[llvm] clarify NaN propagation in fptrunc (PR #68554)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

<details>
<summary>Changes</summary>

Follow-up to https://github.com/llvm/llvm-project/pull/66579: while implementing those semantics in Miri I realized there's a special case to be considered in truncating float casts.

Cc @<!-- -->nunoplopes @<!-- -->nikic @<!-- -->jyknight @<!-- -->jcranmer-intel 

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Full diff: https://github.com/llvm/llvm-project/pull/68554.diff


1 Files Affected:

- (modified) llvm/docs/LangRef.rst (+4-1) 


``````````diff
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 1883e9f6290b151..e27d4e0ed695319 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -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:
 """"""""

``````````

</details>


https://github.com/llvm/llvm-project/pull/68554