Michael,
Thanks for the very thorough analysis.
Yes, sorry, I forgot to state I was running on Windows. The results I got
came from running the command window (the precompiled Windows binary
sqlite-3_7_3.zip). I also got the same results from compiling the
amalgamation (sqlite-amalgamation-3_7_3.zip) using Visual Studio Express
2010.
I took that piece of code you highlighted and compiled and ran it standalone
under both Windows and Linux (gcc 4.4.3). I got the SAME results for each:
realvalue = 8.9484710000000049 and 8.9484710000000103 at the two points you
mentioned. I didn't compile the amalgamation on Linux, nor did I use
optimizations -- maybe the difference lies in there?
BTW, when I traced the amalgamation on Windows for my example, this line of
code
for(idx=precision, rounder=0.5; idx>0; idx--, rounder*=0.1){}
has precision = 14, which suggests that the code is taking into account only
the part after the decimal point.
Here's the standalone code I tested (I hardcoded the values for my example,
plus added some printfs; one thing I wanted to check was that Windows and
gcc were converting the decimal literals to the same values):
#include <stdio.h>
int main(void)
{
double realvalue = 8.948471e15;
int exp = 0, idx;
double rounder;
printf ("1e32 = %.13a\n",1e32);
printf ("1e-32 = %.13a\n",1e-32);
printf ("1e8 = %.13a\n",1e8);
printf ("1e-8 = %.13a\n",1e-8);
while( realvalue>=1e32 && exp<=350 ){ realvalue *= 1e-32; exp+=32; }
while( realvalue>=1e8 && exp<=350 ){ realvalue *= 1e-8; exp+=8; }
while( realvalue>=10.0 && exp<=350 ){ realvalue *= 0.1; exp++; }
while( realvalue<1e-8 ){ realvalue *= 1e8; exp-=8; }
while( realvalue<1.0 ){ realvalue *= 10.0; exp--; }
printf ("Realvalue = %.17g = %.13a\n",realvalue,realvalue);
for(idx=14, rounder=0.5; idx>0; idx--, rounder*=0.1){}
printf ("Rounder = %.17g = %.13a\n",rounder,rounder);
realvalue += rounder;
if( realvalue>=10.0 ){ realvalue *= 0.1; exp++; }
printf ("Realvalue = %.17g = %.13a\n",realvalue,realvalue);
}
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