[Bug c++/40186] New: floating point comparision works wrong ( !(a < b) && (b > a) is true )

[Bug c++/40186] New: floating point comparision works wrong ( !(a < b) && (b > a) is true )

[ich at az2000 dot de]

When comparing the return value of two functions (which both return double and
do always return the same value), I have the case that (a < b) && (b < a)
(whereby they should be equal) in a real world application.

I tried to reproduce it, though in the test case, I have a slightly different
case, which should be also wrong: !(a < b) && (b > a) is true (whereby it
should be a == b).

Probably both problems are related.

The behaviour pretty much depend on the optimisation. With -O0 and -Os, I am
hitting the problem, but not so with -O1, -O2 and -O3.


-- 
           Summary: floating point comparision works wrong ( !(a < b) && (b
                    > a) is true )
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ich at az2000 dot de
 GCC build triplet: tried with GCC 4.1.2, 4.3.2 and 4.4.0
  GCC host triplet: Gentoo Linux i386
GCC target triplet: Linux i386


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparision works wrong ( !(a < b) && (b > a) is true )

[ich at az2000 dot de]

------- Comment #1 from ich at az2000 dot de  2009-05-18 13:11 -------
Created an attachment (id=17887)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17887&action=view)
testcase (c++)

This is the test code. I am hitting the "wrong ordering 2" code.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparision works wrong ( !(a < b) && (b > a) is true )

[ich at az2000 dot de]

------- Comment #2 from ich at az2000 dot de  2009-05-18 13:16 -------
This is a real problem, because in my real application, I tried to use a very
similar order in a std::set<tmp*,CustomOrder>, defined like this:

struct CustomOrder {
        bool operator()(const Tmp* a, const Tmp* b) const {
                return a->get() < b->get();
        }
};

STLport gives this assertion when inserting Tmp pointers with equal values
(returned by get() function):

/usr/include/stlport/stl/debug/_tree.h(63): STL error : Invalid strict weak
ordering predicate, if pred(a, b) then we should have !pred(b, a)
/usr/include/stlport/stl/debug/_tree.h(63): STL assertion failure:    
!_M_non_dbg_cmp(__rhs, __lhs) 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparision works wrong ( !(a < b) && (b > a) is true )

[ich at az2000 dot de]

------- Comment #3 from ich at az2000 dot de  2009-05-18 13:17 -------
Some compiler information:

az@acompneu ~/Programmierung $ g++ -Os -v -ggdb test_order.cpp -o test_order &&
./test_order
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: /var/tmp/portage/sys-devel/gcc-4.4.0/work/gcc-4.4.0/configure
--prefix=/usr --bindir=/usr/i686-pc-linux-gnu/gcc-bin/4.4.0
--includedir=/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include
--datadir=/usr/share/gcc-data/i686-pc-linux-gnu/4.4.0
--mandir=/usr/share/gcc-data/i686-pc-linux-gnu/4.4.0/man
--infodir=/usr/share/gcc-data/i686-pc-linux-gnu/4.4.0/info
--with-gxx-include-dir=/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include/g++-v4
--host=i686-pc-linux-gnu --build=i686-pc-linux-gnu --disable-altivec
--disable-fixed-point --with-ppl --with-cloog --enable-nls
--without-included-gettext --with-system-zlib --disable-checking
--disable-werror --enable-secureplt --disable-multilib --enable-libmudflap
--disable-libssp --enable-libgomp --enable-cld --enable-java-awt=gtk
--with-arch=i686 --enable-languages=c,c++,java,objc,fortran --enable-shared
--enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu
--with-bugurl=http://bugs.gentoo.org/ --with-pkgversion='Gentoo 4.4.0'
Thread model: posix
gcc version 4.4.0 (Gentoo 4.4.0) 
COLLECT_GCC_OPTIONS='-Os' '-v' '-ggdb' '-o' 'test_order' '-shared-libgcc'
'-mtune=generic' '-march=i686'
 /usr/libexec/gcc/i686-pc-linux-gnu/4.4.0/cc1plus -quiet -v -D_GNU_SOURCE
test_order.cpp -quiet -dumpbase test_order.cpp -mtune=generic -march=i686
-auxbase test_order -ggdb -Os -version -o /tmp/cc2REzjQ.s
ignoring nonexistent directory
"/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../../i686-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include/g++-v4
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include/g++-v4/i686-pc-linux-gnu
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include/g++-v4/backward
 /usr/local/include
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/include-fixed
 /usr/include
End of search list.
GNU C++ (Gentoo 4.4.0) version 4.4.0 (i686-pc-linux-gnu)
        compiled by GNU C version 4.4.0, GMP version 4.2.4, MPFR version
2.4.1-p1.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 13a3af7fd37a02ca6104ed29e33fcca6
COLLECT_GCC_OPTIONS='-Os' '-v' '-ggdb' '-o' 'test_order' '-shared-libgcc'
'-mtune=generic' '-march=i686'
 /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../../i686-pc-linux-gnu/bin/as -V
-Qy -o /tmp/cc5XznpU.o /tmp/cc2REzjQ.s
GNU assembler version 2.18 (i686-pc-linux-gnu) using BFD version (GNU Binutils)
2.18
COMPILER_PATH=/usr/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/usr/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/usr/libexec/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/usr/libexec/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/:/usr/lib/gcc/i686-pc-linux-gnu/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../../i686-pc-linux-gnu/bin/
LIBRARY_PATH=/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../../i686-pc-linux-gnu/lib/:/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-Os' '-v' '-ggdb' '-o' 'test_order' '-shared-libgcc'
'-mtune=generic' '-march=i686'
 /usr/libexec/gcc/i686-pc-linux-gnu/4.4.0/collect2 --eh-frame-hdr -m elf_i386
-dynamic-linker /lib/ld-linux.so.2 -o test_order
/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../crt1.o
/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../crti.o
/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/crtbegin.o
-L/usr/lib/gcc/i686-pc-linux-gnu/4.4.0 -L/usr/lib/gcc/i686-pc-linux-gnu/4.4.0
-L/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../../i686-pc-linux-gnu/lib
-L/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../.. /tmp/cc5XznpU.o -lstdc++ -lm
-lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/i686-pc-linux-gnu/4.4.0/crtend.o
/usr/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../crtn.o
wrong ordering 2! 993.747, 993.747
d = 990.91854039579629898
v1 = 1600861594,1600861595
v2 = 1600861596,1600861593
8 chars: 3c7f1bcaf90d8f40
d = 990.91854039579629898
v1 = 5f6b359a,5f6b359b
v2 = 5f6b359c,5f6b3599
8 chars: 3c7f1bcaf90d8f40


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparison is wrong ( !(a < b) && (b > a) is true )

[ich at az2000 dot de]

------- Comment #4 from ich at az2000 dot de  2009-05-18 13:38 -------
Created an attachment (id=17888)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=17888&action=view)
simpler test case (now for wrong ordering case 1)

I was able to reproduce the first case now ("wrong ordering 1"). I also removed
some parts from the test case, it's much shorter now.

This occurs only with -Os now, in all other cases, I don't hit the problem.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparison is wrong ( !(a < b) && (b > a) is true )

[pinskia at gcc dot gnu dot org]

------- Comment #5 from pinskia at gcc dot gnu dot org  2009-05-18 14:11 -------


*** This bug has been marked as a duplicate of 323 ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparison is wrong ( !(a < b) && (b > a) is true )

[pinskia at gcc dot gnu dot org]

------- Comment #6 from pinskia at gcc dot gnu dot org  2009-05-18 14:49 -------
Either use -ffloat-store or -mfpmath=sse .  The issue comes from excessive
precision.  It is not < or > which is causing the problem but keeping one of a
or b in the fp stack register but putting the other one on the normal memory.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparison is wrong ( !(a < b) && (b > a) is true )

[sliwa at cft dot edu dot pl]

------- Comment #7 from sliwa at cft dot edu dot pl  2009-05-18 14:52 -------
The case (a < b) && (b < a) shows that not discarding the extra precision
before performing a comparison leads to serious problems.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186

[Bug c++/40186] floating point comparison is wrong ( !(a < b) && (b > a) is true )

[vincent at vinc17 dot org]

------- Comment #8 from vincent at vinc17 dot org  2009-05-18 14:56 -------
Are you sure that this comes from the extended precision? This would mean that
GCC does implicit extended -> double conversions in an asymmetric way, and
IIRC, I've never seen that.

I can't reproduce the problem with g++-4.4 -mfpmath=387 -Os on an x86_64
machine.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40186