The following code results in an error when optimized using -O2 or -O3 flag,
but works fine with -O1. I've also been able to "confuse" the optimizer by
adding some code just after or before the line "iX += pA2->iX;" but I'm not
sure how safe that method is.
---------------------- Full code starts here ----------------------
#include <iostream>
class A
{
private:
int32_t iX;
public:
explicit A(int x) : iX(x) {}
public:
A& operator*=(const A& a);
public:
int32_t getX() const { return iX; }
};
////////////////////////////////////////////////////////////////
A& A::operator*=(const A& B)
{
const A* pA2 = &B;
int iBothSigns = 0;
if (iX >= 0 && pA2->iX >= 0)
{
iBothSigns = 1;
}
else if (iX < 0 && pA2->iX < 0)
{
iBothSigns = -1;
}
iX += pA2->iX;
if (iX >= 0 && iBothSigns == -1) // Expected
{
std::cout << "OK" << std::endl;
return *this;
}
else if (iX < 0 && iBothSigns == 1) // Not expected
{
std::cout << "Unexpected" << std::endl;
return *this;
}
std::cout << "Error" << std::endl;
return *this;
}
////////////////////////////////////////////////////////////////
int main()
{
A n1(-2147483648);
A n2(-2147483648);
std::cout << "1: expected -2147483648, got " << n1.getX() << std::endl;
std::cout << "2: expected -2147483648, got " << n2.getX() << std::endl;
n1 *= n2;
std::cout << "3: expected previous line to be OK" << std::endl;
return 0;
}
---------------------- end of code ----------------------
---------------------- compiler output ----------------------
g++ -o bugreport.o -c -v -Wall -O3 -pedantic -ansi -save-temps bugreport.cpp
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 4.2.3 (Ubuntu 4.2.3-2ubuntu7)
/usr/lib/gcc/i486-linux-gnu/4.2.3/cc1plus -E -quiet -v -D_GNU_SOURCE
bugreport.cpp -mtune=generic -ansi -Wall -pedantic -O3 -fpch-preprocess -o
bugreport.ii
ignoring nonexistent directory "/usr/local/include/i486-linux-gnu"
ignoring nonexistent directory
"/usr/lib/gcc/i486-linux-gnu/4.2.3/../../../../i486-linux-gnu/include"
ignoring nonexistent directory "/usr/include/i486-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/4.2
/usr/include/c++/4.2/i486-linux-gnu
/usr/include/c++/4.2/backward
/usr/local/include
/usr/lib/gcc/i486-linux-gnu/4.2.3/include
/usr/include
End of search list.
/usr/lib/gcc/i486-linux-gnu/4.2.3/cc1plus -fpreprocessed bugreport.ii -quiet
-dumpbase bugreport.cpp -mtune=generic -ansi -auxbase-strip bugreport.o -O3
-Wall -pedantic -ansi -version -fstack-protector -fstack-protector -o
bugreport.s
GNU C++ version 4.2.3 (Ubuntu 4.2.3-2ubuntu7) (i486-linux-gnu)
compiled by GNU C version 4.2.3 (Ubuntu 4.2.3-2ubuntu7).
GGC heuristics: --param ggc-min-expand=99 --param ggc-min-heapsize=129349
Compiler executable checksum: 9cf91ba46d80e564052e4fbab0d6561b
bugreport.cpp:55: warning: this decimal constant is unsigned only in ISO C90
bugreport.cpp:56: warning: this decimal constant is unsigned only in ISO C90
as --traditional-format -V -Qy -o bugreport.o bugreport.s
GNU assembler version 2.18.0 (i486-linux-gnu) using BFD version (GNU Binutils
for Ubuntu) 2.18.0.20080103
g++ -o bugreport bugreport.o
--
Summary: C++ over-eager optimization when working with a pointer
Product: gcc
Version: 4.2.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jrenggli at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37685
