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[Clang][doc] -ffp-contract options and standard compliance #127621

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24 changes: 16 additions & 8 deletions clang/docs/UsersManual.rst
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Expand Up @@ -1681,19 +1681,27 @@ for more details.
permitted to produce more precise results than performing the same
operations separately.

The C standard permits intermediate floating-point results within an
The C and C++ standards permit intermediate floating-point results within an
expression to be computed with more precision than their type would
normally allow. This permits operation fusing, and Clang takes advantage
of this by default. This behavior can be controlled with the ``FP_CONTRACT``
and ``clang fp contract`` pragmas. Please refer to the pragma documentation
for a description of how the pragmas interact with this option.
of this by default (``on``). Fusion across statements is not compliant with
the C and C++ standards but can be enabled using ``-ffp-contract=fast``.

Fusion can be controlled with the ``FP_CONTRACT`` and ``clang fp contract``
pragmas. Please note that pragmas will be ingored with
``-ffp-contract=fast``, and refer to the pragma documentation for a
description of how the pragmas interact with the different ``-ffp-contract``
option values.

Valid values are:

* ``fast`` (fuse across statements disregarding pragmas, default for CUDA)
* ``on`` (fuse in the same statement unless dictated by pragmas, default for languages other than CUDA/HIP)
* ``off`` (never fuse)
* ``fast-honor-pragmas`` (fuse across statements unless dictated by pragmas, default for HIP)
* ``fast``: enable fusion across statements disregarding pragmas, breaking
compliance with the C and C++ standards (default for CUDA).
* ``on``: enable C and C++ standard complaint fusion in the same statement
unless dictated by pragmas (default for languages other than CUDA/HIP)
* ``off``: disable fusion
* ``fast-honor-pragmas``: fuse across statements unless dictated by pragmas
(default for HIP)

.. option:: -f[no-]honor-infinities

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