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[LangRef] Clarify the behavior of select with FP poison-generating flags #137131

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Merged
merged 4 commits into from
Apr 25, 2025
Merged

[LangRef] Clarify the behavior of select with FP poison-generating flags #137131

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bf12fa7
[LangRef] Clarify the behavior of select with FP poison-generating flags
dtcxzyw Apr 24, 2025
ed85a5a
[LangRef] Rephrase the wording
dtcxzyw Apr 24, 2025
74f823b
[LangRef] Address review comments. NFC.
dtcxzyw Apr 24, 2025
e42b2ba
[LangRef] Address review comments.
dtcxzyw Apr 25, 2025
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9 changes: 7 additions & 2 deletions llvm/docs/LangRef.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -12880,7 +12880,10 @@ class <t_firstclass>` type.
:ref:`fast-math flags <fastmath>`. These are optimization hints to enable
otherwise unsafe floating-point optimizations. Fast-math flags are only valid
for selects that return :ref:`supported floating-point types
<fastmath_return_types>`.
<fastmath_return_types>`. Note that the presence of value which would otherwise result
in poison does not cause the result to be poison if the value is on the non-selected arm.
If :ref:`fast-math flags <fastmath>` are present, they are only applied to the result,
not both arms.

Semantics:
""""""""""
Expand All @@ -12900,7 +12903,9 @@ Example:

.. code-block:: llvm

%X = select i1 true, i8 17, i8 42 ; yields i8:17
%X = select i1 true, i8 17, i8 42 ; yields i8:17
%Y = select nnan i1 true, float 0.0, float NaN ; yields float:0.0
%Z = select nnan i1 false, float 0.0, float NaN ; yields float:poison


.. _i_freeze:
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