跳转到ARM汇编不同的子程序？ [英] Branching to different subroutines in ARM assembly?

问题描述

说我有下面的汇编code：

Say I have the following assembly code:

subroutineA:
   ....some assembly code....

subroutineB:
   ....some assembly code....

subroutineC:
   ....some assembly code....

现在，在另一个子程序中，我想建立的东西，将随机跳转到3子程序上述之一。

Now, in another subroutine, I want to build something that will randomly branch to one of the 3 subroutines above.

是这样的：

subroutineD:
  ....some code to randomly branch to either 
       subroutineA, or subroutineB, or subroutineC ....

如何能做到这一点有效地ARM汇编？

How can do this efficiently in ARM assembler?

下面是我如何结束了解决这个问题，它可能会帮助别人的某一天（伪code以下）：

Here is how I ended up solving the issue and it might help someone else some day (pseudo code below):

我宣布大会阵列来存储所有的子程序标签（它们的内存地址）：

I declared an array in Assembly to store all the subroutine label (their memory address):

.global my_arr
my_arr:
    .long subroutineA
    .long subroutineB  
    .long subroutineC
    .......

我也有在大会（LFSR）一些随机数发生器。说我在 R0随机数（当然，让0和子程序的总数之间肯定随机数），那么我会做这样的事情：

I also have some random number generator in Assembly (LFSR). Say I have have the random number in R0 (of course, make sure random number between 0 and total number of subroutine), then I would do something like:

ldr R4, =my_arr
ldr R5, [R4, R0]

然后在 R5 ，我具有随机子程序的地址。然后，我可以简单地做：

And then in R5, I have address of random subroutine. And then I can simply do:

blr R5

去子程序。

推荐答案

您可以高效地实现这样的跳转表，

You may efficiently implement a jump table like this,

 adr lr, return
 cmp r0, #3
 ldrlo pc, [pc, r0, lsl #2] @ pc is 8 bytes ahead
 b  error                   @ also functions as padding
.long subroutineA
.long subroutineB  
.long subroutineC
return:
 @ one of three routines finished here.
...
error:
 @ random out of array range.

这是非常通用的。大卫密封给予实现交换机类型语句的好方法的 9.2.5多路分支的ARM的ARM第二版，

This is very generic. David Seal gives a nice way to implement a switch type statement in 9.2.5 Multi-Way branch of the ARM ARM 2nd edition,

 cmp   r0,#max
 addlo pc, pc, r0, LSL #routineSizeLog2
 b     outOfRange
index0:
 ...
index1:
 ...
index2:
 ...

编译器通常会做我的第一个版本（函数指针类型），但不需要保存 LR 来实现切换。通常情况陈述不是那么线性的。然而，许多人创建中断处理程序等汇编使用大卫印的方法。

Compilers usually do my first version (function pointer type), but do not need to save lr to implement a switch. Usually case statements are not so linear. However, many people create interrupt handlers, etc in assembler using David Seal's method.

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