optimization/7717: Invalid loop unrolling

optimization/7717: Invalid loop unrolling

[schwab]

>Number:         7717
>Category:       optimization
>Synopsis:       Invalid loop unrolling
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Sun Aug 25 14:06:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     schwab@suse.de
>Release:        3.2
>Organization:
>Environment:
System: Linux sykes 2.4.18-SMP #1 SMP Thu May 16 09:03:14 UTC 2002 ia64 unknown unknown GNU/Linux
Architecture: ia64

host: ia64-suse-linux-gnu
build: ia64-suse-linux-gnu
target: ia64-suse-linux-gnu
configured with: ../configure --enable-threads=posix --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --enable-languages=c,c++,f77,objc,java,ada --enable-libgcj --with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib --with-system-zlib --enable-shared --enable-__cxa_atexit ia64-suse-linux
>Description:
The loop in the attached file is miscompiled with -O2 -funroll-loops.  The problem is that the loop count is miscomputed, so that the loop is executed ULONG_MAX times.
>How-To-Repeat:
$ gcc -O2 -funroll-loops -v takehiro.i
Reading specs from /usr/lib/gcc-lib/ia64-suse-linux/3.2/specs
Configured with: ../configure --enable-threads=posix --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --enable-languages=c,c++,f77,objc,java,ada --enable-libgcj --with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib --with-system-zlib --enable-shared --enable-__cxa_atexit ia64-suse-linux
Thread model: posix
gcc version 3.2
 /usr/lib/gcc-lib/ia64-suse-linux/3.2/cc1 -fpreprocessed takehiro.i -quiet -dumpbase takehiro.i -O2 -version -funroll-loops -o /tmp/ccjOclOf.s
GNU CPP version 3.2 (cpplib) (IA-64) Linux
GNU C version 3.2 (ia64-suse-linux)
        compiled by GNU C version 3.2.
 /usr/lib/gcc-lib/ia64-suse-linux/3.2/../../../../ia64-suse-linux/bin/as -x -o /tmp/ccWx8eKI.o /tmp/ccjOclOf.s
 /usr/lib/gcc-lib/ia64-suse-linux/3.2/collect2 -dynamic-linker /lib/ld-linux-ia64.so.2 /usr/lib/gcc-lib/ia64-suse-linux/3.2/../../../crt1.o /usr/lib/gcc-lib/ia64-suse-linux/3.2/../../../crti.o /usr/lib/gcc-lib/ia64-suse-linux/3.2/crtbegin.o -L/usr/lib/gcc-lib/ia64-suse-linux/3.2 -L/usr/lib/gcc-lib/ia64-suse-linux/3.2/../../../../ia64-suse-linux/lib -L/usr/lib/gcc-lib/ia64-suse-linux/3.2/../../.. /tmp/ccWx8eKI.o -lgcc -lgcc_eh -lc -lgcc -lgcc_eh /usr/lib/gcc-lib/ia64-suse-linux/3.2/crtend.o /usr/lib/gcc-lib/ia64-suse-linux/3.2/../../../crtn.o
$ ./a.out 
Segmentation fault
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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Re: optimization/7717: Invalid loop unrolling

[schwab]

Synopsis: Invalid loop unrolling

State-Changed-From-To: open->closed
State-Changed-By: schwab
State-Changed-When: Tue Oct 22 04:08:45 2002
State-Changed-Why:
    Fixed by this change:
    
    2002-09-27  Alan Modra  <amodra@bigpond.net.au>
    
    	* doloop.c (doloop_modify_runtime <biv skips initial incr>): Adjust
    	by absolute loop increment, not loop increment.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7717