From: Alan Modra <amodra@bigpond•net.au>
To: Jonathan Bartlett <johnnyb@eskimo•com>
Cc: linuxppc-dev@ozlabs•org
Subject: Re: Why the "opd" section?
Date: Tue, 25 Jul 2006 11:45:58 +0930 [thread overview]
Message-ID: <20060725021558.GG6872@bubble.grove.modra.org> (raw)
In-Reply-To: <Pine.SUN.4.58.0607232059070.699@eskimo.com>
On Sun, Jul 23, 2006 at 09:01:38PM -0700, Jonathan Bartlett wrote:
> I'm learning PPC64 assembly language, and I found the existence of the
> "opd" sections containing function descriptors quite odd. What is the use
> of these? Are they used by the linker? Why are they needed in the 64-bit
> ELF platforms and not the 32-bit ones?
OPD is an array of function pointers. Function pointers on powerpc64
are not just simple pointers to some code; They specify the code entry
point, the TOC pointer, and the static chain pointer (unused by C).
To call a function, you need to know all these values because functions
do not initialise their own TOC pointer. This allows for more efficient
code. The compiler/linker can omit the TOC pointer load when both
caller and callee are known to share the same TOC. (In many ways, the
TOC is like the powerpc32 GOT. powerpc32 -fpic/PIC code initialises the
GOT pointer on entry to every function, even when caller and callee are
known to have the same GOT pointer.)
--
Alan Modra
IBM OzLabs - Linux Technology Centre
next prev parent reply other threads:[~2006-07-25 2:59 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-07-24 4:01 Why the "opd" section? Jonathan Bartlett
2006-07-25 2:15 ` Alan Modra [this message]
2006-07-25 13:23 ` Jonathan Bartlett
2006-07-25 14:06 ` Alan Modra
2006-07-28 18:19 ` Linas Vepstas
2006-08-09 13:15 ` .tc entries question Jonathan Bartlett
2006-08-09 13:55 ` Alan Modra
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