[Bug c++/23599] New: Flag -fstrict-aliasing corrupts iterators

[leopardi at bigpond dot net dot au]

Compiling the test case with -O3 or -O3 -fstrict-aliasing causes operator++ on 
a const_iterator to fail to progress. Error occurs with 4.0.1, 4.0.2 20050728 
(prerelease), but not with 3.3.5 or 4.1.0 20050813 (experimental). 
 
I believe this to be a compiler bug rather than a library bug because error 
occurs with 4.0.1 but does not occur with 3.3.5 using reduced preprocessed test 
case. 
 
Original test case (reduced using delta to the smallest test case I can find) 
 
test-case.cpp: 
#include <bitset> 
#include <ext/hash_map> 
 typedef short index_t; 
 const index_t BITS_PER_SET_VALUE = std::numeric_limits<unsigned char>::digits 
* index_t(sizeof(unsigned long)); 
 const index_t DEFAULT_LO = -index_t(BITS_PER_SET_VALUE / 2); 
 class index_set : private std::bitset<BITS_PER_SET_VALUE> { 
 private: typedef std::bitset<BITS_PER_SET_VALUE> bitset_t; 
 public: 
 index_set(const bitset_t& bst) { *this = *static_cast<const 
index_set*>(&bst); } 
 index_set(const index_t& idx)  { this->set(idx); } 
 bool operator== (const index_set& rhs) const; 
 index_set& operator&= (const index_set& rhs);  
 index_set& set(index_t idx); 
 size_t hash_fn() const; 
 }; 
 inline index_set& index_set::operator&= (const index_set& rhs)  
  { bitset_t* pthis = this; const bitset_t* pthat = &rhs; *pthis &= *pthat; 
return *this; } 
 inline bool index_set::operator== (const index_set& rhs) const  
  { const bitset_t* pthis = this; const bitset_t* pthat = &rhs; return *pthis 
== *pthat; } 
 inline const index_set operator& (const index_set& lhs, const index_set& rhs)  
  { index_set result = lhs; return result &= rhs; } 
 inline index_set& index_set::set(index_t idx)  
  { if (idx > 0) bitset_t::set(idx-DEFAULT_LO-1); return *this; } 
 inline size_t index_set::hash_fn() const { 
 static const index_set lo_mask = bitset_t((1UL << -DEFAULT_LO) - 1); 
 const index_set neg_part = *this & lo_mask; 
 const index_set pos_part = *this >> -DEFAULT_LO; 
 const bitset_t* pneg_part = &neg_part; 
 const bitset_t* ppos_part = &pos_part; 
 return size_t((*pneg_part).to_ulong() ^ (*ppos_part).to_ulong()); 
 } 
 class hash { 
 public: inline size_t operator()(index_set val) const { return 
val.hash_fn(); } 
 }; 
 class framed_multi : private __gnu_cxx::hash_map< const index_set, double, 
hash > { 
 public: typedef std::pair< const index_set, double > pair_t; 
 framed_multi(const index_set& ist, const double& crd) 
  { if (crd != double(0)) this->insert(pair_t(ist, crd)); } 
 framed_multi& operator+= (const framed_multi& rhs) 
 { 
  for (const_iterator this_it = this->begin(); this_it != this->end(); 
++this_it) { const_iterator next_it = this_it; next_it++; if (this_it == 
next_it) exit(1); } 
  for (const_iterator rhs_it = rhs.begin(); rhs_it != rhs.end(); ++rhs_it) 
{ const_iterator next_it = rhs_it; next_it++; if (rhs_it == next_it) exit(1); } 
  return *this; 
 } 
 }; 
 framed_multi operator+ (const framed_multi & lhs, const framed_multi & rhs)  
  { framed_multi result = lhs; return result += rhs; } 
 int main() { 
 framed_multi OP = framed_multi(index_set(1),4.0) +  
framed_multi(index_set(2),3.0); 
 } 
 
Compiled using: 
./compile-strict-gcc-3XX.sh: 
$CXX -Werror -Wstrict-aliasing -Wall -ansi -g3 -O3 -fno-check-new 
-fabi-version=0 -fexceptions -fstrict-aliasing $1 -o test01-strict 
./compile-nostrict-gcc-3XX.sh: 
$CXX -Werror -Wstrict-aliasing -Wall -ansi -g3 -O3 -fno-check-new 
-fabi-version=0 -fexceptions -fno-strict-aliasing $1 -o test01-nostrict 
 
Transcript: 
> . ~/.bashrc 
> $CXX -v 
Reading specs from /usr/lib64/gcc-lib/x86_64-suse-linux/3.3.5/specs 
Configured with: ../configure --enable-threads=posix --prefix=/usr 
--with-local-prefix=/usr/local --infodir=/usr/share/info 
--mandir=/usr/share/man --enable-languages=c,c++,f77,objc,java,ada 
--disable-checking --libdir=/usr/lib64 --enable-libgcj --with-slibdir=/lib64 
--with-system-zlib --enable-shared --enable-__cxa_atexit x86_64-suse-linux 
Thread model: posix 
gcc version 3.3.5 20050117 (prerelease) (SUSE Linux) 
> ./compile-strict-gcc-3XX.sh test-case.cpp 
> ./test01-strict 
> echo $? 
0 
> ./compile-nostrict-gcc-3XX.sh test-case.cpp 
> ./test01-nostrict 
> echo $? 
0 
> . ~/.bashrc 
> $CXX -v 
Using built-in specs. 
Target: x86_64-suse-linux 
Configured with: ../gcc-4.0.1/configure --prefix=/usr/local/gcc/gcc-4.0.1 
--enable-threads=posix --disable-libgcj --with-system-zlib --enable-shared 
--enable-__cxa_atexit --enable-languages=c,c++ x86_64-suse-linux 
Thread model: posix 
gcc version 4.0.1 
> ./compile-strict-gcc-3XX.sh test-case.cpp 
> ./test01-strict 
> echo $? 
1 
> ./compile-nostrict-gcc-3XX.sh test-case.cpp 
> ./test01-nostrict 
> echo $? 
0 
> . ~/.bashrc 
> $CXX -v 
Using built-in specs. 
Target: x86_64-suse-linux 
Configured with: ../gcc-4.0-20050728/configure 
--prefix=/usr/local/gcc/gcc-4.0-20050728 --enable-threads=posix 
--disable-libgcj --with-system-zlib --enable-shared --enable-__cxa_atexit 
--enable-languages=c,c++ x86_64-suse-linux 
Thread model: posix 
gcc version 4.0.2 20050728 (prerelease) 
> ./compile-strict-gcc-3XX.sh test-case.cpp 
> ./test01-strict 
> echo $? 
1 
> ./compile-nostrict-gcc-3XX.sh test-case.cpp 
> ./test01-nostrict 
> echo $? 
0 
> . ~/.bashrc 
> $CXX -v 
Using built-in specs. 
Target: x86_64-suse-linux 
Configured with: ../gcc-4.1-20050813/configure 
--prefix=/usr/local/gcc/gcc-4.1-20050813 --enable-threads=posix 
--disable-libgcj --with-system-zlib --enable-shared --enable-__cxa_atexit 
--enable-languages=c,c++ x86_64-suse-linux 
Thread model: posix 
gcc version 4.1.0 20050813 (experimental) 
> ./compile-strict-gcc-3XX.sh test-case.cpp 
> ./test01-strict 
> echo $? 
0 
> ./compile-nostrict-gcc-3XX.sh test-case.cpp 
> ./test01-nostrict 
> echo $? 
0

-- 
           Summary: Flag -fstrict-aliasing corrupts iterators
           Product: gcc
           Version: 4.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: leopardi at bigpond dot net dot au
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: x86_64-suse-linux
  GCC host triplet: x86_64-suse-linux
GCC target triplet: x86_64-suse-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23599