[llvm-commits] X86 vector shift patch

[Eli Friedman]

On Sat, Dec 20, 2008 at 6:22 PM, Mon Ping Wang <monping at apple.com> wrote:
> Hi,
>
> This is a patch that allows us to generate X86 vector shifts (psllq, psrlq
> etc..) for some vector shifts (v2i64, v4i32, and v8i16)  where the shift
> amount is the same for all elements.  This particular patch does as a target
> specific  combine pass because it is easier to identify the shift amounts
> before it gets lowered to something else (e.g., build vector of constants
> can be translated into a load from a constant pool).  If someone has a
> better idea of where it should be done or have any other comments, please
> let me know.

It isn't impossible to do it during legalize... I've had a patch in my
tree for a while that's reasonably successful at doing that.  I'm
attaching my patch, in case it's useful; it does still miss some cases
because isSplatVector isn't clever enough, though.  That patch also
has some suggestions for potentially interesting cases we can handle
in somewhat unconventional ways.

Does you patch handle a shift by a splat shuffle, e.g. shl(x,
shufflevector(y, y, <0,0,0,0>))?  Or does the lowering take care of
this case somehow?

-Eli
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