ARM NEON处理8个uint16_t生成指定q向量：ARMv7优化咨询

Let's break down how to optimize this vector transformation for ARMv7, since its NEON instruction set has different constraints than ARM64 (fewer registers, higher cost for moving data between NEON and general-purpose registers, and limited direct 128-bit lane operations).

First, Recap the Goal #

We start with:

uint16_t input[] = { 1, 2, 3, 4, 11, 22, 33, 44 };

And need to generate four 8-element vectors where each repeats a pair of low/high input values in the pattern [H, H, L, L, H, H, L, L]:

uint16x8_t q1 = { 11, 11, 1, 1, 11, 11, 1, 1 };
uint16x8_t q2 = { 22, 22, 2, 2, 22, 22, 2, 2 };
uint16x8_t q3 = { 33, 33, 3, 3, 33, 33, 3, 3 };
uint16x8_t q4 = { 44, 44, 4, 4, 44, 44, 4, 4 };

The ARMv7 NEON Optimization Approach #

The biggest performance killer in naive ARMv7 implementations is moving data between NEON and general-purpose registers—this is a slow, high-overhead operation. Instead, we'll use NEON's table lookup instruction (vqtbl1q_u16) to keep all operations within NEON registers.

Step 1: Load the Input Vector

First, load the entire input array into a single 128-bit NEON vector in one go:

uint16x8_t input_vec = vld1q_u16(input);

Step 2: Define Lookup Index Vectors

We need index vectors that tell the lookup instruction which elements to pick from input_vec to build our target vectors. Each index vector maps directly to the repeated low/high value pattern we need:

// Indices for q1: pick input[4] (11) twice, input[0] (1) twice, repeat
const uint8_t idx1[] = {4, 4, 0, 0, 4, 4, 0, 0};
// Indices for q2: pick input[5] (22) twice, input[1] (2) twice, repeat
const uint8_t idx2[] = {5, 5, 1, 1, 5, 5, 1, 1};
// Indices for q3: pick input[6] (33) twice, input[2] (3) twice, repeat
const uint8_t idx3[] = {6, 6, 2, 2, 6, 6, 2, 2};
// Indices for q4: pick input[7] (44) twice, input[3] (4) twice, repeat
const uint8_t idx4[] = {7, 7, 3, 3, 7, 7, 3, 3};

// Load indices into NEON vectors (efficient, single-cycle operations)
uint8x8_t idx_vec1 = vld1_u8(idx1);
uint8x8_t idx_vec2 = vld1_u8(idx2);
uint8x8_t idx_vec3 = vld1_u8(idx3);
uint8x8_t idx_vec4 = vld1_u8(idx4);

Step 3: Generate Target Vectors with Table Lookup

Use vqtbl1q_u16 to extract elements from input_vec using our index vectors. This instruction operates entirely within NEON registers, eliminating costly cross-domain data transfers:

uint16x8_t q1 = vqtbl1q_u16(input_vec, idx_vec1);
uint16x8_t q2 = vqtbl1q_u16(input_vec, idx_vec2);
uint16x8_t q3 = vqtbl1q_u16(input_vec, idx_vec3);
uint16x8_t q4 = vqtbl1q_u16(input_vec, idx_vec4);

Why This Works Better on ARMv7 #

• No cross-register overhead: Unlike approaches that extract single elements to ARM general-purpose registers then broadcast them, this keeps all operations in NEON—critical for ARMv7 performance.
• Minimal instruction count: This uses just 1 input load, 4 index loads, and 4 lookup instructions—total 9 NEON operations with no wasted cycles.
• High throughput: vqtbl1q_u16 is a single-cycle instruction on most ARMv7 microarchitectures (like Cortex-A9/A15), so the lookup operations pipeline efficiently.

Compiler Optimization Tips #

To maximize performance, compile with these flags to ensure the compiler leverages NEON fully:

-march=armv7-a -mfpu=neon -mfloat-abi=hard -O3

This targets ARMv7 with NEON enabled, uses the hard floating-point ABI (faster NEON integration), and applies aggressive optimizations.

内容的提问来源于stack exchange，提问作者Pavel P