pitrou commented on code in PR #40374:
URL: https://github.com/apache/arrow/pull/40374#discussion_r1514168156
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docs/source/cpp/env_vars.rst:
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@@ -148,19 +152,29 @@ that changing their value later will have an effect.
platforms currently do not implement any form of runtime dispatch.
.. note::
- In addition to runtime dispatch, the compile-time SIMD level can
- be set using the ``ARROW_SIMD_LEVEL`` CMake configuration variable.
- Unlike runtime dispatch, compile-time SIMD optimizations cannot be
- changed at runtime (for example, if you compile Arrow C++ with AVX512
- enabled, the resulting binary will only run on AVX512-enabled CPUs).
- Setting ``ARROW_USER_SIMD_LEVEL=NONE`` prevents the execution of
- explicit SIMD optimization code, but it does not rule out the execution
- of compiler generated SIMD instructions. E.g., on x86_64 platform,
- Arrow is built with ``ARROW_SIMD_LEVEL=SSE4_2`` by default. Compiler
- may generate SSE4.2 instructions from any C/C++ source code. On legacy
- x86_64 platforms do not support SSE4.2, Arrow binary may fail with
- SIGILL (Illegal Instruction). User must rebuild Arrow and PyArrow from
- scratch by setting cmake option ``ARROW_SIMD_LEVEL=NONE``.
+ In addition to runtime dispatch, Arrow's build system supports two
+ compile-time CMake variables for controlling SIMD: ``ARROW_SIMD_LEVEL``
+ and ``ARROW_RUNTIME_SIMD_LEVEL``.
+
+ The ``ARROW_SIMD_LEVEL`` variable sets the minimum supported SIMD level
at
+ the compiler level. This works by passing the appropriate target
+ architecture flag, such as ``-march=haswell`` for AVX2, to the compiler.
+ Unlike runtime dispatch, compile-time SIMD optimizations cannot be
changed
+ at runtime (for example, if you compile Arrow C++ with AVX512 enabled,
the
+ resulting binary will only run on AVX512-enabled CPUs). Setting
+ ``ARROW_USER_SIMD_LEVEL=NONE`` prevents the execution of explicit SIMD
+ optimization code, but it does not rule out the execution of
+ compiler-generated SIMD instructions. E.g., on x86, Arrow is built with
+ ``ARROW_SIMD_LEVEL=SSE4_2`` by default. The compiler may generate SSE4.2
+ instructions from any C/C++ source code. On legacy x86 platforms which
do
+ not support SSE4.2, an Arrow binary may fail with SIGILL (Illegal
+ Instruction). In this case, one must rebuild Arrow and PyArrow from
Review Comment:
PyArrow shouldn't really be mentioned here, these are the C++ docs.
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