C switch 语句的汇编 - 它是如何工作的? [英] Assembly for a C switch statement - how does it work?

问题描述

我正在阅读一本关于汇编 switch 语句的书，当输入 n 为 case: 100、102、103、104、106 时，代码具有案例/分支.它通过从 n 中减去 100 来简化跳转表，然后如果结果在6以上，则转到L2中的默认情况，否则将转到与％eax中的值匹配的相应分支.

I am reading a book about assembly switch statement, the code has cases/branches when the input n is case: 100, 102, 103, 104, 106. It simplified the jump table by subtracting 100 from n and then if the result is above 6, it goes to the default case in L2, Otherwise it will go to the corresponding branches that match the value in %eax.

我的问题是:

1. 如果是这样，如果跳转表的索引保存在%eaxjmp *.L7(,%eax)吗>?

1. if so, isn't line 7 suppose to be jmp *.L7(,%eax) if the index of jump table is held in %eax?

为什么他们在第 5 行通过 ja .L2 将数字更改为无符号数?

And why did they change the number into unsigned in line 5 by doing ja .L2?

int x 在 %ebp+8，int n 在 %ebp+12

 movl 8(%ebp), %edx 
 movl 12(%ebp), %eax 
 subl $100, %eax 
 cmpl $6, %eax 
 ja .L2 
 jmp *.L7(,%eax,4) 
.L2: 
 movl $0, %eax 
 jmp .L8 
.L5: 
 movl %edx, %eax 
jmp .L9 
 .L3: 
 leal (%edx,%edx,2), %eax 
 leal (%edx,%eax,4), %eax 
 jmp .L8 
.L4: 
 leal 10(%edx), %eax 
.L9: 
 addl $11, %eax 
 jmp .L8 
.L6: 
 movl %edx, %eax 
 imull %edx, %eax 
.L8: 

跳表:

.section .rodata
.align 4 Align       
.L7:
.long .L3         //Case 100: loc_A
.long .L2         //Case 101: loc_def
.long .L4         //Case 102: loc_B
.long .L5        //Case 103: loc_C
.long .L6        //Case 104: loc_D
.long .L2       //Case 105: loc_def
.long .L6        //Case 106: loc_D

推荐答案

如果跳转表的索引保存在%eax中，第7行不应该是jmp *.L7(,%eax)吗?

isn't line 7 suppose to be jmp *.L7(,%eax) if the index of jump table is held in %eax?

跳转表中的每一项都是一个long，即4个字节.因此 eax 被缩放 4.

Each entry in the jump table is a long, which is 4 bytes. Hence eax is scaled by 4.

为什么他们在第 5 行通过 ja .L2 将数字更改为无符号数?

And why did they changed the number into unsigned in line 5 by doing ja .L2?

关键是要排除小于 100 和大于 106 的任何数字.我认为它如何排除大于 106 的值是显而易见的.

The point is to exclude any number that's less than 100 and greater than 106. I assume it's obvious how it excludes values greater than 106.

假设 n 小于 100，例如99. 如果我们再减去 100，我们得到 -1，当被视为一个无符号的 32 位值时，它是 4294967295，这显然是高于"6，然后跳转到 .L2就像它应该的那样.

So let's say n was less than 100, e.g. 99. If we then subtract 100 from that we get -1, which when viewed as an unsigned 32-bit value is 4294967295, which is obviously "above" 6, and the jump to .L2 is taken like it should.

subl $100, %eax   ; eax = 99-100 == -1
cmpl $6, %eax     ; set flags based on -1 - 6 == -7 => ZF=0 and CF=0
ja .L2            ; jump if ZF=0 and CF=0

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