pitrou commented on code in PR #40374:
URL: https://github.com/apache/arrow/pull/40374#discussion_r1520397630
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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
Review Comment:
Ok, perhaps instead of framing this note in terms of CMake configuration
variables, we should explain the two different kinds of SIMD optimizations?
This would be more user-oriented (this is a page about environment variables
after all).
Something like:
> In addition to runtime-selected SIMD optimizations dispatch, Arrow C++ can
also be compiled
> with SIMD optimizations that are cannot be disabled at runtime. For
example, by default, SSE4.2
> optimizations are enabled on x86 builds: therefore, with this default
setting, Arrow C++ does
> not work at all on a CPU without support for SSE4.2. This setting can be
changed using
> the``ARROW_SIMD_LEVEL`` CMake variable so as to either raise or lower the
optimization level.
>
> Finally, the ``ARROW_RUNTIME_SIMD_LEVEL`` CMake variable sets a
compile-time upper bound
> to runtime-selected SIMD optimizations. This useful in cases where a
compiler reports support
> for an instruction set but does not actually support it in full.
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