linux-kernel - Re: [PATCH 4/8] random: remove unused branch in hot code path

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Message-ID: <20170118043536.aeqmxfthmx4h33g7@thunk.org>
Date:   Tue, 17 Jan 2017 23:35:36 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Stephan Müller <smueller@...onox.de>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH 4/8] random: remove unused branch in hot code path

On Tue, Dec 27, 2016 at 11:40:23PM +0100, Stephan Müller wrote:
> The variable ip is defined to be a __u64 which is always 8 bytes on any
> architecture. Thus, the check for sizeof(ip) > 4 will always be true.
> 
> As the check happens in a hot code path, remove the branch.

The fact that it's a hot code path means the compiler will optimize it
out, so the fact that it's on the hot code path is irrelevant.  The
main issue is that on platforms with a 32-bit IP's, ip >> 32 will
always be zero.  It might be that we can just do this via

#if BITS_PER_LONG == 32
  ...
#else
  ...  
#endif

I'm not sure that works for all platforms, though.  More research is
needed...

					- Ted