Inline Assemble Code - A problem relevant to register usage

1. Source code as followings

#include <stdio.h>

int main()

{

int arg1, arg2, add, sub, mul, quo, rem ;

printf( "Enter two integer numbers : " );

scanf( "%d%d", &arg1, &arg2 );

__asm__ ( "movl $0, %%edx;"

"movl %2, %%eax;"

"movl %3, %%ebx;"

"idivl %%ebx;" : "=a" (quo), "=d" (rem) : "g" (arg1), "g" (arg2) );

printf( "%d / %d = %d\n", arg1, arg2, quo );

printf( "%d %% %d = %d\n", arg1, arg2, rem );

return 0 ;

}

2. compile

gcc -o tt1.x tt1.c

3. run

./tt1.x

Floating point exception (core dumped)

4. Root Cause

objdump -D tt1.x >111

vi 111, you will see below

4005c9: e8 d2 fe ff ff callq 4004a0 <__isoc99_scanf@plt>

4005ce: 8b 45 f0 mov -0x10(%rbp),%eax

4005d1: 8b 55 f4 mov -0xc(%rbp),%edx

4005d4: ba 00 00 00 00 mov $0x0,%edx

4005d9: 89 c0 mov %eax,%eax

4005db: 89 d3 mov %edx,%ebx

4005dd: f7 fb idiv %ebx

4005df: 89 45 f8 mov %eax,-0x8(%rbp)

4005e2: 89 55 fc mov %edx,-0x4(%rbp)

4005e5: 8b 55 f4 mov -0xc(%rbp),%edx

4005e8: 8b 45 f0 mov -0x10(%rbp),%eax

4005eb: 8b 4d f8 mov -0x8(%rbp),%ecx

5. Statement

The root cause is that arg2 use register edx, but edx

is cleared zero before idiv

6. improvement

Change arg2 to memory location rather than register.

#include <stdio.h>

int main()

{

int arg1, arg2, add, sub, mul, quo, rem ;

printf( "Enter two integer numbers : " );

scanf( "%d%d", &arg1, &arg2 );

__asm__ ( "movl $0, %%edx;"

"movl %2, %%eax;"

"movl %3, %%ebx;"

"idivl %%ebx;" : "=a" (quo), "=d" (rem) : "m" (arg1), "m" (arg2) );

printf( "%d / %d = %d\n", arg1, arg2, quo );

printf( "%d %% %d = %d\n", arg1, arg2, rem );

return 0 ;

}