Question

I am currently experimenting with the creation of highly-optimized, reusable functions for a library of mine. For instance, I write the function “is power of 2” the following way:

template<class IntType>  
inline bool is_power_of_two( const IntType x )
{
    return (x != 0) && ((x & (x - 1)) == 0);
}

This is a portable, low-maintenance implementation as an inline C++ template. This code is compiled by VC++ 2008 to the following code with branches:

is_power_of_two PROC
    test    rcx, rcx
    je  SHORT $LN3@is_power_o
    lea rax, QWORD PTR [rcx-1]
    test    rax, rcx
    jne SHORT $LN3@is_power_o
    mov al, 1
    ret 0
$LN3@is_power_o:
    xor al, al
    ret 0
is_power_of_two ENDP

I found also the implementation from here: “The bit twiddler”, which would be coded in assembly for x64 as follows:

is_power_of_two_fast PROC
    test rcx, rcx
    je  SHORT NotAPowerOfTwo
    lea rax, [rcx-1]
    and rax, rcx
    neg rax
    sbb rax, rax
    inc rax
    ret
NotAPowerOfTwo:
    xor rax, rax
    ret
is_power_of_two_fast ENDP

I tested both subroutines written separately from C++ in an assembly module (.asm file), and the second one works about 20% faster!

Yet the overhead of the function call is considerable: if I compare the second assembly implementation “is_power_of_two_fast” to the inline’d-version of the template function, the latter is faster despite branches!

Unfortunately, the new conventions for x64 specify that no inline assembly is allowed. One should instead use “intrinsic functions”.

Now the question: can I implement the faster version “is_power_of_two_fast” as a custom intrinsic function or something similar, so that it can be used inline? Or alternatively, is it possible to somehow force the compiler to produce the low-branch version of the function?

Answer 1

Even VC 2005 is capable of producing code with sbb instruction.

for C code

bool __declspec(noinline) IsPowOf2(unsigned int a)
{
    return (a>=1)&((a&(a-1))<1);
}

compiles to the following

00401000  lea         eax,[ecx-1] 
00401003  and         eax,ecx 
00401005  cmp         eax,1 
00401008  sbb         eax,eax 
0040100A  neg         eax  
0040100C  cmp         ecx,1 
0040100F  sbb         ecx,ecx 
00401011  add         ecx,1 
00401014  and         eax,ecx 
00401016  ret