c/2419: GCC 2.96 FP bit manipulation snafu

[cchen at corelation dot com]

>Number:         2419
>Category:       c
>Synopsis:       GCC 2.96 FP bit manipulation snafu
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 28 12:46:01 PST 2001
>Closed-Date:
>Last-Modified:
>Originator:     me
>Release:        gcc version 2.96 20000731 (Red Hat Linux 7.0)
>Organization:
>Environment:
Linux ronaldo 2.2.16-22 #1 Tue Aug 22 16:49:06 EDT 2000 i686 unknown
>Description:
I have enclosed a very simple program which contains two
versions (buena() and _mala()) of a procedure to convert an
array of random 64-bit patterns to valid double-precision
floating-point values in range (-1, 1).  From statistics,
we know that the resulting array should have a mean of 0
and a variance of 1/3.

Essentially, the computation involves passing bit patterns
back and forth between integer and floating-point registers
without conversion of format.  However, in optimizing mode
with GCC 2.96, the _mala() procedure produces incorrect
result.

The program produces expected results under GCC 2.95.3.
>How-To-Repeat:
To generate bad code:
gcc -Wall -O2 -fomit-frame-pointer test.c

To generate good code:
gcc -Wall -g test.c

To run the program:
./a.out [put_any_integer_seed_here]
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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