Use fabs instead of abs to find absolute value of double (or float) data types. Include the <math.h> header for fabs function.

double d1 = fabs(-3.8951);

The speed of a given solution will depend greatly on the architecture, but in C I would say

return (n > 0 ? n : -n);

and let the compiler figure out the best solution.

EDIT: @jonk points out correctly that this will fail for the most-negative possible value of n, assuming that two's-complement arithmetic is used.

Yes, my solution has a conditional branch, but yours has an arithmetic operator and two bitwise operators. Can your microcontroller shift 15 places in a single clock?

There are well-known tricks for computing the absolute value of a two's complement signed number. If the number is negative, flip all the bits and add 1, that is, xor with -1 and subtract -1. If it is positive, do nothing, that is, xor with 0 and subtract 0.

int my_abs(int x)
{
    int s = x >> 31;
    return (x ^ s) - s;
}

As an alternative, you can also use the standard C function abs() (or one of its related functions):

7.22.6.1 The abs, labs and llabs functions

Synopsis

     #include <stdlib.h>
     int abs(int j);
     long int labs(long int j);
     long long int llabs(long long int j);

Description

The abs, labs, and llabs functions compute the absolute value of an integer j. If the result cannot be represented, the behavior is undefined.

Returns

The abs, labs, and llabs, functions return the absolute value.

Footnotes

The absolute value of the most negative number cannot be represented in two's complement.

Note the footnote "The absolute value of the most negative number cannot be represented in two's complement." and "If the result cannot be represented, the behavior is undefined." Strictly speaking, you'd likely need to use long long int and llabs() to avoid undefined behavior in converting INT_MIN to a positive value, assuming a 32-bit int value, and long is often 32-bits, even on 64-bit Windows.

However, since double values are likely implemented in IEEE format with 53 bits of precision, a 32-bit int value can be converted to double with no loss of precision, so you can use the fabs() function to get the absolute value of a 32-bit int value in one call:

7.12.7.2 The fabs functions

Synopsis

    #include <math.h>
    double fabs(double x);
    float fabsf(float x);
    long double fabsl(long double x);

The fabs functions compute the absolute value of a floating-point number x.

So your code would be:

#include <stdio.h>
#include <math.h>

int main (int argc, char *argv[]) {
    int u;

    scanf("%d", &u);
    printf("absValue = %u\n", (unsigned) fabs((double) u));

    return 0;
}

Note that in (unsigned) fabs((double) u), casting u to double is not strictly necessary, as the int value will be implicitly converted to a double because of the double fabs(double) function prototype from stdlib.h. But the cast back to unsigned is exremely necessary to pass the unsigned int value you want to pass to printf().

You could also do this:

#include <stdio.h>
#include <math.h>

int main (int argc, char *argv[]) {
    int u;

    scanf("%d", &u);
    unsigned int absValue = fabs(u);
    printf("absValue = %u\n", absValue);

    return 0;
}

That works because unsigned int absValue is explicitly an unsigned int.

Also, on modern CPUs, conversion between int and double is usually done by a single relatively fast instruction.