[Bug target/50829] avx extra copy for _mm256_insertf128_pd

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50829

--- Comment #10 from Marc Glisse <glisse at gcc dot gnu.org> 2012-12-01 19:50:28 UTC ---
Created attachment 28846
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28846
Use subreg

Hmm, I don't understand why we use UNSPEC_CAST. I tried the attached patch to
use a subreg instead. It passed bootstrap+testsuite and generates optimal code
for the testcase of this PR.

So, any idea what advantage the unspec has over a subreg? And if none, what is
the best way to use a subreg?