docs

Author: Thorsten Schütt

llvm/docs/GlobalISel/GenericOpcode.rst

@@ -760,7 +760,8 @@ Create a scalable vector where all lanes are linear sequences starting at 0
 with a given unsigned step.
 
 The type of the operand must be equal to the vector element type. Arithmetic
-is performed modulo the bitwidth of the element.
+is performed modulo the bitwidth of the element. The step must be > 0.
+Otherwise the vector is zero. The canonical is splat vector of zeros.
 
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