On 07/10/2011 03:10 PM, cryptocat wrote:
I'm trying to multiply in assembler by shifting and adding and I get the
weirdest output from the following code. If the last two function calls in
the int main are commented out I get a normal result but otherwise I get a
normal result for the first call and two garbage results for the second and
third?
#include<iostream>
using namespace std;
int times_ten(int multiply_by_ten)
{
int multiplied_by_ten = 0;
//this multiplies by 10
__asm__("shld %%eax,%1;"
"movl %%eax,%%ebx;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"addl %%ebx,%%eax;"
: "=r" (multiplied_by_ten)
: "a" (multiply_by_ten)
);
return multiplied_by_ten;
}
int times_hundred(int multiply_by_hundred)
{
int multiplied_by_hundred = 0;
//this multiplies by 100
__asm__("shld %%eax,%1;"
"shld %%eax,%1;"
"movl %%eax,%%ebx;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"movl %%eax,%%edx;"
"shld %%eax,%1;"
"addl %%ebx,%%eax;"
"addl %%edx,%%eax;"
: "=r" (multiplied_by_hundred)
: "a" (multiply_by_hundred)
);
return multiplied_by_hundred;
}
int main()
{
cout<<times_hundred(1)<<endl;
cout<<times_ten(1)<<endl;
cout<<times_hundred(1)<<endl;
return 0;
}
The following has the clobber list but it wont compile. the errors are below
it.
#include<iostream>
using namespace std;
int times_ten(int multiply_by_ten)
{
int multiplied_by_ten = 0;
//this multiplies by 10
__asm__("shld %%eax,%1;"
"movl %%eax,%%ebx;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"addl %%ebx,%%eax;"
: "=r" (multiplied_by_ten)
: "a" (multiply_by_ten)
: "%%eax", "%%ebx"
);
return multiplied_by_ten;
}
int times_hundred(int multiply_by_hundred)
{
int multiplied_by_hundred = 0;
//this multiplies by 100
__asm__("shld %%eax,%1;"
"shld %%eax,%1;"
"movl %%eax,%%ebx;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"shld %%eax,%1;"
"movl %%eax,%%edx;"
"shld %%eax,%1;"
"addl %%ebx,%%eax;"
"addl %%edx,%%eax;"
: "=r" (multiplied_by_hundred)
: "a" (multiply_by_hundred)
: "%%eax", "%%ebx", "%%edx"
);
return multiplied_by_hundred;
}
int main()
{
cout<<times_hundred(1)<<endl;
cout<<times_ten(1)<<endl;
cout<<times_hundred(1)<<endl;
return 0;
}
|In function 'int times_ten(int)':|
error: unknown register name '%%ebx' in 'asm'|
error: unknown register name '%%eax' in 'asm'|
In function 'int times_hundred(int)':|
error: unknown register name '%%edx' in 'asm'|
error: unknown register name '%%ebx' in 'asm'|
error: unknown register name '%%eax' in 'asm'|
In function 'int main()':|
error: unknown register name '%%edx' in 'asm'|
error: unknown register name '%%ebx' in 'asm'|
error: unknown register name '%%eax' in 'asm'|
error: unknown register name '%%ebx' in 'asm'|
error: unknown register name '%%eax' in 'asm'|
error: unknown register name '%%edx' in 'asm'|
error: unknown register name '%%ebx' in 'asm'|
error: unknown register name '%%eax' in 'asm'|
||=== Build finished: 13 errors, 0 warnings ===|
that works here for me.
Might not be correct, I don't put the condition register in
the clobber list, and maybe other problems too which I am not
aware of...
HTH.
#include <iostream>
using namespace std;
int times_ten(int multiply_by_ten)
{
int multiplied_by_ten = 0;
//this multiplies by 10
__asm__("shl $1, %%eax;"
"movl %%eax,%%ebx;"
"shl $2, %%eax;"
"addl %%ebx,%%eax;"
: "=a" (multiplied_by_ten)
: "0" (multiply_by_ten) : "%ebx"
);
return multiplied_by_ten;
}
int times_hundred(int multiply_by_hundred)
{
int multiplied_by_hundred = 0;
//this multiplies by 100
__asm__("shl $2, %%eax;"
"movl %%eax,%%ebx;"
"shl $3, %%eax;"
"movl %%eax,%%edx;"
"shl $1, %%eax;"
"addl %%ebx,%%eax;"
"addl %%edx,%%eax;"
: "=a" (multiplied_by_hundred)
: "0" (multiply_by_hundred) : "%ebx","%edx"
);
return multiplied_by_hundred;
}
int main()
{
cout<<times_hundred(1)<<endl;
cout<<times_ten(1)<<endl;
cout<<times_hundred(1)<<endl;
return 0;
}
