C++ attribute: assume (since C++23)

assert (x > 0); // trigger an assertion when NDEBUG is not defined and x > 0 is false
[[assume(x > 0)]]; // provide optimization opportunities when NDEBUG is defined

#include <cmath>
 
void f(int& x, int y)
{
 void g(int);
 void h();
 
 [[assume(x > 0)]]; // Compiler may assume x is positive
 
 g(x / 2); // More efficient code possibly generated
 
 x = 3;
 int z = x;
 
 [[assume((h(), x == z))]]; // Compiler may assume x would have the same value after
 // calling h
 // The assumption does not cause a call to h
 
 h();
 g(x); // Compiler may replace this with g(3);
 
 h();
 g(x); // Compiler may NOT replace this with g(3);
 // An assumption applies only at the point where it appears
 
 z = std::abs(y);
 
 [[assume((g(z), true))]]; // Compiler may assume g(z) will return
 
 g(z); // Due to above and below assumptions, compiler may replace this with g(10);
 
 [[assume(y == -10)]]; // Undefined behavior if y != -10 at this point
 
 [[assume((x - 1) * 3 == 12)]];
 
 g(x); // Compiler may replace this with g(5);
}