Bug 164154

Summary:	gcc floating point has problems when -O according to paranoia.c
Product:	Red Hat Enterprise Linux 3	Reporter:	gpk
Component:	gcc	Assignee:	Jakub Jelinek <jakub>
Status:	CLOSED NOTABUG	QA Contact:
Severity:	medium	Docs Contact:
Priority:	medium
Version:	3.0
Target Milestone:	---
Target Release:	---
Hardware:	i686
OS:	Linux
Whiteboard:
Fixed In Version:		Doc Type:	Bug Fix
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2005-07-25 21:34:14 UTC	Type:	---
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
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Embargoed:

Description gpk 2005-07-25 13:45:42 UTC

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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.

Comment 1 Jakub Jelinek 2005-07-25 21:34:14 UTC

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.