[Bug tree-optimization/50413] Incorrect instruction is used to shift value of 128 bit xmm0 registrer

[ubizjak at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50413

Uros Bizjak <ubizjak at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #3 from Uros Bizjak <ubizjak at gmail dot com> 2011-09-15 13:29:23 UTC ---
(In reply to comment #0)
> After compilation an attached code with -O2 and -ftree-vectorize flags, it
> doesn't work properly.
> 
> Assembler code shows that G++ tries to replace the following code 
> 
>   V.uint128.uint64_lower = (V.uint128.uint64_lower >> 1);
>   V.bitmap.b63 = V.bitmap.b64;
>   V.uint128.uint64_upper = (V.uint128.uint64_upper >> 1);
> 
> with SSE instructions:
> 
>   400a10:   movdqa 0x103d8(%rip),%xmm0        # 410df0 <V>
>   400a17:   and    $0x1,%edi
>   400a1b:   psrlq  $0x1,%xmm0
>   400a20:   movdqa %xmm0,0x103c8(%rip)        # 410df0 <V>
> 
> 
> But psrlq shifts 64 bit value, it's necessary to use psrldq here

You are wrong. The code above describes shift of two 64bit elements, each by
one, so psrlq [1] is correct.

[1] http://www.rz.uni-karlsruhe.de/rz/docs/VTune/reference/vc259.htm