[PATCH] D148637: [LangRef] Update shufflevector's semantics to return poison if the mask is undef

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

I'm fine with the semantics change, but I think there are too many words here.



================
Comment at: llvm/docs/LangRef.rst:10026
+using ``undef`` for ``val`` is not recommended because it prohibits
+optimizations to '``shufflevector``' with ``poison`` masks.
+
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I don't think this sentence really adds anything. It just introduces a new ill-defined term "don't-care vector".


================
Comment at: llvm/docs/LangRef.rst:10086-10101
+If the shuffle mask is ``poison``, the result vector is ``poison``. If
+the shuffle mask selects a ``poison`` element from one of the input vectors,
+the resulting element is ``poison``. Similarly, if an ``undef`` element
+is selected, the result is ``undef`` as well. A ``poison`` element
+in the mask vector specifies that the resulting element is ``poison``.
+
+If the shuffle mask is ``undef``, the result vector is ``poison``. An ``undef``
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I feel like we could use a lot less words here and be less confusing at the same time.


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https://reviews.llvm.org/D148637