[PATCH][AVX] Lower v4i64->v4i32 ISD::TRUNCATE for minimal shuffles

[Nadav Rotem]

Hi Cameron, 

Your change looks like a win. If I remember correctly Sandybridge only has  a single shuffle port (port5) and reducing the pressure on port5 should be a win. You are also reducing the latency, which is great. 

Thanks, 
Nadav

On Mar 5, 2014, at 7:44 AM, Cameron McInally <cameron.mcinally at nyu.edu> wrote:

> Hey guys,
> 
> For AVX, v4i64->v4i32 truncates are currently lowered into two
> shuffles plus a movlh:
> 
>> vpshufd $8, %xmm1, %xmm1        ## xmm1 = xmm1[0,2,0,0]
>> vpshufd $8, %xmm0, %xmm0        ## xmm0 = xmm0[0,2,0,0]
>> vmovlhps %xmm1, %xmm0, %xmm0 ## xmm0 = xmm0[0],xmm1[0]
> 
> This could also be done using a vshufps:
> 
>> vshufps $-120, %xmm1, %xmm0, %xmm0 ## xmm0 = xmm0[0,2],xmm1[0,2]
> 
> Please note that this does change the execution domain of the shuffle,
> but as far as I can tell this should be okay. My understanding, from
> looking at Fog's tables, is that the shuffles should be a wash and
> avoiding the movlh is a win.
> 
> Any insights into whether this change is a good idea or not?
> 
> Tia,
> Cameron
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