linux-ext4 - Re: [PATCH] lib/ext2fs: Use __builtin_popcount when available Signed-off-by: Cristian Rodríguez <crrodriguez@...nsuse.org>

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Message-ID: <20130107013156.GB12838@thunk.org>
Date:	Sun, 6 Jan 2013 20:31:56 -0500
From:	Theodore Ts'o <tytso@....edu>
To:	Cristian Rodríguez <crrodriguez@...nsuse.org>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] lib/ext2fs: Use __builtin_popcount when available Signed-off-by: Cristian Rodríguez <crrodriguez@...nsuse.org>

On Sun, Jan 06, 2013 at 09:53:47PM -0300, Cristian Rodríguez wrote:
> 
> Yeah, I asked GCC developers exactly this, was told to fill a
> enhancement request.

If you could also sned them a bug/enhancement request to use a more
optimized version of __popcountdi2, that would be great.  I'm not sure
it helps e2fsprogs much, since it's too hard for us to tell whether we
are using a version of the gcc runtime that has a optimized or
unuptomized version of builtin_popcount().

But since it doesn't make that much difference, my preference is to
just ignore builtin_popcount() for now.  If someone is really using
128TB ext4 file systems, and cares about that extra 6 seconds of CPU,
it's probably going to require the ugly approach of using x86 asm
statements to determine whether or not we're running on a CPU that
supports the popcount instruction or not....

						- Ted

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