x87浮点指令转换为C代码

Question

I have this assignment, and x, y, and z are double type variables:

fldl    x
fldl    y
fmulp   st, st(1)
fldl    z
faddp   st, st (1)
fstpl   z

Translating this into C, I got this:

z+= x*y

I am not sure about z tho? Did I translated it correctly? Thanks, any help appreciated.

Answer 1

Yes, you got the meaning correctly figured out.

For later reference, you can just stick such code into an assembly file and step through in a debugger. From the at&t syntax I assume it is intended for gas , the gnu assembler. You can test it like this:

$ cat > test.s
.att_syntax noprefix
.globl _start
_start:
fldl    x
fldl    y
fmulp   st, st(1)
fldl    z
faddp   st, st (1)
fstpl   z
.data
x: .double 2
y: .double 3
z: .double 4
$ gcc -m32 -nostdlib -g test.s
$ gdb ./a.out
(gdb) b _start
Breakpoint 1 at 0x80480b8: file test.s, line 4.
(gdb) r
Starting program: /var/tmp/./a.out

Breakpoint 1, _start () at test.s:4
4       fldl    x
(gdb) s
5       fldl    y
(gdb)
6       fmulp   st, st(1)
(gdb)
7       fldl    z
(gdb)
8       faddp   st, st(1)
(gdb)
9       fstpl   z
(gdb)
0x080480d4 in ?? ()
(gdb) x/gf &z
0x80490e4:      10

Note I have added some test input at the end, and some needed stuff at the top.

You can of course examine all registers during the execution at any point too, see gdb help for other commands.