[llvm] [GlobalISel] Update the documentation of abd. (PR #164594)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-globalisel

Author: David Green (davemgreen)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/164594.diff


1 Files Affected:

- (modified) llvm/docs/GlobalISel/GenericOpcode.rst (+1-1) 


``````````diff
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
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/164594