Error with gcc inline assembly

Question

I'm trying to learn how to write gcc inline assembly.

The following code is supposed to perform an shl instruction and return the result.

#include <stdio.h>
#include <inttypes.h>

uint64_t rotate(uint64_t x, int b)
{
    int left = x;
    __asm__ ("shl %1, %0"
             :"=r"(left)
             :"i"(b), "0"(left));

   return left;
}

int main()
{
    uint64_t a = 1000000000;
    uint64_t res = rotate(a, 10);
    printf("%llu\n", res);
    return 0;
}

Compilation fails with error: impossible constraint in asm

The problem is basically with "i"(b) . I've tried "o" , "n" , "m" among others but it still doesn't work. Either its this error or operand size mismatch .

What am I doing wrong?

Answer 1

As written, you code compiles correctly for me (I have optimization enabled). However, I believe you may find this to be a bit better:

#include <stdio.h>
#include <inttypes.h>

uint64_t rotate(uint64_t x, int b)
{
    __asm__ ("shl %b[shift], %[value]"
             : [value] "+r"(x)
             : [shift] "Jc"(b)
             : "cc");

   return x;
}

int main(int argc, char *argv[])
{
    uint64_t a = 1000000000;
    uint64_t res = rotate(a, 10);
    printf("%llu\n", res);
    return 0;
}

Note that the 'J' is for 64bit. If you are using 32bit, 'I' is the correct value.

Other things of note:

• You are truncating your rotate value from uint64_t to int? Are you compiling for 32bit code? I don't believe shl can do 64bit rotates when compiled as 32bit.
• Allowing 'c' on the input constraint means you can use variable rotate amounts (ie not hard-coded at compile time).
• Since shl modifies the flags, use "cc" to let the compiler know.
• Using the [name] form makes the asm easier to read (IMO).
• The %b is a modifier. See https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#i386Operandmodifiers

If you want to really get smart about inline asm, check out the latest gcc docs: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html