From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dmitry Potapov <dpotapov@gmail.com>
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Date: Thu, 28 Apr 2011 20:36:27 +0400
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References: <20110427225114.GA16765@elte.hu>
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Cc: Erik Faye-Lund <kusmabite@gmail.com>,
	Junio C Hamano <gitster@pobox.com>, git@vger.kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
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	Arnaldo Carvalho de Melo <acme@redhat.com>,
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	Pekka Enberg <penberg@cs.helsinki.fi>
To: Ingo Molnar <mingo@elte.hu>
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2011/4/28 Ingo Molnar <mingo@elte.hu>:
>
> If unsigned char arrays are allocated unaligned then that's another bug i
> suspect that should be fixed. Unaligned access on x86 is not free either -
> there's cycle penalties.

Unsigned char arrays can be stored unaligned. Basically, it depends on
the context in what they were declared. If a preceding field in some
structure ended unaligned then the byte array will start unaligned.
For example:

struct foo
{
   char ch;
   unsigned char sha1[20];
};

The same on the stack, except the compiler may pack them as it wishes.
So, you have no guarantee here. If you want to make sure all SHA-1 are
aligned properly, sha1 should be declared as ui32: 'uint32_t sha1[5]'.


Dmitry