Geometry shaders can be invoked more than one time for each input
primitive. This is known as geometry shader instancing and is requested by
including an OpExecutionMode instruction with mode specified as
Invocations and the number of invocations specified as an integer
literal.
In this mode, the geometry shader will execute
$n$
times for
each input primitive, where
$n$
is the number of invocations
specified in the OpExecutionMode instruction. The instance number is
available to each invocation as a built-in input using InvocationId.