wjones127 commented on code in PR #40374:
URL: https://github.com/apache/arrow/pull/40374#discussion_r1513400339
##########
docs/source/cpp/env_vars.rst:
##########
@@ -148,19 +152,27 @@ 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 maximum SIMD level at the
+ compiler level. Unlike runtime dispatch, compile-time SIMD optimizations
Review Comment:
For clarity, maybe we should share the mechanism. Also, I think it's more
appropriate to call this the *minimum* SIMD level, as it is older computers
that won't support this.
```suggestion
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
```
https://github.com/apache/arrow/blob/3d467ac7bfae03cf2db09807054c5672e1959aec/cpp/cmake_modules/SetupCxxFlags.cmake#L47-L67
##########
docs/source/cpp/env_vars.rst:
##########
@@ -148,19 +152,27 @@ 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 maximum SIMD level at the
+ compiler level. 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
Review Comment:
```suggestion
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
```
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