http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55393
Bug #: 55393
Summary: gcc/g++ multiplies two unsigned integers using the
IMULQ instruction
Classification: Unclassified
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: [email protected]
ReportedBy: [email protected]
g++ -Wall -Wextra -O2 -o mult mult.cpp
g++ (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2)
64bit
mult.cpp:
#include <iostream>
void
display(
unsigned long int num,
unsigned long int mult)
{
unsigned long int tmp = num * mult;
std::cout << "mult " << mult << "\n num " << num
<< "\n tmp " << tmp << std::endl;
if (tmp < num)
std::cout << "overflow" << std::endl;
}
int
main(
int /* argc */,
char ** /* argv */)
{
unsigned long int num = 999999999999999999;
unsigned long int mult = 1024;
display(num, mult);
return 0;
}
Problem:
"overflow" is not displayed as expected.
Analysis:
gcc generates an IMULQ instruction to calculate the value of tmp.
The value of num has bit 63 set. Since IMULQ sees that argument
as signed, it results in an incorrect number that happens to be
greater than num.
IMULQ will generate the wrong result when the result just fits
into 64 bits too, even though the result would have been correct
(with no overflow) had the proper instruction been used.
Fix:
Whenever the multiplication operands are both unsigned, gcc should
generate an unsigned multiply instruction (MULQ in this case), unless
it can prove that the result would fit into 63 bits.
