Re: [PATCH 2/4] rs6000: Support variable insert and Expand vec_insert in expander [PR79251]

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Sat, Oct 10, 2020 at 03:08:23AM -0500, Xionghu Luo wrote:
> vec_insert accepts 3 arguments, arg0 is input vector, arg1 is the value
> to be insert, arg2 is the place to insert arg1 to arg0.  Current expander
> generates stxv+stwx+lxv if arg2 is variable instead of constant, which
> causes serious store hit load performance issue on Power.  This patch tries
>  1) Build VIEW_CONVERT_EXPR for vec_insert (i, v, n) like v[n&3] = i to
> unify the gimple code, then expander could use vec_set_optab to expand.
>  2) Expand the IFN VEC_SET to fast instructions: lvsr+insert+lvsl.
> In this way, "vec_insert (i, v, n)" and "v[n&3] = i" won't be expanded too
> early in gimple stage if arg2 is variable, avoid generating store hit load
> instructions.
> 
> For Power9 V4SI:
> 	addi 9,1,-16
> 	rldic 6,6,2,60
> 	stxv 34,-16(1)
> 	stwx 5,9,6
> 	lxv 34,-16(1)
> =>
> 	rlwinm 6,6,2,28,29
> 	mtvsrwz 0,5
> 	lvsr 1,0,6
> 	lvsl 0,0,6
> 	xxperm 34,34,33
> 	xxinsertw 34,0,12
> 	xxperm 34,34,32

It still takes me quite some time to verify this, tricky bit-fiddling!

But the code that generates this is easier to read :-)

> +/* Insert VAL into IDX of TARGET, VAL size is same of the vector element, IDX
> +   is variable and also counts by vector element size.  */

"Set vector element IDX of TARGET to VAL.  IDX is not a constant
integer."?


Okay for trunk (with an improved comment).  Thanks!


Segher