difference between the function performance when passing parameter as compile time constant or variable

Question

In Linux kernel code there is a macro used to test bit ( Linux version 2.6.2 ):

#define test_bit(nr, addr)                      \
        (__builtin_constant_p((nr))             \
         ? constant_test_bit((nr), (addr))      \
         : variable_test_bit((nr), (addr)))

where constant_test_bit and variable_test_bit are defined as:

static inline int constant_test_bit(int nr, const volatile unsigned long *addr  )
{       
        return ((1UL << (nr & 31)) & (addr[nr >> 5])) != 0;
}


static __inline__ int variable_test_bit(int nr, const volatile unsigned long *addr)
{       
        int oldbit;

        __asm__ __volatile__(
                "btl %2,%1\n\tsbbl %0,%0"
                :"=r" (oldbit)
                :"m" (ADDR),"Ir" (nr));
        return oldbit;
}

I understand that __builtin_constant_p is used to detect whether a variable is compile time constant or unknown. My question is: Is there any performance difference between these two functions when the argument is a compile time constant or not? Why use the C version when it is and use the assembly version when it's not?

UPDATE: The following main function is used to test the performance:

constant, call constant_test_bit:

int main(void) {
        unsigned long i, j = 21;
        unsigned long cnt = 0;
        srand(111)
        //j = rand() % 31;
        for (i = 1; i < (1 << 30); i++) {
                j = (j + 1) % 28;
                if (constant_test_bit(j, &i))
                        cnt++;
        }
        if (__builtin_constant_p(j))
                printf("j is a compile time constant\n");
        return 0;
}

This correctly outputs the sentence j is a...

For the other situations just uncomment the line which assigns a "random" number to j and change the function name accordingly. When that line is uncommented the output will be empty, and this is expected.

I use gcc test.c -O1 to compile, and here is the result:

constant, constant_test_bit:

$ time ./a.out 

j is compile time constant

real    0m0.454s
user    0m0.450s
sys     0m0.000s

constant, variable_test_bit( omit time ./a.out, same for the following ):

j is compile time constant

real    0m0.885s
user    0m0.883s
sys     0m0.000s

variable, constant_test_bit:

real    0m0.485s
user    0m0.477s
sys     0m0.007s

variable, variable_test_bit:

real    0m3.471s
user    0m3.467s
sys     0m0.000s

I have each version runs several times, and the above results are the typical values of them. It seems the constant_test_bit function is always faster than the variable_test_bit function, no matter whether the parameter is a compile time constant or not... For the last two results( when j is not constant ) the variable version is even dramatically slower than the constant one. Am I missing something here?

Answer 1

Using godbolt we can do a experiment using of constant_test_bit, the following two test functions are compiled gcc with the -O3 flag:

// Non constant expression test case
int func1(unsigned long i, unsigned long j)
{
  int x = constant_test_bit(j, &i) ;
  return x ;
}

// constant expression test case
int func2(unsigned long i)
{
  int x = constant_test_bit(21, &i) ;
  return x ;
}

We see the optimizer is able to optimize the constant expression case to the following:

shrq    $21, %rax
andl    $1, %eax

while the non-constant expression case ends up as follows:

sarl    $5, %eax
andl    $31, %ecx
cltq
leaq    -8(%rsp,%rax,8), %rax
movq    (%rax), %rax
shrq    %cl, %rax
andl    $1, %eax

So the optimizer is able to produce much better code for the constant expression case and we can see that the non-constant case for constant_test_bit is pretty bad compared to the hand rolled assembly in variable_test_bit and the implementer must believe the constant expression case for constant_test_bit ends up being better than:

btl %edi,8(%rsp)
sbbl %esi,%esi 

for most cases.

As to why your test case seems to show a different conclusion is that your test case it is flawed. I have not been able to suss out all the issues. But if we look at this case using constant_test_bit with a non-constant expression we can see the optimizer is able to move all the work outside the look and reduce the work related to constant_test_bit inside the loop to:

movq    (%rax), %rdi

even with an older gcc version, but this case may not be relevant to the cases test_bit is being used in. There may be more specific cases where this kind of optimization won't be possible.