presto-main/src/main/java/io/prestosql/sql/gen/PageFunctionCompiler.java/PageFunctionCompiler/declareBlockVariables
class PageFunctionCompiler:    private static void declareBlockVariables(RowExpression expression, Parameter page, Scope scope, BytecodeBlock body)
    {
        for (int channel : getInputChannels(expression)) {
            scope.declareVariable("block_" + channel, body, page.invoke("getBlock", Block.class, constantInt(channel)));
        }
    }