[Bug c++/62080] Suboptimal code generation with eigen library

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62080

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
(note that a minimal, self-contained testcase would be much better and
shouldn't be hard to produce)

We write to memory with:

(insn 10 8 11 2 (set (mem:V2DI (reg/v/f:DI 97 [ vec ]) [0 MEM[(__m128i *
{ref-all})vec_4(D)]+0 S16 A128])
        (subreg:V2DI (reg:V4SI 98) 0))
/usr/lib/gcc-snapshot/lib/gcc/x86_64-linux-gnu/4.10.0/include/emmintrin.h:706
1147 {*movv2di_internal}
     (expr_list:REG_DEAD (reg:V4SI 98)
        (nil)))

and then read back with:

(insn 15 12 17 2 (set (reg:V2DF 100)
        (vec_concat:V2DF (mem:DF (reg/v/f:DI 97 [ vec ]) [5 MEM[(const double
*)vec_4(D)]+0 S8 A64])
            (mem:DF (plus:DI (reg/v/f:DI 97 [ vec ])
                    (const_int 8 [0x8])) [0  S8 A8])))
/usr/lib/gcc-snapshot/lib/gcc/x86_64-linux-gnu/4.10.0/include/emmintrin.h:925
2016 {*vec_concatv2df}
     (nil))

The vec_concat of the 2 adjacent memory locations is not merged into a single
memory read, although from the previous insn it looks like it is suitably
aligned.