netdev - Use of Indirect function calls

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Date:   Tue, 6 Mar 2018 19:35:46 -0800
From:   Rao Shoaib <rao.shoaib@...cle.com>
To:     netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Use of Indirect function calls

I am working on a change which introduces a couple of indirect function 
calls in the fast path. I used indirect function calls instead of 
"if/else" as it keeps the code cleaner and more readable and provides 
for extensibility.

I do not expect any measurable overhead as modern CPU's use pre-fetching 
and multiple parallel execution engines. Indirect jump prediction has 
gotten very good too. Assembly shows that the difference is an 
instruction to get the table address followed by a call to *callq 
instead of callq. I could not find official Intel numbers[1] but third 
party numbers suggest that *callq may have an over head of 1 clock 
cycle. Over head of accessing the table address is similar to reading a 
variable or testing a flag.

Will a patch that introduces indirect function calls be rejected just 
because it uses indirect function calls in the fast path or is the 
use/impact evaluated on a case by case basis ? I do see usage of 
indirect function calls in TCP fast path and Ethernet drivers so it 
can't be that bad.

Regards,

Shoaib

[1] I believe this is because the instruction is broken down into 
multiple uops and due to perfecting and parallel execution the latency 
is hard to measure.