Simd Intrinsics

SIMD Intrinsics are low-level C/C++ functions that map directly to SIMD (Single Instruction Multiple Data) CPU instructions — bypassing the compiler to explicitly exploit vector registers for processing 4, 8, 16, or 32 data elements per instruction.

SIMD Evolution on x86

Example: AVX2 Vectorized Addition

#include <immintrin.h>

void add_arrays(float* a, float* b, float* c, int n) {
    for (int i = 0; i < n; i += 8) {
        __m256 va = _mm256_loadu_ps(a + i);    // Load 8 floats
        __m256 vb = _mm256_loadu_ps(b + i);
        __m256 vc = _mm256_add_ps(va, vb);     // Add 8 pairs in parallel
        _mm256_storeu_ps(c + i, vc);           // Store 8 results
    }
}

Key Intrinsic Categories

• Load/Store: `_mm256_load_ps`, `_mm256_loadu_ps` (unaligned).
• Arithmetic: `_mm256_add_ps`, `_mm256_mul_ps`, `_mm256_fmadd_ps` (FMA).
• Compare: `_mm256_cmp_ps` → mask for conditional operations.
• Shuffle/Permute: `_mm256_permute_ps` — rearrange elements within vector.
• Masked (AVX-512): `_mm512_mask_add_ps` — lane-selective operations.

FMA (Fused Multiply-Add)

• `_mm256_fmadd_ps(a, b, c)` = a×b + c in single instruction.
• 2x throughput vs. separate mul+add.
• Key for GEMM, dot products, convolution.

When to Use Intrinsics vs. Auto-Vectorization

• Compiler auto-vec: Often sufficient for simple loops.
• Intrinsics: When compiler fails (complex control flow, precision requirements, special shuffles).
• Profile first: Ensure vectorization is the actual bottleneck.

SIMD intrinsics are the highest-performance path for compute-intensive loops — critical path optimization in media codecs, ML inference engines, database scans, and scientific simulations routinely requires explicit vectorization to approach peak hardware throughput.