从基于源的索引转换为基于目标的索引 [英] Converting from Source-based Indices to Destination-based Indices

问题描述

我在一些C代码中使用AVX2指令.

I'm using AVX2 instructions in some C code.

VPERMD 指令采用两个8位整数向量a和idx并通过基于idx置换a来生成第三个dst.这似乎等效于dst[i] = a[idx[i]] for i in 0..7.我称此为基于源，因为移动是基于源进行索引的.

The VPERMD instruction takes two 8-integer vectors a and idx and generates a third one, dst, by permuting a based on idx. This seems equivalent to dst[i] = a[idx[i]] for i in 0..7. I'm calling this source based, because the move is indexed based on the source.

但是，我有基于目的地形式的计算索引.这对于设置数组是很自然的，等效于dst[idx[i]] = a[i] for i in 0..7.

However, I have my calculated indices in destination based form. This is natural for setting an array, and is equivalent to dst[idx[i]] = a[i] for i in 0..7.

如何从基于源的表单转换为基于目标的表单?一个示例测试用例是:

How can I convert from source-based form to destination-based form? An example test case is:

{2 1 0 5 3 4 6 7}    source-based form. 
{2 1 0 4 5 3 6 7}    destination-based equivalent

对于此转换，我将保留在ymm寄存器中，这意味着基于目标的解决方案不起作用.即使我要分别插入每个对象，因为它只能在常量索引上运行，所以不能只设置它们.

For this conversion, I'm staying in ymm registers, so that means that destination-based solutions don't work. Even if I were to insert each separately, since it only operates on constant indexes, you can't just set them.

推荐答案

我想您是在隐式地说，您不能修改代码以首先计算基于源的索引?除了采用基于dst的索引的AVX512分散指令之外，我想不出对x86 SIMD可以做的任何事情.

I guess you're implicitly saying that you can't modify your code to calculate source-based indices in the first place? I can't think of anything you can do with x86 SIMD, other than AVX512 scatter instructions that take dst-based indices.

将向量存储，反转和重新加载实际上可能是最好的. (或者直接传送到整数寄存器，而不是通过内存，可能是在vextracti128/packusdw之后传送，因此您只需要两次从矢量到整数reg的64位传送:movq和pextrq.)

Storing to memory, inverting, and reloading a vector might actually be best. (Or transferring to integer registers directly, not through memory, maybe after a vextracti128 / packusdw so you only need two 64-bit transfers from vector to integer regs: movq and pextrq).

但是无论如何，然后将它们用作索引，以将计数器存储到内存中的数组中，并将其作为向量重新加载.这仍然是缓慢而丑陋的，并且包括存储转发失败延迟.因此，可能值得您花些时间更改索引生成代码以生成基于源的混洗向量.

But anyway, then use them as indices to store a counter into an array in memory, and reload that as a vector. This is still slow and ugly, and includes a store-forwarding failure delay. So it's probably worth your while to change your index-generating code to generate source-based shuffle vectors.

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