Resolved: dotnet/runtime#104001

ConditionalSelectReproduce

Reproduce for the issue of JIT optimization for the VectorT.ConditionalSelect (particularly 128-bit) in comparison with Sse41.BlendVariable.

Benchmark results

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22631.3737/23H2/2023Update/SunValley3)
AMD Ryzen 7 5800X3D, 1 CPU, 16 logical and 8 physical cores
.NET SDK 9.0.100-preview.5.24307.3
  [Host]     : .NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2
  DefaultJob : .NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2

Method	Mean	Error	StdDev	Code Size
ConditionalSelectField	0.3298 ns	0.0038 ns	0.0034 ns	54 B
BlendVariable	0.1897 ns	0.0067 ns	0.0059 ns	52 B

Disassembly

Disassembly diff

## .NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2
```assembly
-; ConditionalSelectBenchmark.BlendVariable()
+; ConditionalSelectBenchmark.ConditionalSelectField()
       sub       rsp,28
       mov       rax,vectorOneAddr
       mov       rax,[rax]
-      vmovups   xmm0,[base]
+      vmovups   xmm0,[rax+10]
-      vxorps    xmm1,xmm1,xmm1
+      vpand     xmm1,xmm0,[7FFE613076D0]
+      vxorps    xmm2,xmm2,xmm2
-      vmovups   xmm2,[rax+10]
-      vpblendvb xmm0,xmm0,xmm1,xmm2
+      vpandn    xmm0,xmm0,xmm2
+      vpor      xmm0,xmm0,xmm1
       vmovd     eax,xmm0
       movzx     eax,al
       add       rsp,28
       ret
- ; Total bytes of code 52
+ ; Total bytes of code 54

So, we've got vpand + vpandn + vpor instead of vmovups (not sure why we need it anyway, probably can be optimized?) + vpblendvb.

Full disassembly

.NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2

; ConditionalSelectBenchmark.ConditionalSelectField()
       sub       rsp,28
       mov       rax,21971404218
       mov       rax,[rax]
       vmovups   xmm0,[rax+10]
       vpand     xmm1,xmm0,[7FFE613076D0]
       vxorps    xmm2,xmm2,xmm2
       vpandn    xmm0,xmm0,xmm2
       vpor      xmm0,xmm0,xmm1
       vmovd     eax,xmm0
       movzx     eax,al
       add       rsp,28
       ret
; Total bytes of code 54

.NET 9.0.0 (9.0.24.30607), X64 RyuJIT AVX2

; ConditionalSelectBenchmark.BlendVariable()
       sub       rsp,28
       mov       rax,1D49BC04218
       mov       rax,[rax]
       vmovups   xmm0,[7FFE612E76C0]
       vxorps    xmm1,xmm1,xmm1
       vmovups   xmm2,[rax+10]
       vpblendvb xmm0,xmm0,xmm1,xmm2
       vmovd     eax,xmm0
       movzx     eax,al
       add       rsp,28
       ret
; Total bytes of code 52