[Bug c++/110283] New: Bogus Warray-bounds

[Bug c++/110283] New: Bogus Warray-bounds

[pilarlatiesa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110283

            Bug ID: 110283
           Summary: Bogus Warray-bounds
           Product: gcc
           Version: 13.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pilarlatiesa at gmail dot com
  Target Milestone: ---

$ ~/gcc-13/bin/g++ -v
Usando especificaciones internas.
COLLECT_GCC=~/gcc-13/bin/g++
COLLECT_LTO_WRAPPER=/home/pililatiesa/gcc-13/libexec/gcc/x86_64-pc-linux-gnu/13.1.0/lto-wrapper
Objetivo: x86_64-pc-linux-gnu
Configurado con: ../gcc-13.1.0/configure --enable-languages=c,c++
--disable-multilib --prefix=/home/pililatiesa/gcc-13/
Modelo de hilos: posix
Algoritmos de compresión LTO admitidos: zlib
gcc versión 13.1.0 (GCC)


$ ~/gcc-13/bin/g++ -std=c++23 -Wall -O3 -c ElemBase.ii 
En el fichero incluido desde ../../MEF/ElemBase.h:11,
                 desde ../ElemBase.cpp:2:
In member function ‘constexpr const std::array<_Tp, _Nm>::value_type&
std::array<_Tp, _Nm>::operator[](size_type) const [with _Tp = const TNodo<3>*;
long unsigned int _Nm = 2]’,
    inlined from ‘const TNodo<TElemTraits<TElem>::Ngdl>&
TElemBase<TElem>::Nodo(int) const [with TElem = TElemViga]’ at
../../MEF/ElemBase.h:64:25,
    inlined from ‘void TElemBase<TElem>::CalcCCTerm(TTripletVec&) const [with
TElem = TElemViga]’ at ../ElemBase.cpp:10:21:
/home/pililatiesa/gcc-13/include/c++/13.1.0/array:213:17: aviso: el subíndice 2
de la matriz está por encima de los límites de matriz de
‘std::__array_traits<const TNodo<3>*, 2>::_Type’ {aka ‘const TNodo<3>* const
[2]’} [-Warray-bounds=]
/home/pililatiesa/gcc-13/include/c++/13.1.0/array: In member function ‘void
TElemBase<TElem>::CalcCCTerm(TTripletVec&) const [with TElem = TElemViga]’:
/home/pililatiesa/gcc-13/include/c++/13.1.0/array:109:48: nota: while
referencing ‘std::array<const TNodo<3>*, 2>::_M_elems’

I couldn't get rid of the eigen dependency, sorry.

It only happens with O3.

It appears that it is only a bogus warning, but I don't know if wrong code is
actually generated.

[Bug c++/110283] Bogus Warray-bounds

[pilarlatiesa at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110283

--- Comment #1 from Pilar Latiesa <pilarlatiesa at gmail dot com> ---
Created attachment 55345
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55345&action=edit
preprocessed code

[Bug tree-optimization/110283] Bogus Warray-bounds

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110283

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
As an aside, Eigen has some maybe wrong code in it:
```
# 55 "../../eigen-3.3.4/Eigen/src/Core/arch/SSE/PacketMath.h" 3 4
typedef __m128 Packet4f;
typedef __m128i Packet4i;
typedef __m128d Packet2d;


template<> struct is_arithmetic<__m128> { enum { value = true }; };
template<> struct is_arithmetic<__m128i> { enum { value = true }; };
template<> struct is_arithmetic<__m128d> { enum { value = true }; };
...
template<> struct unpacket_traits<Packet4f> { typedef float type; enum {size=4,
alignment=Aligned16}; typedef Packet4f half; };
template<> struct unpacket_traits<Packet2d> { typedef double type; enum
{size=2, alignment=Aligned16}; typedef Packet2d half; };
template<> struct unpacket_traits<Packet4i> { typedef int type; enum {size=4,
alignment=Aligned16}; typedef Packet4i half; };
```
Because __m128/__m128i/__m128d are all declared as may_alias but the template
will lose that attribute.

[Bug tree-optimization/110283] Bogus Warray-bounds

[c_antonio_sanchez at msn dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110283

Antonio Sanchez <c_antonio_sanchez at msn dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |c_antonio_sanchez at msn dot com

--- Comment #3 from Antonio Sanchez <c_antonio_sanchez at msn dot com> ---
@Andrew Pinsky can you elaborate on what's potentially wrong in the Eigen code?

The template argument will ignore the `may_alias` attribute, but the `typedef`s
do not, so `Packet4f` and `unpacket_traits<Packet4f>::half` are still marked as
`may_alias` and do allow aliasing without strict-aliasing warnings.

[Bug tree-optimization/110283] Bogus Warray-bounds

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110283

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Antonio Sanchez from comment #3)
> @Andrew Pinsky can you elaborate on what's potentially wrong in the Eigen
> code?
> 
> The template argument will ignore the `may_alias` attribute, but the
> `typedef`s do not, so `Packet4f` and `unpacket_traits<Packet4f>::half` are
> still marked as `may_alias` and do allow aliasing without strict-aliasing
> warnings.

As I said potentially wrong because of the template arguments ignorning the
attributes and I didn't look further into the code to see if Eigen used that
template argument in anyway except for matching.


But that is a different issue overall.

Anyways the warning here is complex due to how complex the code Eigen causes to
be inlined and someone will have to figure out why the warning is happening if
it is a true bug or just warning about dead code or something GCC could not
figure out is not out of bounds.