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{{Short description|A compiler's ability to execute a function at compile time rather than runtime}}
In [[computing]], '''
If the value of only some of the arguments are known, the compiler may still be able to perform some level of compile-time function execution ([[partial evaluation]]), possibly producing more optimized code than if no arguments were known.
==Examples==
===Lisp===
The [[Macro_(computer_science)#Syntactic_macros|Lisp macro system]] is an early example of the use of compile-time evaluation of user-defined functions in the same language.
===C++===
The Metacode extension to C++ (Vandevoorde 2003)<ref>{{cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2003/n1471.pdf|title=Reflective Metaprogramming in C++|author=Daveed Vandevoorde, Edison Design Group|date=April 18, 2003|accessdate=July 19, 2015}}</ref> was an early experimental system to allow compile-time function evaluation (CTFE) and code injection as an improved syntax for C++ [[template metaprogramming]].
In earlier versions of [[C++]], [[template metaprogramming]] is often used to compute values at compile time, such as:
<syntaxhighlight lang="cpp">
template <int N>
struct Factorial {
enum { value = N * Factorial<N - 1>::value };
};
template <>
struct Factorial<0> {
enum { value = 1 };
};
// Factorial<4>::value == 24
// Factorial<0>::value == 1
void
int x = Factorial<0>::value; // == 1
}
</syntaxhighlight>
<syntaxhighlight lang="cpp">
#include <cstdio>
constexpr int Factorial(int n) { return n ? (n * Factorial(n - 1)) : 1; }
constexpr int f10 = Factorial(10);
int main() {
printf("%d\n", f10);
return 0;
}
</syntaxhighlight>
In [[C++11]], this technique is known as [[C++11#constexpr – Generalized constant expressions|generalized constant expressions]] (<code>constexpr</code>).<ref>{{cite web|url=http://www.stroustrup.com/sac10-constexpr.pdf|author=Gabriel Dos Reis and Bjarne Stroustrup | title=General Constant Expressions for System Programming Languages. SAC-2010. The 25th ACM Symposium On Applied Computing. | date=March 2010}}</ref> [[C++14]] [[C++14#Relaxed constexpr restrictions|relaxes the constraints]] on constexpr – allowing local declarations and use of conditionals and loops (the general restriction that all data required for the execution be available at compile-time remains).
Here's an example of compile-time function evaluation in C++14:
<syntaxhighlight lang="cpp">
// Iterative factorial at compile time.
constexpr int Factorial(int n) {
int result = 1;
while (n > 1) {
result *= n--;
}
return result;
}
int main() {
constexpr int f4 = Factorial(4); // f4 == 24
}
</syntaxhighlight>
===Immediate functions (C++)===
In [[C++20]], immediate functions were introduced, and compile-time function execution was made more accessible and flexible with relaxed <code>constexpr</code> restrictions.
<syntaxhighlight lang="cpp">
// Iterative factorial at compile time.
consteval int Factorial(int n) {
int result = 1;
while (n > 1) {
result *= n--;
}
return result;
}
int main() {
int f4 = Factorial(4); // f4 == 24
}
</syntaxhighlight>
Since function <code>Factorial</code> is marked <code>consteval</code>, it is guaranteed to invoke at compile-time without being forced in another manifestly constant-evaluated context. Hence, the usage of immediate functions offers wide uses in metaprogramming, and compile-time checking (used in C++20 text formatting library).
Here's an example of using immediate functions in compile-time function execution:
<syntaxhighlight lang="cpp">
void you_see_this_error_because_assertion_fails() {}
consteval void cassert(bool b) {
if (!b)
you_see_this_error_because_assertion_fails();
}
consteval void test() {
int x = 10;
cassert(x == 10); // ok
x++;
cassert(x == 11); // ok
x--;
cassert(x == 12); // fails here
}
int main() { test(); }
</syntaxhighlight>
In this example, the compilation fails because the immediate function invoked function which is not usable in constant expressions. In other words, the compilation stops after failed assertion.
The typical compilation error message would display:
<syntaxhighlight lang="cpp">
In function 'int main()':
in 'constexpr' expansion of 'test()'
in 'constexpr' expansion of 'cassert(x == 12)'
error: call to non-'constexpr' function 'you_see_this_error_because_assertion_fails()'
you_see_this_error_because_assertion_fails();
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
[ ... ]
</syntaxhighlight>
Here's another example of using immediate functions as constructors which enables compile-time argument checking:
<syntaxhighlight lang="cpp">
#include <string_view>
#include <iostream>
void you_see_this_error_because_the_message_ends_with_exclamation_point() {}
struct checked_message {
std::string_view msg;
consteval checked_message(const char* arg)
: msg(arg) {
if (msg.ends_with('!'))
you_see_this_error_because_the_message_ends_with_exclamation_point();
}
};
void send_calm_message(checked_message arg) {
std::cout << arg.msg << '\n';
}
int main() {
send_calm_message("Hello, world");
send_calm_message("Hello, world!");
}
</syntaxhighlight>
The compilation fails here with the message:
<syntaxhighlight lang="cpp">
In function 'int main()':
in 'constexpr' expansion of 'checked_message(((const char*)"Hello, world!"))'
error: call to non-'constexpr' function 'void you_see_this_error_because_the_message_ends_with_exclamation_point()'
you_see_this_error_because_the_message_ends_with_exclamation_point();
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
[ ... ]
</syntaxhighlight>
===D===
Here's an example of compile-time function evaluation in the [[D programming language]]:<ref>[http://d-programming-language.org/function.html#interpretation D 2.0 language specification: Functions]</ref>
<syntaxhighlight lang="d">
int factorial(int n) {
if (n == 0)
return 1;
Line 50 ⟶ 168:
}
// computed at compile time
enum y = factorial(0); // == 1
</syntaxhighlight>
This example specifies a valid D function called "factorial" which would typically be evaluated at run time. The use of <code>enum</code> tells the compiler that the initializer for the variables must be computed at compile time. Note that the arguments to the function must be able to be resolved at compile time as well.<ref>[http://d-programming-language.org/attribute.html#const D 2.0 language specification: Attributes]</ref>
CTFE can be used to populate data structures at compile-time in a simple way (D version 2):
<syntaxhighlight lang="d">
int[] genFactorials(int n) {
auto result = new int[n];
result[0] = 1;
foreach (i; 1 .. n)
result[i] = result[i - 1] * i;
return result;
}
enum factorials = genFactorials(13);
void main() {}
// 'factorials' contains at compile-time:
// [1, 1, 2, 6, 24, 120, 720, 5_040, 40_320, 362_880, 3_628_800,
// 39_916_800, 479_001_600]
</syntaxhighlight>
CTFE can be used to generate strings which are then parsed and compiled as D code in D.
===Zig===
Here's an example of compile-time function evaluation in the [[Zig programming language]]:<ref>[https://ziglang.org/documentation/0.11.0/#Compile-Time-Expressions Zig 0.11.0 Language Reference: Compile-Time Expressions]</ref>
<syntaxhighlight lang="zig">
pub fn factorial(n: usize) usize {
var result = 1;
for (1..(n + 1)) |i| {
result *= i;
}
return result;
}
pub fn main() void {
const x = comptime factorial(0); // == 0
const y = comptime factorial(4); // == 24
}
</syntaxhighlight>
This example specifies a valid Zig function called "factorial" which would typically be evaluated at run time. The use of <code>comptime</code> tells the compiler that the initializer for the variables must be computed at compile time. Note that the arguments to the function must be able to be resolved at compile time as well.
Zig also support Compile-Time Parameters.<ref>[https://ziglang.org/documentation/0.11.0/#Compile-Time-Parameters Zig 0.11.0 Language Reference: Compile-Time Parameters]</ref>
<syntaxhighlight lang="zig">
pub fn factorial(comptime n: usize) usize {
var result: usize = 1;
for (1..(n + 1)) |i| {
result *= i;
}
return result;
}
pub fn main() void {
const x = factorial(0); // == 0
const y = factorial(4); // == 24
}
</syntaxhighlight>
CTFE can be used to create generic data structures at compile-time:
<syntaxhighlight lang="zig">
fn List(comptime T: type) type {
return struct {
items: []T,
len: usize,
};
}
// The generic List data structure can be instantiated by passing in a type:
var buffer: [10]i32 = undefined;
var list = List(i32){
.items = &buffer,
.len = 0,
};
</syntaxhighlight>
==References==
<references/>
==External links==
* [http://rosettacode.org/wiki/Compile-time_calculation Rosettacode examples of compile-time function evaluation in various languages]
{{Compiler optimizations}}
[[Category:Compiler construction]]
[[Category:Articles with example C++ code]]
[[Category:Articles with example D code]]
[[Category:Compiler optimizations]]
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