[Bug target/49133] [4.6/4.7 Regression] modification of aliased __m128d miscompiles

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49133

--- Comment #3 from Uros Bizjak <ubizjak at gmail dot com> 2011-05-24 10:51:25 UTC ---
(In reply to comment #2)
> I applied the patch to the latest 4.6 snapshot. I confirm that it fixes the
> bug. Also, there are no regressions in my testsuite.
> 
> Just for confirmation, the patched sse.md looks like this for me now (starting
> from line 4952):
> (define_insn "sse2_loadhpd"
>   [(set (match_operand:V2DF 0 "nonimmediate_operand"     "=x,x,x,o,o,o")
>     (vec_concat:V2DF
>       (vec_select:DF
>         (match_operand:V2DF 1 "nonimmediate_operand" " 0,0,x,0,0,0")
>         (parallel [(const_int 0)]))
>       (match_operand:DF 2 "nonimmediate_operand"     " m,x,0,x,*f,r")))]
>   "TARGET_SSE2 && !(MEM_P (operands[1]) && MEM_P (operands[2]))"
>   "@
>    movhpd\t{%2, %0|%0, %2}
>    unpcklpd\t{%2, %0|%0, %2}
>    shufpd\t{$0, %1, %0|%0, %1, 0}
>    #
> 
> Question, why not use unpcklpd instead of shufpd $0? On older CPUs unpcklpd
> should be slightly faster than shufpd.

OTOH, it looks that this alternative is wrong entirely. Unmodified operand can
only be passed in lower half (operand 1 in the pattern above). GCC will then
generate unpcklpd, as suggested.