[Bug target/79709] Subobtimal code with -mavx and explicit vector

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79709

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
We have trouble with your VSET macro (known issue):
  two_28 = BIT_INSERT_EXPR <two_27(D), 2.0e+0, 0 (64 bits)>;
  two_29 = BIT_INSERT_EXPR <two_28, 2.0e+0, 64 (64 bits)>;
  two_30 = BIT_INSERT_EXPR <two_29, 2.0e+0, 128 (64 bits)>;
  two_31 = BIT_INSERT_EXPR <two_30, 2.0e+0, 192 (64 bits)>;
it is easier for gcc if you write:
  v4do two={2,2,2,2};
or you could even replace two with 2 in the expressions, gcc handles it just
fine.

In reload, subregs are extracted via the stack, whereas the low subreg should
already be available (NOP) and the high one can be extracted by a single insn.
That's probably the first thing to investigate. (-mtune doesn't change what
happens)

res could be kept in a register (or even better a pair of registers) through
the whole loop.