https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104412
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
On x86_64 we get
movq %rsi, %xmm0
movq %rdi, %xmm1
punpcklqdq %xmm1, %xmm0
ret
> ./cc1 -quiet t.c -I include -O2 -fopt-info-vec
t.c:10:7: optimized: basic block part vectorized using 16 byte vectors
and at -O1
movq %rsi, -24(%rsp)
movq %rdi, -16(%rsp)
movdqa -24(%rsp), %xmm0
ret
costing is a bit difficult since we get
t.c:10:7: note: Cost model analysis:
i2_4(D) 1 times scalar_store costs 12 in body
i1_6(D) 1 times scalar_store costs 12 in body
i2_4(D) 1 times vector_store costs 12 in body
<unknown> 1 times vec_construct costs 8 in prologue
t.c:10:7: note: Cost model analysis for part in loop 0:
Vector cost: 20
Scalar cost: 24
as we do not have an idea how costly the construction is (depends on
calling conventions) or how the return d.v allows us to elide the store
and the load.
