Re: cygwin-b20 gcc: "A - floor(A)" returns a negative value

[Earnie Boyd <earnie_boyd at yahoo dot com>]

Hi Peter,

Although your example indeed shows a potential bug in the optimizer
IMHO your example is a little buggy itself.  I believe your intent
with the printf is to show the values that gave you the result and to
do this you need two variables.  When I modified your example to use
two variables, AV1 and AV2, the result was the same for all values of
optimization.

If this was not your intent then please forgive my claims. 

---Peter Kabal <Kabal@ece.mcgill.ca> wrote:
>
> Switching from cygwin-b19 to cygwin-b20 caused a C-routine to give
> incorrect results.  This was traced to a calculation (in double) of
> "A - float(A)" which returns a negative value.  By definition it
should
> always be positive.  A short test program which demonstrates this
> problem is included.  If the program is further simplified, the
problem
> goes away.  To exercise the bug, the program must be compiled with
> optimization level -O3.
> 
> % gcc -O3 tfloor.c -o tfloor
> % ./tfloor
> AV = 20, AV - floor(AV) = -1.11022e-15
> 
> ----------------
> #include <math.h>
> #include <stdio.h>
> 
> int
> main (argc, argv)
>      int argc;
>      char *argv[];
> {
>   double AV;
> 
>   AV = 0.05;
>   AV = 1.0 / (AV - floor (AV));
>   printf ("AV = %g, AV - floor(AV) = %g\n", AV, AV - floor(AV));
> 
>   return 0;
> };
> ----------------
> Peter Kabal    kabal@ECE.McGill.CA
> Dept. Electrical & Computer Engineering
> McGill University
> -
> For help on using this list (especially unsubscribing), send a
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> 

==
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