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From: "edulix at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/39143] New: Incorrect compilation involving assignment by addition/substraction Date: Mon, 09 Feb 2009 18:42:00 -0000 [thread overview] Message-ID: <bug-39143-17288@http.gcc.gnu.org/bugzilla/> (raw) A friend of mine have noticed an error in GCC when he was developing his own C compiler. The problem happens when using -O0 (no optimization) and local vars. See sample code: #include <stdio.h> int jj = 0, ii = 0; //global vars int main() { int j = 0, i = 0; // local vars i -= i += 2; // i = 0 is correct ii -= ii += 2; // ii = -2 => incorrect, should be 0 too printf("i: %d ii: %d\n", i, ii); return 0; } The output is: $ gcc main.c -o main -O0 && ./main i: 0 ii: -2 As you can see, the same code (using code "var -= var += value;") returns a different value depending if the var is local or global. Also note that, oddly enough, when we use optimizations then the compilated code turns correct in both cases: $ gcc main.c -o main -O1 && ./main i: 0 ii: 0 Same happens when using C++. This is the gcc version: $ gcc -v Using built-in specs. Target: i486-linux-gnu Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-targets=all --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu Thread model: posix gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3) -- Summary: Incorrect compilation involving assignment by addition/substraction Product: gcc Version: 4.2.4 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: edulix at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39143
next reply other threads:[~2009-02-09 18:42 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2009-02-09 18:42 edulix at gmail dot com [this message] 2009-02-09 18:45 ` [Bug c/39143] " pinskia at gcc dot gnu dot org
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