When casting a double to a long long causes GCC to overflow the output value.
Imagine;
#include <iostream>
double foo() { return 9223372036854775807.0; }
int main() {
std::wcout << (long long)foo(); // Prints -9223372036854775808
}
Additionally, std::pow(double, int) and std::pow(double, double) seem to behave
differently, presumably for the same reason;
#include <iostream>
#include <cmath>
int main() {
std::wcout << (long long)std::pow(2.0, 64); // -9223372036854775808
std::wcout << (long long)std::pow(2.0, 64.0); // 9223372036854775807
}
Note that this behaviour is not the same if the return value of foo(),
std::pow() (in both cases) are stored in variables first.
Compiling with "g++ -Wall -ansi -std=gnu++0x -pedantic test.cpp", with GCC
4.4.1. Note that compiling with "g++ -Wall -ansi -std=gnu++0x -pedantic -O3
test.cpp" does NOT exhibit this behaviour; foo() and both std::pow() results in
9223372036854775807.
Note that (unsigned long long)std::pow(2.0, 64) == 0, and (unsigned long
long)std::pow(2.0, 64.0) == 18446744073709551615.
--
Summary: Explicit casting of double to long long causes value to
overflow
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: ullner at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43284
