Red Hat Bugzilla – Bug 164154
gcc floating point has problems when -O according to paranoia.c
Last modified: 2007-11-30 17:07:07 EST
From Bugzilla Helper: User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7) Gecko/20040616 Description of problem: Running paranoia.c (a standard floating point test program), compiled with optimization, reports many flaws and defects. Version-Release number of selected component (if applicable): gcc-3.2.3-52 How reproducible: Always Steps to Reproduce: 1. Download from http://www.netlib.org/paranoia/paranoia.c 2. gcc -O -o paranoia paranoia.c -lm 3. ./paranoia Actual Results: Much explanatory text, then: Searching for Radix and Precision. Radix = 2.000000 . Closest relative separation found is U1 = 5.4210109e-20 . Recalculating radix and precision gets better closest relative separation U1 = 9.2157185e-18 . MYSTERY: recalculated Radix = 0.0000000e+00 . FLAW: Radix is not as good as 2 or 10. FAILURE: (1-U1)-1/2 < 1/2 is FALSE, prog. fails?. The number of significant digits of the Radix is 0.000000 . FAILURE: Precision test appears to be inconsistent... PLEASE NOTIFY KARPINKSI! U1 = 9.2157185e-18, Z1 - U1 = -9.1615084e-18 U2 = 1.0842022e-19, Z2 - U2 = 0.0000000e+00 Checking for guard digit in *, /, and -. SERIOUS DEFECT: Division lacks a Guard Digit, so X/1 != X. FAILURE: * and/or / gets too many last digits wrong. SERIOUS DEFECT: - lacks Guard Digit, so cancellation is obscured. Expected Results: Much explanatory text without the DEFECT warnings, or the appeak to KARPINSKI. Additional info: Similar problems were seen on a Debian system, using gcc3.3, so I'm pretty sure it's not a hardware problem.
paranoia.c relies on no excess precision computation. So, on IA-32 because of its misdesigned FPU, you either need to use -ffloat-store (which is slow, but more standard conforming (though still not fully, see GCC PR323), or, if you have SSE2 support in hardware, -msse2 -mfpmath=sse.