Why use abs() or fabs() instead of conditional negation?

Question

In C/C++, why should one use abs() or fabs() to find the absolute value of a variable without using the following code?

int absoluteVal         


        

Answer 1

The intent behind abs() is "(unconditionally) set the sign of this number to positive". Even if that had to be implemented as a conditional based on the current state of the number, it's probably more useful to be able to think of it as a simple "do this", rather than a more complex "if… this… that".

Answer 2

The compiler will most likely do the same thing for both at the bottom layer - at least a modern competent compiler.

However, at least for floating point, you'll end up writing a few dozen lines if you want to handle all the special cases of infinity, not-a-number (NaN), negative zero and so on.

As well as it's easier to read that abs is taking the absolute value than reading that if it's less than zero, negate it.

If the compiler is "stupid", it may well end up doing worse code for a = (a < 0)?-a:a, because it forces an if (even if it's hidden), and that could well be worse than the built-in floating point abs instruction on that processor (aside from complexity of special values)

Both Clang (6.0-pre-release) and gcc (4.9.2) generates WORSE code for the second case.

I wrote this little sample:

#include <cmath>
#include <cstdlib>

extern int intval;
extern float floatval;

void func1()
{
    int a = std::abs(intval);
    float f = std::abs(floatval);
    intval = a;
    floatval = f;
}


void func2()
{
    int a = intval < 0?-intval:intval;
    float f = floatval < 0?-floatval:floatval;
    intval = a;
    floatval = f;
}

clang makes this code for func1:

_Z5func1v:                              # @_Z5func1v
    movl    intval(%rip), %eax
    movl    %eax, %ecx
    negl    %ecx
    cmovll  %eax, %ecx
    movss   floatval(%rip), %xmm0   # xmm0 = mem[0],zero,zero,zero
    andps   .LCPI0_0(%rip), %xmm0
    movl    %ecx, intval(%rip)
    movss   %xmm0, floatval(%rip)
    retq

_Z5func2v:                              # @_Z5func2v
    movl    intval(%rip), %eax
    movl    %eax, %ecx
    negl    %ecx
    cmovll  %eax, %ecx
    movss   floatval(%rip), %xmm0   
    movaps  .LCPI1_0(%rip), %xmm1 
    xorps   %xmm0, %xmm1
    xorps   %xmm2, %xmm2
    movaps  %xmm0, %xmm3
    cmpltss %xmm2, %xmm3
    movaps  %xmm3, %xmm2
    andnps  %xmm0, %xmm2
    andps   %xmm1, %xmm3
    orps    %xmm2, %xmm3
    movl    %ecx, intval(%rip)
    movss   %xmm3, floatval(%rip)
    retq

g++ func1:

_Z5func1v:
    movss   .LC0(%rip), %xmm1
    movl    intval(%rip), %eax
    movss   floatval(%rip), %xmm0
    andps   %xmm1, %xmm0
    sarl    $31, %eax
    xorl    %eax, intval(%rip)
    subl    %eax, intval(%rip)
    movss   %xmm0, floatval(%rip)
    ret

g++ func2:

_Z5func2v:
    movl    intval(%rip), %eax
    movl    intval(%rip), %edx
    pxor    %xmm1, %xmm1
    movss   floatval(%rip), %xmm0
    sarl    $31, %eax
    xorl    %eax, %edx
    subl    %eax, %edx
    ucomiss %xmm0, %xmm1
    jbe .L3
    movss   .LC3(%rip), %xmm1
    xorps   %xmm1, %xmm0
.L3:
    movl    %edx, intval(%rip)
    movss   %xmm0, floatval(%rip)
    ret

Note that both cases are notably more complex in the second form, and in the gcc case, it uses a branch. Clang uses more instructions, but no branch. I'm not sure which is faster on which processor models, but quite clearly more instructions is rarely better.

Answer 3

...and would you make it into a macro, you can have multiple evaluations that you may not want (side efffects). Consider:

#define ABS(a) ((a)<0?-(a):(a))

and use:

f= 5.0;
f=ABS(f=fmul(f,b));

which would expand to

f=((f=fmul(f,b)<0?-(f=fmul(f,b)):(f=fmul(f,b)));

Function calls won't have this unintended side-effects.