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From: Jeff King <peff@peff.net>
To: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Cc: "Junio C Hamano" <gitster@pobox.com>,
	"Stefan Beller" <stefanbeller@gmail.com>,
	"Arjun Sreedharan" <arjun024@gmail.com>,
	"Git Mailing List" <git@vger.kernel.org>,
	"Christian Couder" <chriscool@tuxfamily.org>,
	"Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>
Subject: Re: [PATCH] bisect: save heap memory. allocate only the required amount
Date: Tue, 26 Aug 2014 08:14:00 -0400	[thread overview]
Message-ID: <20140826121359.GA29180@peff.net> (raw)
In-Reply-To: <53FC7621.7090102@ramsay1.demon.co.uk>

On Tue, Aug 26, 2014 at 12:57:21PM +0100, Ramsay Jones wrote:

> > +	ret = xcalloc(1, base + extra);
> > +	va_start(ap, fmt);
> > +	vsnprintf(ret + offset, extra, fmt, ap);
> 
> What is the relationship between 'base' and 'offset'?
> 
> Let me assume that base is always, depending on your compiler, either
> equal to offset or offset+1. Yes? (I'm assuming base is always the
> sizeof(struct whatever)). Do you need both base and offset?

It's much more complicated than that. Take "struct name_decoration", for
instance, which looks like this:

  struct name_decoration {
	struct name_decoration *next;
	int type;
	char name[FLEX_ARRAY];
  };

On my 64-bit system using gcc, sizeof() returns 16; it has to pad the
whole thing to 64-bit alignment in case I put two of them in an array.
But offsetof(name) is 12, since the array of char does not need the same
alignment; it can go right after the type and make use of the padding
bits.

As a side note, that means that the original "char name[1]" (before it
became FLEX_ARRAY) was not any less efficient on 64-bit machines (the
1-byte went into the padding, and sizeof() was the same). It did matter
on 32-bit systems, though where it bumped the empty struct size from 12
to 16.

-Peff

  reply	other threads:[~2014-08-26 12:14 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-08-24 14:17 [PATCH] bisect: save heap memory. allocate only the required amount Arjun Sreedharan
2014-08-24 15:10 ` Stefan Beller
2014-08-24 23:39   ` Junio C Hamano
2014-08-25 13:07     ` Jeff King
2014-08-25 21:36       ` Junio C Hamano
2014-08-26 11:03         ` Jeff King
2014-08-26 11:57           ` Ramsay Jones
2014-08-26 12:14             ` Jeff King [this message]
2014-08-26 12:37               ` Ramsay Jones
2014-08-26 12:43                 ` Jeff King
2014-08-26 12:59                   ` Ramsay Jones
2014-08-24 15:32 ` Ramsay Jones
2014-08-24 21:55   ` Arjun Sreedharan
2014-08-25 13:35 ` Jeff King
2014-08-25 14:06   ` Christian Couder
2014-08-25 15:00     ` Jeff King
2014-08-25 18:26       ` Junio C Hamano
2014-08-25 19:35         ` Jeff King
2014-08-25 20:27           ` Arjun Sreedharan
2014-08-25 21:11           ` Junio C Hamano
2014-08-26 10:20             ` [PATCH 0/3] name_decoration cleanups Jeff King
2014-08-26 10:23               ` [PATCH 1/3] log-tree: make add_name_decoration a public function Jeff King
2014-08-26 11:48                 ` Ramsay Jones
2014-08-26 12:17                   ` Jeff King
2014-08-26 10:23               ` [PATCH 2/3] log-tree: make name_decoration hash static Jeff King
2014-08-26 17:40                 ` Junio C Hamano
2014-08-26 17:43                   ` Jeff King
2014-08-26 19:25                     ` Junio C Hamano
2014-08-26 10:24               ` [PATCH 3/3] log-tree: use FLEX_ARRAY in name_decoration Jeff King
2014-08-27  0:30                 ` Eric Sunshine
2014-08-25 21:14 ` [PATCH] bisect: save heap memory. allocate only the required amount Junio C Hamano
2014-08-26  7:30   ` Arjun Sreedharan

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