Avx2.AlignRight does not generate VPPALIGNR intrinsic

[zvrba]

Description

I have a piece of code that looks like this:

    using V = Vector256<int>;

        [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
        unsafe V Load8(int* v, int c) {
            var m = Avx2.AlignRight(AlternatingMaskLo128, Complement, (byte)((8 - c) << 2));
            return Avx2.BlendVariable(Max, Avx2.MaskLoad(v, m), m);
        }

which is called from various places in the code. When looking at the disassembly, I find the following code:

00007FF8FCD0A78F  vextractf128 xmm7,ymm6,1  
00007FF8FCD0A795  call        Method stub for: System.Runtime.Intrinsics.X86.Avx2.AlignRight(System.Runtime.Intrinsics.Vector256`1<Int32>, System.Runtime.Intrinsics.Vector256`1<Int32>, Byte) (07FF8FCCFAF58h)  
00007FF8FCD0A79A  vmovupd     ymm0,ymmword ptr [rsi+0D8h]  
00007FF8FCD0A7A2  vmovupd     ymm1,ymmword ptr [rsp+130h]  
00007FF8FCD0A7AB  mov         rax,qword ptr [rsp+168h]  
00007FF8FCD0A7B3  vpmaskmovd  ymm1,ymm1,ymmword ptr [rax]  
00007FF8FCD0A7B8  vmovupd     ymm2,ymmword ptr [rsp+130h]  
00007FF8FCD0A7C1  vpblendvb   ymm8,ymm0,ymm1,ymm2  

i.e., everything gets nicely inlined and optimized, except PALIGNR instruction not being generated. I cannot even step into the CALL in visual studio to see what it does. The code had executed for a while before I set a breakpoint to disassemble it.

Configuration

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19042.1348 (20H2/October2020Update)
Intel Core i7-8650U CPU 1.90GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET SDK=5.0.303
[Host] : .NET Core 3.1.21 (CoreCLR 4.700.21.51404, CoreFX 4.700.21.51508), X64 RyuJIT
DefaultJob : .NET Core 3.1.21 (CoreCLR 4.700.21.51404, CoreFX 4.700.21.51508), X64 RyuJIT

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Issue Details

---

Description

I have a piece of code that looks like this:

        [MethodImpl(MethodImplOptions.AggressiveInlining | MethodImplOptions.AggressiveOptimization)]
        unsafe V Load8(int* v, int c) {
            var m = Avx2.AlignRight(AlternatingMaskLo128, Complement, (byte)((8 - c) << 2));
            return Avx2.BlendVariable(Max, Avx2.MaskLoad(v, m), m);
        }

which is called from various places in the code. When looking at the disassembly, I find the following code:

00007FF8FCD0A78F  vextractf128 xmm7,ymm6,1  
00007FF8FCD0A795  call        Method stub for: System.Runtime.Intrinsics.X86.Avx2.AlignRight(System.Runtime.Intrinsics.Vector256`1<Int32>, System.Runtime.Intrinsics.Vector256`1<Int32>, Byte) (07FF8FCCFAF58h)  
00007FF8FCD0A79A  vmovupd     ymm0,ymmword ptr [rsi+0D8h]  
00007FF8FCD0A7A2  vmovupd     ymm1,ymmword ptr [rsp+130h]  
00007FF8FCD0A7AB  mov         rax,qword ptr [rsp+168h]  
00007FF8FCD0A7B3  vpmaskmovd  ymm1,ymm1,ymmword ptr [rax]  
00007FF8FCD0A7B8  vmovupd     ymm2,ymmword ptr [rsp+130h]  
00007FF8FCD0A7C1  vpblendvb   ymm8,ymm0,ymm1,ymm2  

i.e., everything gets nicely inlined and optimized, except PALIGNR instruction not being generated. I cannot even step into the CALL in visual studio to see what it does. The code had executed for a while before I set a breakpoint to disassemble it.

Configuration

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19042.1348 (20H2/October2020Update)
Intel Core i7-8650U CPU 1.90GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET SDK=5.0.303
[Host] : .NET Core 3.1.21 (CoreCLR 4.700.21.51404, CoreFX 4.700.21.51508), X64 RyuJIT
DefaultJob : .NET Core 3.1.21 (CoreCLR 4.700.21.51404, CoreFX 4.700.21.51508), X64 RyuJIT

Author:	zvrba
Assignees:	-
Labels:	area-System.Runtime.Intrinsics, tenet-performance, untriaged
Milestone:	-

[saucecontrol]

vpalignr requires the byte count to be an immediate (constant) value. Since you are using a variable count, JIT must generate a stub to emit the code with the value as a constant.

[zvrba]

Oh, good catch, thanks. I overlooked that. And C# doesn't have an expressive enough type system to require that this parameter must be a compile-time constant.

[jkotas]

We plan to add analyzer that warns about these situations. See #33771 .

[zvrba]

@jkotas One last question: why won't visual studio step into stub methods? If I could have seen the disassembled stub (and the generated jump table), I'd figure this out on my own. I even tried to paste the address of the CALL target into the disassembly window, but I got no meaningful result.

[jkotas]

why won't visual studio step into stub methods?

I do not know. I have opened #61890 . It is not clear to me whether it is a problem in the runtime or in Visual Studio.