Explore Auto-Vectorization by Modern Compilers

Compiler Vectorization

Modern Clang and GCC scan loops for SIMD opportunities when -O3 -march=native and #pragma omp simd or __restrict__ prove independence. The compiler emits AVX2/NEON instructions you never wrote in source.

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When Autovec Fails

• Aliasing (c might overlap a)
• Complex control flow inside the loop
• Non-countable loops or function calls
• Reduction ordering the compiler cannot prove safe

For example, adding __restrict__ on pointers often unlocks vectorization identical to hand intrinsics.

For this exercise, you will toggle pragmas and compiler flags on the same kernel and dump assembly with objdump -d. This task asks you to document one loop the compiler vectorizes and one it refuses, with the reported reason from optimization remarks.

Working Through the Exercise

Keep the relevant datasheet, ISA manual, or architecture textbook chapter open while you implement. When your output disagrees with the reference trace on the same program, the bug is usually a mis-decoded opcode, a stale register read, or a flag bit left unchanged after arithmetic.

• Trace first: walk one failing input by hand before changing random lines
• Read the manual: skim the official doc for the mechanism named in this task
• Compare baselines: save measurements from the prior step so speedups are honest

For this exercise, you will use those habits while implementing the requirement in the starter code. Microarchitectural product names change across CPU generations, but the control ideas (fetch, bypass, cache lines, vector lanes) stay stable enough to debug from first principles.