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

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #5)
> With the intrinsics patch, I notice that we don't simplify in gimple either:
> 
>   _40 = VIEW_CONVERT_EXPR<__m128i>(_39);
>   MEM[(__m128i * {ref-all})vec_4(D)] = _40;
>   _60 = MEM[(const double *)vec_4(D)];
>   _61 = MEM[(const double *)vec_4(D) + 8B];
>   _62 = {_60, _61};
>   _63 = VIEW_CONVERT_EXPR<__v4si>(_62);
> 
> (_39 and _63 have the same type)

value-numbering has difficulties in seeing through so much stmts (read:
not implemented) and it doesn't have a way of expressing "partial"
values.  That is, it knows that at MEM[(__m128i * {ref-all})vec_4(D)] we
stored _39 but when value-numbering the partial reads it can't assign
the value _39 to them (as said, "partial" values are not supported).

So one way to optimize this is to special-case the composition
operations and try looking up a proper memory operation.

Another possibility is to value-number compound operations also as
piecewise operations, introducing fake value-numbers (that is,
"lower" everything to component-wise operations internally).

I suppose pattern-matching

>   _60 = MEM[(const double *)vec_4(D)];
>   _61 = MEM[(const double *)vec_4(D) + 8B];
>   _62 = {_60, _61};

and generating a single read (with eventually a permute?) would be
more profitable and easier to implement.