c/10684: register variable update disappears when using -O

c/10684: register variable update disappears when using -O

[Joerg-Cyril.Hoehle]

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>Number:         10684
>Category:       c
>Synopsis:       register variable update disappears when using -O
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Thu May 08 16:06:01 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Jörg-Cyril Höhle
>Release:        3.2 (also seen with 3.0)
>Organization:
>Environment:
Linux sql-server 2.4.19-4GB #1 Fri Sep 13 13:14:56 UTC 2002 i686 unknown
Suse-Linux 8.1, using binary distribution of gcc from CD-ROM.

$ gcc -v
Reading specs from /usr/lib/gcc-lib/i486-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 i486-suse-linux
Thread model: posix
gcc version 3.2
[Bug was also seen for 3.0]
>Description:
Using -O2 or -O1, a case is exhibited where an update to a global register variable disappears from the assembly output.
>How-To-Repeat:
In the attached program the function convert_from_foreign_array_fill() is miscompiled.

$ gcc -O0 bugO.c; a.out ; echo $?
0
$ gcc -O2 bugO.c; a.out ; echo $?
1

Please look at the code generated by gcc -O2 -S bugO.c for convert_from_foreign_array_fill() and search for lines matching ebx.
You'll find three of them, but 4 in the -O0 case.

The
        addl    $4, %ebx
disappeared. That was the update part of pushSTACK().

Please add this sample program to your testsuite.

>Fix:

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