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Why is the standard "abs" function faster than mine? – Debian

I wanted to try making my own absolute value function. I figured that the fastest way to calculate absolute value would be to simply mask out the sign bit (the last bit in IEEE 754). I wanted to compare it’s speed to the standard abs function. Here is my implementation:

// Union used for type punning
union float_uint_u
{
    float f_val;
    unsigned int ui_val;
};

// 'MASK' has all bits == 1 except the last one
constexpr unsigned int MASK = ~(1 << (sizeof(int) * 8 - 1));

float abs_bitwise(float value)
{
    float_uint_u ret;
    ret.f_val = value;
    ret.ui_val &= MASK;
       
    return ret.f_val;
}

For the record, I know that this sort of type punning is not standard C++. However, this is just for educational purposes, and according to the docs, this is supported in GCC.

I figured this should be the fastest way to calculate absolute value, so it should at the very least be as fast as the standard implementation. However, timing 100000000 iterations of random values, I got the following results:

Bitwise time: 5.47385 | STL time: 5.15662
Ratio: 1.06152

My abs function is about 6% slower.

Assembly output

I compiled with -O2 optimization and the -S option (assembly output) to help determine what was going on. I have extracted the relevant portions:

; 16(%rsp) is a value obtained from standard input
movss   16(%rsp), %xmm0
andps   .LC5(%rip), %xmm0 ; .LC5 == 2147483647
movq    %rbp, %rdi
cvtss2sd    %xmm0, %xmm0

movl    16(%rsp), %eax
movq    %rbp, %rdi
andl    $2147483647, %eax
movd    %eax, %xmm0
cvtss2sd    %xmm0, %xmm0

Observations

I’m not great at assembly, but the main thing I noticed is that the standard function operates directly on the xmm0 register. But with mine, it first moves the value to eax (for some reason), performs the and, and then moves it into xmm0. I’m assuming the extra mov is where the slow down happens. I also noticed that, for the standard, it stores the bit mask elsewhere in the program vs an immediate. I’m guessing that’s not significant, however. The two versions also use different instructions (e.g. movl vs movss).

System info

This was compiled with g++ on Debian Linux (unstable branch). g++ --version output:

g++ (Debian 10.2.1-6) 10.2.1 20210110

If these two versions of the code both calculate absolute value the same way (via an and), why doesn’t the optimizer generate the same code? Specifically, why does it feel the need to include an extra mov when it optimizes my implementation?

Tags:

2

Answers


  1. 0

    I got a bit different assembly. According to the x86_64 Linux ABI, a float argument is passed via xmm0. With standard fabs, the bitwise AND operation is performed directly on this register (Intel syntax):

    andps xmm0, XMMWORD PTR .LC0[rip] # .LC0 contains 0x7FFFFFFF
ret

    However, in your case, the bitwise AND is performed on objects of type unsigned int. Therefore, GCC does the same which requires to move xmm0 to eax first:

    movd eax, xmm0
and  eax, 2147483647
movd xmm0, eax
ret

    Live demo: https://godbolt.org/z/xj8MMo

    I haven’t found any way how to force the GCC optimizer to perform AND directly on xmm0 with only pure C/C++ source code. It seems that efficient implementations need to be built upon assembler code or Intel intrinsic.

    Relevant question: How to perform a bitwise operation on floating point numbers. All the proposed solutions basically result in the same outcome.

    I also tried to use the copysign function, but the result was even worse. The generated machine code then conatiend x87 instructions.

    Anyway, it is quite interesting that the Clang optimizer was clever enough to make the assembly in all 3 cases equivalent: https://godbolt.org/z/b6Khv5.

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  2. 0

    Why is the standard “abs” function faster than mine?

    Because with most optimizing compilers (in particular GCC or Clang), it would use a specialized machine instruction known by the compiler

    The GCC compiler has even a builtin for abs

    Be sure to compile with gcc -O3 and perhaps -ffast-math.

    You could study the assembler code: compile your example.c as gcc -Wall -O3 -ffast-math -fverbose-asm example.c and look inside the emitted example.s assembler file.

    On Linux systems (e.g. Debian), you could study the source code of GNU libc and look inside the math.h standard header (and use g++ -O3 -C -E to get the preprocessed form)

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