Quickest way to change endianness

Question

What is the quickest way to reverse the endianness of a 16 bit and 32 bit integer. I usually do something like (this coding was done in Visual Studio in C++):

un         


        

Answer 1

I used the following code for the 16bit version swap function:

_int16 changeEndianness16(__int16 val)
{
    return ((val & 0x00ff) << 8) | ((val & 0xff00) >> 8);
}    

With g++ (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5 the above code when compiled with g++ -O3 -S -fomit-frame-pointer test.cpp results in the following (non-inlined) assembler code:

movzwl  4(%esp), %eax
rolw    $8, %ax
ret

The next code is equivalent but g++ is not as good at optimizing it.

__int16 changeEndianness16_2(__int16 val)
{
    return ((val & 0xff) << 8) | (val >> 8);
}

Compiling it gives more asm code:

movzwl  4(%esp), %edx
movl    %edx, %eax
sarl    $8, %eax
sall    $8, %edx
orl     %edx, %eax
ret