Branching elimination using bitwise operators

问题

I have some critical branching code inside a loop that's run about 2^26 times. Branch prediction is not optimal because m is random. How would I remove the branching, possibly using bitwise operators?

bool m;
unsigned int a;
const unsigned int k = ...; // k >= 7
if(a == 0)
    a = (m ? (a+1) : (k));
else if(a == k)
    a = (m ?     0 : (a-1));
else
    a = (m ? (a+1) : (a-1));

And here is the relevant assembly generated by gcc -O3:

.cfi_startproc
movl    4(%esp), %edx
movb    8(%esp), %cl
movl    (%edx), %eax
testl   %eax, %eax
jne L15
cmpb    $1, %cl
sbbl    %eax, %eax
andl    $638, %eax
incl    %eax
movl    %eax, (%edx)
ret
L15:
cmpl    $639, %eax
je  L23
testb   %cl, %cl
jne L24
decl    %eax
movl    %eax, (%edx)
ret
L23:
cmpb    $1, %cl
sbbl    %eax, %eax
andl    $638, %eax
movl    %eax, (%edx)
ret
L24:
incl    %eax
movl    %eax, (%edx)
ret
.cfi_endproc

回答1:

The branch-free division-free modulo could have been useful, but testing shows that in practice, it isn't.

const unsigned int k = 639;
void f(bool m, unsigned int &a)
{
    a += m * 2 - 1;
    if (a == -1u)
        a = k;
    else if (a == k + 1)
        a = 0;
}

Testcase:

unsigned a = 0;
f(false, a);
assert(a == 639);
f(false, a);
assert(a == 638);
f(true, a);
assert(a == 639);
f(true, a);
assert(a == 0);
f(true, a);
assert(a == 1);
f(false, a);
assert(a == 0);

Actually timing this, using a test program:

int main()
{
    for (int i = 0; i != 10000; i++)
    {
        unsigned int a = k / 2;
        while (a != 0) f(rand() & 1, a);
    }
}

(Note: there's no srand, so results are deterministic.)

My original answer: 5.3s

The code in the question: 4.8s

Lookup table: 4.5s (static unsigned lookup[2][k+1];)

Lookup table: 4.3s (static unsigned lookup[k+1][2];)

Eric's answer: 4.2s

This version: 4.0s

回答2:

The fastest I've found is now the table implementation

Timings I got (UPDATED for new measurement code)

HVD's most recent: 9.2s

Table version: 7.4s (with k=693)

Table creation code:

    unsigned int table[2*k];
    table_ptr = table;
    for(int i = 0; i < k; i++){
      unsigned int a = i;
      f(0, a);
      table[i<<1] = a;

      a = i;
      f(1, a);
      table[i<<1 + 1] = a;
    }

Table runtime loop:

void f(bool m, unsigned int &a){
  a = table_ptr[a<<1 | m];
}

With HVD's measurement code, I saw the cost of the rand() dominating the runtime, so that the runtime for a branchless version was about the same range as these solutions. I changed the measurement code to this (UPDATED to keep random branch order, and pre-computing random values to prevent rand(), etc. from trashing the cache)

int main(){
  unsigned int a = k / 2;
  int m[100000];
  for(int i = 0; i < 100000; i++){
    m[i] = rand() & 1;
  }

  for (int i = 0; i != 10000; i++
  {
    for(int j = 0; j != 100000; j++){
      f(m[j], a);  
    }
  }
}

回答3:

I don't think you can remove the branches entirely, but you can reduce the number by branching on m first.

if (m){
    if (a==k) {a = 0;} else {++a;}
}
else {
    if (a==0) {a = k;} else {--a;}
}

回答4:

Adding to Antimony's rewrite:

if (a==k) {a = 0;} else {++a;}

looks like an increase with wraparound. You can write this as

a=(a+1)%k;

which, of course, only makes sense if divisions are actually faster than branches.

Not sure about the other one; too lazy to think about what the (~0)%k will be.

回答5:

This has no branches. Because K is constant, compiler might be able to optimize the modulo depending on it's value. And if K is 'small' then a full lookup table solution would probably be even faster.

bool m;
unsigned int a;
const unsigned int k = ...; // k >= 7
const int inc[2] = {1, k};

a = a + inc[m] % (k+1);

回答6:

If k isn't large enough to cause overflow, you could do something like this:

int a; // Note: not unsigned int
int plusMinus = 2 * m - 1;
a += plusMinus;
if(a == -1) 
    a = k; 
else if (a == k+1) 
    a = 0;

Still branches, but the branch prediction should be better, since the edge conditions are rarer than m-related conditions.

来源：https://stackoverflow.com/questions/12030022/branching-elimination-using-bitwise-operators

标签

c++

optimization

bit-manipulation