assembly - ARM assembly 中的分支

Question

我正在使用 GNU ARM 工具链 (arm-elf-gcc)。我有一个程序集文件和一个调用程序集文件中定义的全局方法的 c 文件。我可以成功地从 c 调用单个汇编方法，但我无法弄清楚如何调用本身调用另一个汇编方法的汇编方法。我希望这两种汇编方法都可以从 c 调用并在同一个文件中。

这是两个文件。当我运行程序时，它永远不会返回，所以我知道分支有问题。

mathLib.s

@ ARM Assembler Library for absolute value

    .align 2                    @ Align to word boundary
    .arm                        @ This is ARM code
    .global asm_abs             @ This makes it a real symbol
    .global asm_two_abs         @ This makes it a real symbol

@ ABS Func. Compiler just does "rsblt   r0, r0, #0" but this is more fun.
    asm_abs:                        @ Start of function definition
        mov     r2, #1              @ Set the least bit high in r2
        mov     r2, r2, lsl#31      @ Shift that bit all the way to the sign slot
        orr     r2, r2, r0          @ XOR with input (r0) to set ensure negative
        cmp     r2, r0              @ compare a for-sure neg input with input
        bne     asm_abs_ret         @ if not equal, input was pos, so return
        sub     r0, r0, r0, lsl#1   @ else set input = input - (2*input)
    asm_abs_ret:
        mov     pc, lr              @ Set program counter to lr (was set by caller)

    asm_two_abs:                        @ Start of function definition
        bl      asm_abs                 @ Should set r0 = abs(r0)
        mov     r2, r0                  @ set r2 = r0
        mov     r0, r1                  @ set r0 = r1
        bl      asm_abs                 @ Should set r0 = abs(r0)
        add     r0, r0, r2              @ set r0 = r0 + r2  
        mov     pc, lr                  @ Set program counter to lr (was set by caller)

程序.c

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
    extern int asm_abs(int a);
    extern int asm_two_abs(int a, int b);

    int m = atoi(argv[1]);
    int n = atoi(argv[2]);
    int r = asm_two_abs(m, n); 

    printf("%d\n", r);

    return 0;
}

编辑

这是我最后想到的

@ ARM Assembler Library for absolute value

    .align 2                    @ Align to word boundary
    .arm                        @ This is ARM code
    .global asm_abs             @ This makes it a real symbol
    .global asm_mod             @ This makes it a real symbol


@ ABS Func. Compiler just does "rsblt   r0, r0, #0" but this is more fun.
@int asm_abs(int a)

asm_abs:                        @ start of function definition
    mov     r2, #1              @ set the least bit high in r2
    mov     r2, r2, lsl#31      @ shift that bit all the way to the sign slot
    orr     r2, r2, r0          @ XOR with input a (r0) to set ensure negative
    cmp     r2, r0              @ compare a for-sure neg input with input a
    bne     asm_abs_ret         @ if not equal, input was pos, so return
    sub     r0, r0, r0, lsl#1   @ else set a = a - (2*a)
asm_abs_ret:
    mov     pc, lr              @ set program counter to lr (was set by caller)

@ Take two numbers, and return abs(a) + abs(b)
@ int asm_two_abs(int a, int b)

asm_two_abs:                        @ start of function definition
    stmfd   sp!, {r4-r6}        @ push the non-scratch registers we'll use on the stack
    mov     r4, lr              @ store link register in r4
    mov     r6, r1              @ store second argument b in r6
    bl      asm_abs             @ set a (r0) = abs(a)
    mov     r5, r0              @ store abs(a) in r5
    mov     r0, r6              @ set r0 = b
    bl      asm_abs             @ should set b = abs(b)
    add     r0, r0, r5          @ set r0 = abs(b) + abs(a)  
    mov     lr, r4              @ restore link register
    ldmfd   sp!, {r4-r6}        @ restore non-scratch registers we used
    mov     pc, lr              @ set program counter to lr (was set by caller)

Answer 1

你没有保留你的链接寄存器。

bl 分支到一个地址并将链接寄存器 r14 设置为返回地址

bl hello
asdf <- lr points here


hello:
  push {lr}
  bl there
  pop {lr}
  bx lr


there:
  stuff
  bx lr

由于 hello 调用另一个函数，它需要保存链接寄存器以了解如何返回调用它的人。没有，因为它不会向其他任何人发送 bl(不修改 lr)。