[GlobalISel] Update the documentation of abd. #164594
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The abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs. Update the description of the instruction to avoid misunderstandings.
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@llvm/pr-subscribers-llvm-globalisel Author: David Green (davemgreen) ChangesThe abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs and so produce different results. Update the description of the instruction to avoid misunderstandings. Full diff: https://github.com/llvm/llvm-project/pull/164594.diff 1 Files Affected:
diff --git a/llvm/docs/GlobalISel/GenericOpcode.rst b/llvm/docs/GlobalISel/GenericOpcode.rst
index b055327466739..661a11537cf57 100644
--- a/llvm/docs/GlobalISel/GenericOpcode.rst
+++ b/llvm/docs/GlobalISel/GenericOpcode.rst
@@ -504,7 +504,7 @@ undefined.
G_ABDS, G_ABDU
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-Compute the absolute difference (signed and unsigned), e.g. abs(x-y).
+Compute the absolute difference (signed and unsigned), e.g. trunc(abs(ext(x)-ext(y)).
.. code-block:: none
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arsenm
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Oct 22, 2025
I mean thet do if x - y does not overflow. But yeah. |
dvbuka
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Oct 27, 2025
The abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs and so produce different results. Update the description of the instruction to avoid misunderstandings.
Lukacma
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Oct 29, 2025
The abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs and so produce different results. Update the description of the instruction to avoid misunderstandings.
aokblast
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Oct 30, 2025
The abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs and so produce different results. Update the description of the instruction to avoid misunderstandings.
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The abd nodes do not perform the same as abs(x-y), although they are often mistaken to do so. They extend into a larger bit size before performing the sub / abs and so produce different results. Update the description of the instruction to avoid misunderstandings.