From: Dmitry Potapov <dpotapov@gmail.com>
Date: Thu, 28 Apr 2011 16:36:27 GMT
Subject: Re: [PATCH] git gc: Speed it up by 18% via faster hash comparisons
Message-ID: <BANLkTinfc9J4vhWd9V+ZpXb5tumtZM1jZA@mail.gmail.com>
In-Reply-To: <20110428093703.GB15349@elte.hu>

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