11.4.4. Run-Time ISA Selection - 5.2 English - 57404

For select functions, AOCL-LAPACK supports automatic processor dispatching to suitable code paths based on the target CPU ISA architecture. However, you can enable different ISA code path using environment variable, AOCL_ENABLE_INSTRUCTIONS. Valid values for AOCL_ENABLE_INSTRUCTIONS are SSE2, AVX, AVX2, AVX512 and GENERIC. All values are case-insensitive.

When you set AOCL_ENABLE_INSTRUCTIONS to an ISA value higher than that supported by a target CPU, AOCL-LAPACK chooses the code path that uses ISA supported by the CPU and which is also optimal for that CPU. If you choose a lower level ISA, then the code path using the lower level ISA will be chosen. Any ISA selection lower than AVX2 defaults to the generic reference code path.

Case 1: On an AVX2-only machine (example: AMD Zen1 / Zen2 / Zen3)

• Setting AOCL_ENABLE_INSTRUCTIONS=AVX2 will take avx2 path.

• Setting AOCL_ENABLE_INSTRUCTIONS=AVX512 will take avx2 path

• Setting AOCL_ENABLE_INSTRUCTIONS=generic or sse2 or avx will take reference path.

Case 2: On an AVX512 machine (example: Zen4 / Zen5)

• Setting AOCL_ENABLE_INSTRUCTIONS=AVX512 will take avx512 path

• Setting AOCL_ENABLE_INSTRUCTIONS=AVX2 will take avx2 path

• Setting AOCL_ENABLE_INSTRUCTIONS=generic or sse2 or avx will run reference path.

Case 3: Setting AOCL_ENABLE_INSTRUCTIONS to values other than avx512, avx2, avx, sse2, or generic will result in an error

Please note that enabling AMD flags for compilation sets the minimum ISA requirement to be AVX2 (see previous section). Hence, in these cases SSE2 and AVX options for AOCL_ENABLE_INSTRUCTIONS are not meaningful. They are applicable if the library was built without -mavx2 option.