https://github.com/sjoerdmeijer created
https://github.com/llvm/llvm-project/pull/127621
We had an internal discussion about -ffp-contract, how it compared to GCC which
defaults to fast, and standard compliance. Looking at our docs, I think most
information is there, but also thought it could be a little bit more explicit
about when it is and isn't standard compliant. Let me know if you think this is
an improvement or not; happy to abandon this if you think it is not helping
much.
>From fd2cb4b7d69fd1c8d29ea8066e707f48e0576f96 Mon Sep 17 00:00:00 2001
From: Sjoerd Meijer <smei...@nvidia.com>
Date: Tue, 18 Feb 2025 03:37:08 -0800
Subject: [PATCH] [Clang][doc] -ffp-contract options and standard compliance
We had an internal discussion about -ffp-contract, how it compared to
GCC which defaults to fast, and standard compliance. Looking at our
docs, I think most information is there, but also thought it could be a
little bit more explicit about when it is and isn't standard compliant.
Let me know if you think this is an improvement or not; happy to abandon
this if you think it is not helping much.
---
clang/docs/UsersManual.rst | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index d977868b8a2c6..8809632479a00 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -1681,19 +1681,25 @@ 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/C++ standard permits 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 non-C/C++ standard
+ compliant behavior and can be enabled (``fast``).
+
+ Fusion 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.
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 non-C/C++ standard compliant behavior and fusion across
+ statements disregarding pragmas (default for CUDA)
+ * ``on``: enable C/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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