Apologies if this is a duplicate: I searched existing reports and found tons of hits on "virtual inheritance" and "reference", but couldn't find anything like this. The following program illustrates the problem:
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< struct Lower { const int& ref; Lower(const int& ref) : ref(ref) { } }; struct Middle : public virtual Lower { Middle(const int& ref) : Lower(ref) { } }; struct Upper : public Middle { Upper(const int& ref) : Lower(ref), Middle(ref) { } int get() { return ref; } }; int main() { int i = 0; Upper upper(i); return upper.get(); } >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Compiling this using 4.2.2 or 4.2.3 with -O2 or higher causes the resulting binary to return a non-zero value; 3.4.2 produces a bug-free binary. I've seen the same effect in more complex code, where gdb gives an obviously wrong address for the "ref" member. Dropping to -O fixes the problem, as do several other changes: * Making "ref" an int (as opposed to an int&) * Making Middle inherit non-virtually from Lower * Accessing "ref" from Middle instead of Upper The constness of the reference has no effect. Issued command-line: g++ -Wall -W -Wundef -Wpointer-arith -g -O2 VirtuallyInheritedReference.cpp Full output from running with "-v --save-temps" is as follows: g++ -v -save-temps -Wall -W -Wundef -Wpointer-arith -g -O2 -c -o /dev/null VirtuallyInheritedReference.cpp <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< Using built-in specs. Target: i686-pc-linux-gnu Configured with: /mnt/taw/usr/Taw/tmp/gcc-4.2.2/configure --prefix=/usr --disable-nls --libexecdir=/usr/lib --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --enable-languages=c,c++ --disable-bootstrap Thread model: posix gcc version 4.2.2 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/cc1plus -E -quiet -v -D_GNU_SOURCE VirtuallyInheritedReference.cpp -mtune=generic -Wall -W -Wundef -Wpointer-arith -fworking-directory -O2 -fpch-preprocess -o VirtuallyInheritedReference.ii ignoring nonexistent directory "/usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../i686-pc-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2 /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2/i686-pc-linux-gnu /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/../../../../include/c++/4.2.2/backward /usr/local/include /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/include /usr/include End of search list. /usr/lib/gcc/i686-pc-linux-gnu/4.2.2/cc1plus -fpreprocessed VirtuallyInheritedReference.ii -quiet -dumpbase VirtuallyInheritedReference.cpp -mtune=generic -auxbase-strip /dev/null -g -O2 -Wall -W -Wundef -Wpointer-arith -version -o VirtuallyInheritedReference.s GNU C++ version 4.2.2 (i686-pc-linux-gnu) compiled by GNU C version 4.2.2. GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 6ba594205d388e98f3b46dee442d61ac as -V -Qy -o /dev/null VirtuallyInheritedReference.s GNU assembler version 2.18 (i686-pc-linux-gnu) using BFD version (GNU Binutils) 2.18 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Contents of VirtuallyInheritedReference.ii: <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< # 1 "VirtuallyInheritedReference.cpp" # 1 "/home/raymond/src/C++/gcc-bugs//" # 1 "<built-in>" # 1 "<command-line>" # 1 "VirtuallyInheritedReference.cpp" struct Lower { int& ref; virtual ~Lower() { } Lower(int& ref) : ref(ref) { } }; struct Middle : public virtual Lower { Middle(int& ref) : Lower(ref) { } }; struct Upper : public Middle { Upper(int& ref) : Lower(ref), Middle(ref) { } int get() { return ref; } }; int main() { int i = 0; Upper upper(i); return upper.get(); } >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> -- Summary: Reference variable in virtually inherited base corrupted under optimization Product: gcc Version: 4.2.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: raymond at corvil dot com GCC build triplet: i686-pc-linux-gnu GCC host triplet: i686-pc-linux-gnu GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36960