What Every Computer Scientist Should Know About Floating-Point Arithmetic: http://www.esatechnology.com/library/math/floatingmath.pdf IEEE double has a 53 bit precision. Hence it can resolve 1 part in 2^53. This comes out to a resolution of 1 part in 9007199254740992. Count them. That's 16 decimal digits. All of you answers match in the first 15 digits: 2666682129.57945 You are attempting to manipulate numbers specified to 29 digits. Places where variations will creep in: - conversion from text to binary - rounding mode - possible use of x86 80-bit floating point - if product is performed at compile time does compiler produce bit-for-bit identical results - conversion from binary to text /john -----Original Message----- From: Venkatesh Nagarajan [mailto:venkat@xxxxxxxxxxxx] Sent: Thursday, December 25, 2003 6:20 PM To: gcc-help@xxxxxxxxxxx Subject: Floating point differences between solaris and linux Hi I am using Sun SPARC machine running Solaris 5.6 and an Intel Xeon machine running 7.3 linux with GCC 2.96.113 compiler. I am multiplying 2 doubles with a simple program and the answers are different. What am I missing? Is this a difference in the compiler? Architecture? Operating System? Even Linux itself gives two different results with Optimization turned on and off!!! Thanks Venkat /******************* SOLARIS ************************************************/ evaadmin03:/usr/evahome/venkat/test 49 uname -a SunOS evaadmin03 5.6 Generic_105181-35 sun4u sparc sun4u evaadmin03:/usr/evahome/venkat/test 50 cat foo.C #include <stdio.h> int main() { double a = 51859.435045995065593160688877; double b = 51421.349407573121425230056047; double c = a * b; printf("%24.24f * %24.24f = %24.24f\n", a, b, c); return 1; } evaadmin03:/usr/evahome/venkat/test 51 CC -o foo.solaris foo.C evaadmin03:/usr/evahome/venkat/test 52 ./foo.solaris 51859.435045995065593160688877 * 51421.349407573121425230056047 = 2666682129.579454898834228515625000 evaadmin03:/usr/evahome/venkat/test 53 /************************************ LINUX *****************************************/ evadevel07:/usr/evahome/venkat/test 27 uname -a Linux evadevel07 2.4.20-20.7smp #1 SMP Mon Aug 18 14:46:14 EDT 2003 i686 unknown evadevel07:/usr/evahome/venkat/test 28 g++ -o foo.linux foo.C evadevel07:/usr/evahome/venkat/test 29 ./foo. foo.linux* foo.solaris* evadevel07:/usr/evahome/venkat/test 29 ./foo.linux 51859.435045995065593160688877 * 51421.349407573121425230056047 = 2666682129.579455375671386718750000 evadevel07:/usr/evahome/venkat/test 30 /************************* LINUX differences with and without optimization flag ***********************************/ evadevel07:/usr/evahome/venkat/test 38 g++ -o foo.linux foo.C evadevel07:/usr/evahome/venkat/test 39 ./foo.linux 51859.435045995065593160688877 * 51421.349407573121425230056047 = 2666682129.579455375671386718750000 evadevel07:/usr/evahome/venkat/test 40 g++ -O -o foo.linux foo.C evadevel07:/usr/evahome/venkat/test 41 ./foo.linux 51859.435045995065593160688877 * 51421.349407573121425230056047 = 2666682129.579454898834228515625000 evadevel07:/usr/evahome/venkat/test 42
