将_mm_shuffle_epi32转换为C表达式以进行排列? [英] Convert _mm_shuffle_epi32 to C expression for the permutation?

问题描述

我正在使用SSE2到NEON的端口.该端口尚处于早期阶段，并且会产生不正确的结果.错误结果的部分原因是_mm_shuffle_epi32和我选择的NEON指令.

I'm working on a port of SSE2 to NEON. The port is early stage and it's producing incorrect results. Part of the reason for the incorrect results is _mm_shuffle_epi32 and the NEON instructions I selected.

_mm_shuffle_epi32的文档来自英特尔文档更好，但我不清楚某些伪代码在做什么.

The documentation for _mm_shuffle_epi32 is on the lean side from Microsoft. The Intel documentation is better, but it's not clear to me what some of the pseudo-code is doing.

SELECT4(src, control)
{
    CASE(control[1:0])
        0: tmp[31:0] := src[31:0]
        1: tmp[31:0] := src[63:32]
        2: tmp[31:0] := src[95:64]
        3: tmp[31:0] := src[127:96]
    ESAC
    RETURN tmp[31:0]
}

dst[31:0] := SELECT4(a[127:0], imm8[1:0])
dst[63:32] := SELECT4(a[127:0], imm8[3:2])
dst[95:64] := SELECT4(a[127:0], imm8[5:4])
dst[127:96] := SELECT4(a[127:0], imm8[7:6])

我需要设想_mm_shuffle_epi32的功能.或更正确地说，排列是立即数应用于值的.我想我需要将其视为基本的C和AND与OR.

I need help envisioning what _mm_shuffle_epi32 does. Or more correctly, the permutation applied to the value by the immediate. I guess I need to see it as basic C and ANDs and ORs.

给出C语句和宏，例如:

Given C statements and macros like:

v2 = _mm_shuffle_epi32(v1, _MM_SHUFFLE(i1,i2,i3,i4));

将结果C表达式展开为基本C语句后会是什么样?

What does the resulting C expression look like when it's unrolled into basic C statements?

推荐答案

除非您需要解压缩包含四个2bit索引的8bit整数，否则不会进行AND/OR操作.

There's no AND/OR going on, unless you need to unpack the 8bit integer holding four 2bit indices.

为_MM_SHUFFLE定义自己的定义，该定义扩展为四个arg，而不是打包它们.

Make your own definition for _MM_SHUFFLE that expands to four args, instead of packing them.

就像

// dst = _mm_shuffle_epi32(src, _MM_SHUFFLE(d,c,b,a))
void pshufd(int dst[4], int src[4], int d,int c,int b,int a)
{   // note that the _MM_SHUFFLE args are high-element-first order
    dst[0] = src[a];
    dst[1] = src[b];
    dst[2] = src[c];
    dst[3] = src[d];
}

向量从低位元素= 0开始索引.低位元素是存储在内存中最低地址的元素，但是当值位于寄存器中时，应将其视为[ 3 2 1 0 ].在这种表示法中，矢量向右移(例如psrldq)实际上向右移.

Vectors are indexed from low element = 0. The low element is the one that stores into memory at the lowest address, but when values are in registers you should think about them as [ 3 2 1 0 ]. In this notation, vector right-shifts (like psrldq) actually shift to the right.

这就是为什么_mm_set_epi32(3, 2, 1, 0)以与int foo[] = { 0, 1, 2, 3 };相反的顺序获取其args的原因.

This is why _mm_set_epi32(3, 2, 1, 0) takes its args in reverse order from int foo[] = { 0, 1, 2, 3 };.

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