Developers compiling for AVX2 on Windows ARM under Prism emulation face a surprising performance penalty. Contrary to expectations that wider operations would compensate for emulation overhead, AVX2 code runs at 2/3 the speed of equivalent SSE2-SSE4.x optimized code. This finding emerged from testing math benchmarks using a vectorised math library integration.

Windows 11's Prism emulation layer translates x86/64 code into ARM instructions on the fly, with recent versions supporting AVX2 and FMA instructions. The author, testing with a multi-language compiler toolchain that targets native CPUs via LLVM, discovered this performance gap while investigating emulation overhead. Their benchmarks compared 21 different math operations running on both real x64 hardware and emulated environments.

The results show that despite AVX2's theoretical advantages for vectorised operations, the emulation layer introduces significant overhead that negates these benefits. For developers targeting Windows ARM, the clear recommendation is to compile for SSE2-4.x instruction sets rather than AVX2 when performance matters. This finding challenges assumptions about emulation efficiency and highlights the importance of instruction set selection in cross-platform development scenarios.