Does static constexpr variable inside a function make sense?

Question

If I have a variable inside a function (say, a large array), does it make sense to declare it both static and constexpr? constexpr guarantees that the array is created at compile time, so would the static be useless?

void f() {
    static constexpr int x [] = {
        // a few thousand elements
    };
    // do something with the array
}

Is the static actually doing anything there in terms of generated code or semantics?

Answer 1

The short answer is that not only is static useful, it is pretty well always going to be desired.

First, note that static and constexpr are completely independent of each other. static defines the object's lifetime during execution; constexpr specifies that the object should be available during compilation. Compilation and execution are disjoint and discontiguous, both in time and space. So once the program is compiled, constexpr is no longer relevant.

Every variable declared constexpr is implicitly const but const and static are almost orthogonal (except for the interaction with static const integers.)

The C++ object model (§1.9) requires that all objects other than bit-fields occupy at least one byte of memory and have addresses; furthermore all such objects observable in a program at a given moment must have distinct addresses (paragraph 6). This does not quite require the compiler to create a new array on the stack for every invocation of a function with a local non-static const array, because the compiler could take refuge in the as-if principle provided it can prove that no other such object can be observed.

That's not going to be easy to prove, unfortunately, unless the function is trivial (for example, it does not call any other function whose body is not visible within the translation unit) because arrays, more or less by definition, are addresses. So in most cases, the non-static const(expr) array will have to be recreated on the stack at every invocation, which defeats the point of being able to compute it at compile time.

On the other hand, a local static const object is shared by all observers, and furthermore may be initialized even if the function it is defined in is never called. So none of the above applies, and a compiler is free not only to generate only a single instance of it; it is free to generate a single instance of it in read-only storage.

So you should definitely use static constexpr in your example.

However, there is one case where you wouldn't want to use static constexpr. Unless a constexpr declared object is either ODR-used or declared static, the compiler is free to not include it at all. That's pretty useful, because it allows the use of compile-time temporary constexpr arrays without polluting the compiled program with unnecessary bytes. In that case, you would clearly not want to use static, since static is likely to force the object to exist at runtime.

Answer 2

In addition to given answer, it's worth noting that compiler is not required to initialize constexpr variable at compile time, knowing that the difference between constexpr and static constexpr is that to use static constexpr you ensure the variable is initialized only once.

Following code demonstrates how constexpr variable is initialized multiple times (with same value though), while static constexpr is surely initialized only once.

In addition the code compares the advantage of constexpr against const in combination with static.

#include <iostream>
#include <string>
#include <cassert>
#include <sstream>

const short const_short = 0;
constexpr short constexpr_short = 0;

// print only last 3 address value numbers
const short addr_offset = 3;

// This function will print name, value and address for given parameter
void print_properties(std::string ref_name, const short* param, short offset)
{
    // determine initial size of strings
    std::string title = "value \\ address of ";
    const size_t ref_size = ref_name.size();
    const size_t title_size = title.size();
    assert(title_size > ref_size);

    // create title (resize)
    title.append(ref_name);
    title.append(" is ");
    title.append(title_size - ref_size, ' ');

    // extract last 'offset' values from address
    std::stringstream addr;
    addr << param;
    const std::string addr_str = addr.str();
    const size_t addr_size = addr_str.size();
    assert(addr_size - offset > 0);

    // print title / ref value / address at offset
    std::cout << title << *param << " " << addr_str.substr(addr_size - offset) << std::endl;
}

// here we test initialization of const variable (runtime)
void const_value(const short counter)
{
    static short temp = const_short;
    const short const_var = ++temp;
    print_properties("const", &const_var, addr_offset);

    if (counter)
        const_value(counter - 1);
}

// here we test initialization of static variable (runtime)
void static_value(const short counter)
{
    static short temp = const_short;
    static short static_var = ++temp;
    print_properties("static", &static_var, addr_offset);

    if (counter)
        static_value(counter - 1);
}

// here we test initialization of static const variable (runtime)
void static_const_value(const short counter)
{
    static short temp = const_short;
    static const short static_var = ++temp;
    print_properties("static const", &static_var, addr_offset);

    if (counter)
        static_const_value(counter - 1);
}

// here we test initialization of constexpr variable (compile time)
void constexpr_value(const short counter)
{
    constexpr short constexpr_var = constexpr_short;
    print_properties("constexpr", &constexpr_var, addr_offset);

    if (counter)
        constexpr_value(counter - 1);
}

// here we test initialization of static constexpr variable (compile time)
void static_constexpr_value(const short counter)
{
    static constexpr short static_constexpr_var = constexpr_short;
    print_properties("static constexpr", &static_constexpr_var, addr_offset);

    if (counter)
        static_constexpr_value(counter - 1);
}

// final test call this method from main()
void test_static_const()
{
    constexpr short counter = 2;

    const_value(counter);
    std::cout << std::endl;

    static_value(counter);
    std::cout << std::endl;

    static_const_value(counter);
    std::cout << std::endl;

    constexpr_value(counter);
    std::cout << std::endl;

    static_constexpr_value(counter);
    std::cout << std::endl;
}

Possible program output:

value \ address of const is               1 564
value \ address of const is               2 3D4
value \ address of const is               3 244

value \ address of static is              1 C58
value \ address of static is              1 C58
value \ address of static is              1 C58

value \ address of static const is        1 C64
value \ address of static const is        1 C64
value \ address of static const is        1 C64

value \ address of constexpr is           0 564
value \ address of constexpr is           0 3D4
value \ address of constexpr is           0 244

value \ address of static constexpr is    0 EA0
value \ address of static constexpr is    0 EA0
value \ address of static constexpr is    0 EA0

As you can see yourself constexpr is initilized multiple times (address is not the same) while static keyword ensures that initialization is performed only once.

Answer 3

Not making large arrays static, even when they're constexpr can have dramatic performance impact and can lead to many missed optimizations. It may slow down your code by orders of magnitude. Your variables are still local and the compiler may decide to initialize them at runtime instead of storing them as data in the executable.

Consider the following example:

template <int N>
void foo();

void bar(int n)
{
    // array of four function pointers to void(void)
    constexpr void(*table[])(void) {
        &foo<0>,
        &foo<1>,
        &foo<2>,
        &foo<3>
    };
    // look up function pointer and call it
    table[n]();
}

You probably expect gcc-10 -O3 to compile bar() to a jmp to an address which it fetches from a table, but that is not what happens:

bar(int):
        mov     eax, OFFSET FLAT:_Z3fooILi0EEvv
        movsx   rdi, edi
        movq    xmm0, rax
        mov     eax, OFFSET FLAT:_Z3fooILi2EEvv
        movhps  xmm0, QWORD PTR .LC0[rip]
        movaps  XMMWORD PTR [rsp-40], xmm0
        movq    xmm0, rax
        movhps  xmm0, QWORD PTR .LC1[rip]
        movaps  XMMWORD PTR [rsp-24], xmm0
        jmp     [QWORD PTR [rsp-40+rdi*8]]
.LC0:
        .quad   void foo<1>()
.LC1:
        .quad   void foo<3>()

This is because GCC decides not to store table in the executable's data section, but instead initializes a local variable with its contents every time the function runs. In fact, if we remove constexpr here, the compiled binary is 100% identical.

This can easily be 10x slower than the following code:

template <int N>
void foo();

void bar(int n)
{
    static constexpr void(*table[])(void) {
        &foo<0>,
        &foo<1>,
        &foo<2>,
        &foo<3>
    };
    table[n]();
}

Our only change is that we have made table static, but the impact is enormous:

bar(int):
        movsx   rdi, edi
        jmp     [QWORD PTR bar(int)::table[0+rdi*8]]
bar(int)::table:
        .quad   void foo<0>()
        .quad   void foo<1>()
        .quad   void foo<2>()
        .quad   void foo<3>()

In conclusion, never make your lookup tables local variables, even if they're constexpr. Clang actually optimizes such lookup tables well, but other compilers don't. See Compiler Explorer for a live example.