Simd Instructions

SIMD / Vectorization — executing a Single Instruction on Multiple Data elements simultaneously using wide vector registers, achieving 4–16x speedup on data-parallel operations without multiple cores.

How SIMD Works

Scalar (1 at a time):      SIMD (4 at a time):
a[0] = b[0] + c[0]        a[0..3] = b[0..3] + c[0..3]
a[1] = b[1] + c[1]        (single instruction!)
a[2] = b[2] + c[2]
a[3] = b[3] + c[3]
4 instructions             1 instruction

x86 SIMD Evolution

• SSE (1999): 128-bit registers → 4× float32 or 2× float64
• AVX (2011): 256-bit registers → 8× float32
• AVX-512 (2017): 512-bit registers → 16× float32
• AMX (2023): Matrix extensions for AI inference on CPU

ARM SIMD

• NEON: 128-bit. Available on all modern ARM (phones, Apple Silicon)
• SVE/SVE2: Scalable Vector Extension. Variable-width (128–2048 bit). Used in ARM server CPUs

Auto-Vectorization

• Modern compilers (GCC, Clang, MSVC) automatically vectorize simple loops
• Flags: `-O2 -march=native` (GCC/Clang)
• Limitations: Complex control flow, data dependencies, non-contiguous access patterns prevent auto-vectorization

Manual SIMD

• Intrinsics: `__m256 result = _mm256_add_ps(a, b);`
• Used in performance-critical libraries (NumPy, OpenBLAS, FFmpeg)

SIMD is free parallelism within a single core — essential for high-performance numeric computation, media processing, and AI inference on CPUs.