As detailed on https://reviews.llvm.org/rL340813, many recent machines have better throughput for the 'per-element' variable vector shifts than the old style 'scalar-count-in-xmm' variable shifts if we know that the shift amount is already splatted:

Probably the wrong place to report this, but I looked at some other sequences:

; AVX-LABEL: splatvar_shift_v4i32:
; AVX:       # %bb.0:
; AVX-NEXT:    vpmovzxdq {{.*#+}} xmm1 = xmm1[0],zero,xmm1[1],zero   # 1 uop / 1c latency
; AVX-NEXT:    vpsrad %xmm1, %xmm0, %xmm0                # 2 uops / 2c latency on Intel since Haswell at least
; AVX-NEXT:    retq

For Skylake, variable-shifts (vpsraVd) are single uop, but count-in-xmm shifts are 2 uops. Probably they're implemented internally as broadcast to feed the SIMD variable-shift hardware.

The above is 3 uops / 3c latency on SKL.

So for AVX2 Skylake (but not Broadwell or earlier) we want this 2 uop / 2c latency implementation:

vpbroadcastd %xmm1, %xmm1         = xmm1[0],xmm1[1],xmm1[2],xmm1[3]   # 1 uop / 1c latency
vpsravd      %xmm1, %xmm0, %xmm0                          # 1 uop / 1c latency on SKL.   3 / 3 on BDW and earlier.

Same for SKX AVX512 with vpsravw and so on. There are some test cases where we use the same shift-count register multiple times, and it would be significantly better to broadcast it and use variable-shifts instead of count-from-the-low-element shifts.

But on Ryzen, and Broadwell and earlier, variable-shifts cost more. (Interestingly, on Ryzen they run on a different execution port from normal count-in-xmm shifts; still a single uop (per lane) but 3c latency and not fully pipelined. Ryzen has shift-in-xmm shifts as efficient as immediate shifts, unlike Intel where shift-in-xmm is always 2 uops (port5 + shift port).

KNL is horrible for pslld xmm,xmm (13c throughput/latency), but it has the same throughput as immediate for variable shifts like VPSRLVD z,z,z. I don't totally trust Agner's numbers for x,x shifts; maybe he only used the non-VEX encoding?

Anyway, for AVX512 we should prefer broadcast + variable-shift instead of pmovzxb/wq / regular shift, because it's better on SKX and at least as good on KNL. This includes 16-bit elements for AVX512BW, unlike AVX2.

(With AVX1, we don't have variable shifts so the earlier implementation with vpsrad is our best option.)