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c++cpuminimuminstruction-set

In C++, is it better to cap a value using std::min or an if branch?

A very common pattern in programming is to cap a value at a maximum after some kind of update. What I'd like to know, is if there's a difference between the following two pieces of code, and if one should be preferred:

value += increment;
value = std::min(value, valueMax);

vs

value += increment;

if (value > valueMax)
    value = valueMax;

My thinking is that this comes down to whether CPUs have instructions for taking two values and producing the minumum. If so, the call to std::min should result in this instruction and avoid an unnecessary branch. If not, the second version avoids an unnecessary assignment when value <= valueMax.

I'm not very good with this kind of thing, but I'm sure there's old-school assembly hackers who would know this. To them I ask: which is better?

Solution

  • Modern compilers are smart enough to generate the same code in both cases. For example, 32-bit GCC generates:

    addl    %esi, %edi
cmpl    %edx, %edi
movl    %edi, %eax
cmovgl  %edx, %eax

    64-bit Clang:

    %1 = add nsw i32 %increment, %value
%2 = icmp sgt i32 %1, %valueMax
%value = select i1 %2, i32 %valueMax, i32 %1