X86 程序集 - 处理 IDIV 指令 [英] X86 assembly - Handling the IDIV instruction
问题描述
我目前正在编写一个简单的 C 编译器,它将 .c 文件作为输入并生成汇编代码(X86、AT&T 语法).一切都很好,但是当我尝试执行 IDIVQ 指令时,我得到了一个浮点异常.这是我的输入:
I am currently writing a simple C compiler, that takes a .c file as input and generates assembly code (X86, AT&T syntax). Everyting is good, but when I try to execute a IDIVQ instruction, I get a floating-point exception. Here's my input:
int mymain(int x){
int d;
int e;
d = 3;
e = 6 / d;
return e;
}
这是我生成的代码:
mymain:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
movq %rsp, %rbp
.cfi_offset 6, -16
.cfi_def_cfa_register 6
movq %rdi, -40(%rbp)
movq $3, -8(%rbp)
movq $6, %rax
movq -8(%rbp), %rdx
movq %rdx, %rbx
idivq %rbx
movq %rax, -16(%rbp)
movq -16(%rbp), %rax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size mymain, .-mymain
根据http://www.cs.virginia.edu/~evans/cs216/guides/x86.html,idivq %rbx 应该在 %rax 中产生 6/d(商).但是我遇到了一个浮点异常,而且我似乎找不到问题所在.
According to http://www.cs.virginia.edu/~evans/cs216/guides/x86.html, idivq %rbx should produce 6/d (the quotient) in %rax. But I'm getting a floating-point exception, and I can't seem to find the problem.
任何帮助将不胜感激!
推荐答案
Mysticials 第一部分回答正确,idiv
做了128/64位除法,所以rdx的值
,它保存被除数的高 64 位,不能包含随机值.但是零扩展是错误的方法.
The first part of Mysticials answer is correct, idiv
does a 128/64 bit division, so the value of rdx
, which holds the upper 64 bit from the dividend must not contain a random value. But a zero extension is the wrong way to go.
由于您有 signed 变量,您需要 sign 将 rax
扩展为 rdx:rax
.对此有一个特定的指令,AT&T 中的 cqto
(convert quad to oct) 和 Intel 语法中的 cqo
.AFAIK 较新版本的气体接受这两个名称.
As you have signed variables, you need to sign extend rax
to rdx:rax
. There is a specific instruction for this, cqto
(convert quad to oct) in AT&T and cqo
in Intel syntax. AFAIK newer versions of gas accept both names.
movq %rdx, %rbx
cqto # sign extend rax to rdx:rax
idivq %rbx
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