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

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Date:	Sun, 06 Jan 2013 21:53:47 -0300
From:	Cristian Rodríguez <crrodriguez@...nsuse.org>
To:	Theodore Ts'o <tytso@....edu>
CC:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] lib/ext2fs: Use __builtin_popcount when available Signed-off-by: Cristian Rodríguez <crrodriguez@...nsuse.org>

El dom 06 ene 2013 19:20:20 CLST, Theodore Ts'o escribió:


> I could imagine a patch which explicitly checks the CPU type using the
> x86 cpuinfo instructure, and only optionally uses the popcount
> instruction if it exists, and uses the optimized code if it doesn't,
> but then again, it's not clear how much it's worth it.

The first incarnation of this patch did exactly that, too ugly didnt 
not send it. ;)

> Better yet would be if gcc was taught how to recognize the C
> statements in popcount32(), since that's one of the standard,
> intelligent ways of implementing popcount32 (as opposed to the stupid
> way which is apparently used by gcc's runtime library, sigh), and
> inserted code which automatically did the cpuinfo check and fallback
> to popcount if the CPU supports it.  This would be even better since
> then gcc could do a single cpuinfo check, and then automatically use
> the faster SSE[234] instructions as necessary, without having to ask
> application programmers to put in gcc-specific __builtin_* functions

Yeah, I asked GCC developers exactly this, was told to fill a 
enhancement request.

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