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Re: REL vs. RELA: how to choose?
- To: Greg McGary <greg at mcgary dot org>
- Subject: Re: REL vs. RELA: how to choose?
- From: Alan Modra <alan at linuxcare dot com dot au>
- Date: Thu, 15 Feb 2001 11:25:50 +1100 (EST)
- cc: binutils at sources dot redhat dot com
On Wed, 14 Feb 2001, Greg McGary wrote:
> What are the processor-architecture features that might make one or
> the other of REL vs. RELA necessary or more convenient? This has been
> a nagging question for some time now.
REL is a bad choice for architectures that store values in instructions in
odd ways. eg. parisc branch insn looks like
| | w1 | | w2 | | w |
| 11 bits | 5 bit | 3 bits | 11 bits | 1 bit | 1 bit |
with the actual offset being calculated as
sign_extend ((w << 16) | (w1 << 10) | ((w2 & 1) << 9) | (w2 >> 1)) << 2
or something like that.
Can you imagine the hoops the linker would have to go through dealing with
relocations for a REL parisc object file?
REL is also bad because it complicates overflow checking in the linker as
it's typically not possible to store a full address in the instruction.
RELA is nearly always the better choice, except where smaller object file
size is very important. What's the price of 40G disks today?
Alan Modra
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