simd-intrinsics

SIMD Intrinsics

Purpose

Guide agents through SIMD: reading auto-vectorization output, writing SSE2/AVX2/NEON intrinsics, runtime CPU feature detection, and choosing between compiler auto-vectorization and manual intrinsics.

Triggers

• "How do I check if my loop is being auto-vectorized?"
• "How do I write SSE2/AVX2 intrinsics?"
• "Auto-vectorization failed — how do I fix it?"
• "How do I check for CPU features at runtime?"
• "Should I use intrinsics or let the compiler vectorize?"
• "How do I write NEON intrinsics for ARM?"

Workflow

1. Check auto-vectorization

# GCC: show vectorization info
gcc -O2 -march=native -fopt-info-vec src/hot.c -o hot

# Verbose: show missed + successful
gcc -O2 -march=native -fopt-info-vec-missed -fopt-info-vec-optimized src/hot.c

# Clang: vectorization remarks
clang -O2 -march=native \
    -Rpass=loop-vectorize \
    -Rpass-missed=loop-vectorize \
    -Rpass-analysis=loop-vectorize \
    src/hot.c -o hot

# Example missed message:
# hot.c:15:5: remark: loop not vectorized: value that could not be identified as
# reduction is used outside the loop [-Rpass-missed=loop-vectorize]

Common auto-vectorization blockers:

Blocker	Fix
Loop-carried dependency	Restructure to remove dependency
Data-dependent exit (early return)	Move exit after loop
Non-contiguous memory	Use gather/scatter or restructure
Aliasing (pointer may alias)	Add __restrict__ or restrict
Unknown trip count	Add __builtin_expect or hint
Function call in loop body	Inline the function

// Help the compiler by adding restrict
void add_arrays(float * __restrict__ dst,
                const float * __restrict__ a,
                const float * __restrict__ b,
                size_t n) {
    for (size_t i = 0; i < n; i++)
        dst[i] = a[i] + b[i];  // Now vectorizable
}

2. Runtime CPU feature detection

// Linux: use __builtin_cpu_supports (GCC/Clang)
if (__builtin_cpu_supports("avx2")) {
    process_avx2(data, len);
} else if (__builtin_cpu_supports("sse4.2")) {
    process_sse42(data, len);
} else {
    process_scalar(data, len);
}

// Check specific features:
__builtin_cpu_supports("sse2")
__builtin_cpu_supports("sse4.1")
__builtin_cpu_supports("sse4.2")
__builtin_cpu_supports("avx")
__builtin_cpu_supports("avx2")
__builtin_cpu_supports("avx512f")
__builtin_cpu_supports("bmi")
__builtin_cpu_supports("bmi2")
__builtin_cpu_supports("fma")