  x0, x0, x1
    br   x0

jmp_table:
    .int  .L.case1 - jmp_table
    .int  .L.case2 - jmp_table
    .int  .L.case3 - jmp_table

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