optimization/5781: "-O -funroll-loops" breaks machine code output

[jodaman at ee1 dot bradley dot edu]

>Number:         5781
>Category:       optimization
>Synopsis:       "-O -funroll-loops" breaks machine code output
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Mon Feb 25 19:16:01 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     jodaman@ee1.bradley.edu
>Release:        gcc version 3.0.4
>Organization:
>Environment:
Linux version 2.2.20 #1 Wed Feb 6 06:07:24 CST 2002
arch: i686
>Description:
Under certain conditions, the loop unrolling optimization
will yield faulty code.  The resulting code may overwrite
the bounds of an array in a loop.  Attached is a small test
case that will crash because of this problem.

Another problem test case is the LAME audio encoder
(http://lame.sourceforge.net/) with "-O3 -funroll-loops"
or "-O2 -funroll-loops" enabled.  The developers of LAME
are aware of the this gcc issue and have recently removed
"-funroll-loops" for all gcc version 3 compiles.

Disabling "-funroll-loops" until this problem is resolved
may be wise because of the nature of the problem.  In some
cases, GCC may silently overstep the bounds of an array
without any apparent immediate failure.  All code compiled
with optimization and "-funroll-loops" with GCC 3.0.3 or
3.0.4 is suspect.  (GCC 3.0.2 might not have this problem.)
>How-To-Repeat:
Test case fails (crashes):
 gcc-3.0.4 -O -funroll-loops test-O_unroll.c -o test; ./test

Test case completes:
 gcc-3.0.4 -O test-O_unroll.c -o test; ./test
>Fix:

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