optimization/7175: Calculation yields different results for version optimized with -O2

[weber at informatik dot uni-kl dot de]

>Number:         7175
>Category:       optimization
>Synopsis:       Calculation yields different results for version optimized with -O2
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Mon Jul 01 04:56:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Gunther Weber
>Release:        gcc version 3.1
>Organization:
>Environment:
RedHat Linux 7.3 running on a 800 MHz Celeron (Coppermine) processor. Problem also occurs on an AMD Athlon(tm) XP 1700+ system.
GCC Build option:
Reading specs from /usr/global/lib/gcc-lib/i586-pc-linux-gnu/3.1/specs
Configured with: ./configure --prefix=/usr/global --enable-threads=posix
Thread model: posix
gcc version 3.1
>Description:
A program that calculates an expression yields different results when compiled with -O2 option as compared to -O1 or no optimization. The file attachment contains code with everything but the necessary code to reproduce the bug was removed. Note that trying to print variable conntents for debugging purposes gets rid of the bug. A complete log of compiling the program and its results is listed under How-To-Reperat.
>How-To-Repeat:
[weber@stella TestGCC]$ g++3.1 test.cc
[weber@stella TestGCC]$ ./a.out
Using second solution t=0
0.131997, 0.69765
[weber@stella TestGCC]$ g++3.1 -O1 test.cc
[weber@stella TestGCC]$ ./a.out
Using second solution t=0
0.131997, 0.69765
[weber@stella TestGCC]$ g++3.1 -O2 test.cc
[weber@stella TestGCC]$ ./a.out
Using second solution t=4.14165e-17
0.568462, -0.302107
[weber@stella TestGCC]$ g++3.1 -O2 -DPRINT_T test.cc
[weber@stella TestGCC]$ ./a.out
t1=2 t2=0
Using second solution t=0
0.131997, 0.69765
>Fix:
None that I know of except not to use -O2 option.
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: application/octet-stream; name="test.cc"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="test.cc"

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