CODEWITHSUNDEEP
c++11g++compiler-optimization
Maksim Surov
Maksim Surov

Reputation: 613

constexpr keywork doesn't affect on code generation

I wrote a simple program:

constexpr int strlen_c(char const* s)
{
    return *s ? 1 + strlen_c(s + 1) : 0;
}

int main()
{
    return strlen_c("hello world");
}

I expected that the compiler optimizes the function and evaluates its result in compile time. But actually the generated machine code evaluates the result in a loop:

    mov     edx, offset aHelloWorld ; "hello world"
loc_408D00:
    add     edx, 1
    mov     eax, edx
    sub     eax, offset aHelloWorld ; "hello world"
    cmp     byte ptr [edx], 0
    jnz     short loc_408D00
    leave
    retn

The program is being compiled with g++ version 5.3 with flags -std=c++11 -Ofast -O2. The same result I obtain in Visual Studio 2013, and g++ 4.9.

Quaestion what is the reason the compiler couldn't optimize the given code?

Upvotes: 1

Views: 182

Answers (1)

Dinesh Khandelwal
Dinesh Khandelwal

Reputation: 327

A constexpr function is not necessarily always evaluated at compile time. However, it must be evaluated at compile time if used in a constexpr context, So, following will work regardless of the compiler optimizations:

int main()
{
   constexpr auto len = strlen_c("hello world");
   return len;
}

Following is the assembly generated for the above code:

main:
        mov     eax, 11
        ret

Demo

Upvotes: 3

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