Date: Mon, 07 Nov 1994 11:42:24 +0100
From: Peter Csizmadia <cspt AT ludens DOT elte DOT hu>
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Subject: Re: 2.4/3-2.4/3!=0

	I wrote a second message about the second bug, (in reply to Stephen 
Turnbull's, who totally misunderstood the problem), but it seems it wasn't 
arrived, so I write again.
	I just received Bill Metzenthen's and Eli Zaretskii's messages.
Their explanation for the first bug is acceptable. Bill partly explained the 
anomalous behaviour of the second program, but the bug is still unexplained.
I simpified the program:

#include <stdlib.h>
main(int argc, char* argv[]) {
   double a = atof(argv[1]);
   double x = a/sqrt(1+a*a);
   printf("1-x=%le\n", 1.0-x);
   printf("x%s1\n", (x==1.0)? "==":"!="); !!! simple comparision !!!
   printf("1-x=%le\n", 1.0-x);
}

gcc g.c -o g -O1
go32 g 10
1-x=4.96280979001086e-03
x!=1
1-x=4.96280979001096e-03  !!! IT CHANGED !!!

go32 g 100
1-x=4.99962503124712e-05
x!=1
1-x=4.99962503125451e-05  !!! IT CHANGED !!!

go32 g 1e17
1-x=-3.25260651745651e-19
x==1                              ???
1-x=0.00000000000000e+00  !!! IT CHANGED !!!

	A simple comparison changed the value of a variable (x) when I 
compiled the program with -O1. When I compiled a program with these operations 
without optimizations (or with optimizations, but declaring all double 
variables volatile), 1-x was always >=0. When I optimized it, there was this 
bug, which caused the program to terminate with a floating point exception 
when it tried to calculate sqrt(1-x). I hardly found this bug, because when I 
compared 1-x to 0, (if(1-x<0) printf("akurvaeletbe\n"); or something like that 
before sqrt), the program did not hang (the comparison corrected the value 
of x, so it became 0).

	So my question is:
Is it possible to compile this program with -O1 (and -ffloat-store), but 
without this bug? 
If it is, which optimization flag must I switch off after -O1?