optimization/115: Re: horrible bug in gcc 2.95

["Martin v. Loewis" <martin at loewis dot home dot cs dot tu-berlin dot de>]

>Number:         115
>Category:       optimization
>Synopsis:       horrible bug in gcc 2.95
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          analyzed
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 15 01:36:01 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     Dan Egnor <egnor@ofb.net>
>Release:        2.95.2
>Organization:
>Environment:
>Description:
 Original-Message-Id: <199908092225.AAA14823@mail.cs.tu-berlin.de>
 Date: Mon, 09 Aug 1999 15:25:09 -0700

 The gcc 2.95 loop optimizer (?) seems to have some kind of horrible flaw.
 Reproduction case is below; note that every other compiler, and indeed gcc
 2.95 with optimization off, gives the expected result.

 ** compile command **
 <ofb> gcc -v --save-temps -O2 bug.c
 Reading specs from /usr/lib/gcc-lib/i386-pc-linux/2.95/specs
 gcc version 2.95 19990728 (release)
  /usr/lib/gcc-lib/i386-pc-linux/2.95/cpp -lang-c -v -D__GNUC__=2 -D__GNUC_MINOR__=95 -D__ELF__ -Dunix -D__i386__ -Dlinux -D__ELF__ -D__unix__ -D__i386__ -D__linux__ -D__unix -D__linux -Asystem(posix) -D__OPTIMIZE__ -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ bug.c bug.i
 GNU CPP version 2.95 19990728 (release) (i386 Linux/ELF)
 #include "..." search starts here:
 #include <...> search starts here:
  /usr/local/include
  /usr/lib/gcc-lib/i386-pc-linux/2.95/include
  /usr/include
 End of search list.
 The following default directories have been omitted from the search path:
  /usr/lib/gcc-lib/i386-pc-linux/2.95/../../../../include/g++-3
  /usr/lib/gcc-lib/i386-pc-linux/2.95/../../../../i386-pc-linux/include
 End of omitted list.
  /usr/lib/gcc-lib/i386-pc-linux/2.95/cc1 bug.i -quiet -dumpbase bug.c -O2 -version -o bug.s
 GNU C version 2.95 19990728 (release) (i386-pc-linux) compiled by GNU C version 2.95 19990728 (release).
  as -V -Qy -o bug.o bug.s
 GNU assembler version 2.9.4 (i686-pc-linux-gnu) using BFD version 2.9.4.0.6
  /usr/lib/gcc-lib/i386-pc-linux/2.95/collect2 -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o /usr/lib/gcc-lib/i386-pc-linux/2.95/crtbegin.o -L/usr/lib/gcc-lib/i386-pc-linux/2.95 bug.o -lgcc -lc -lgcc /usr/lib/gcc-lib/i386-pc-linux/2.95/crtend.o /usr/lib/crtn.o

 ** expected result of execution **
 <ofb> ./a.out
 count = 0

 ** actual result of execution **
 <ofb> ./a.out
 count = -123

 ** source file, preprocessed output, assembler output **
>How-To-Repeat:
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`
end

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