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clarify NaN propagation in fptrunc #68554

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Oct 26, 2023
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clarify NaN propagation in fptrunc #68554

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5 changes: 4 additions & 1 deletion llvm/docs/LangRef.rst
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Expand Up @@ -11311,7 +11311,10 @@ environment <floatenv>`.
NaN values follow the usual :ref:`NaN behaviors <floatnan>`, except that _if_ a
NaN payload is propagated from the input ("Quieting NaN propagation" or
"Unchanged NaN propagation" cases), then the low order bits of the NaN payload
which cannot fit in the resulting type are discarded.
which cannot fit in the resulting type are discarded. Note that if discarding
the low order bits leads to an all-0 payload, this cannot be represented as a
signaling NaN (it would represent an infinity instead), so in that case
"Unchanged NaN propagation" is not possible.

Example:
""""""""
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