[LLVMdev] [ARM] Mixing rel/rela relocations

[Jack Carter]

________________________________________
> From: Shankar Easwaran [shankare at codeaurora.org]
> Sent: Monday, November 04, 2013 11:44 AM
> To: Jack Carter; Eric Christopher
> Cc: llvmdev Dev
> Subject: Re: [ARM] Mixing rel/rela relocations
>
> On 11/4/2013 1:40 PM, Jack Carter wrote:
>> On 11/04/2013 11:15 AM, Eric Christopher wrote:
>>>
>>>
>>>
>>> On Mon, Nov 4, 2013 at 11:05 AM, Shankar Easwaran
>>> <shankare at codeaurora.org <mailto:shankare at codeaurora.org>> wrote:
>>>
>>>     Hi,
>>>
>>>     I was looking at the ARM ABI
>>> docs(http://infocenter.arm.__com/help/topic/com.arm.doc.__ihi0044e/IHI0044E_aaelf.pdf
>>> <http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044e/IHI0044E_aaelf.pdf>)
>>>     and they mention.
>>>
>>>     "A binary file may use REL or RELA relocations or a mixture of the
>>>     two (but multiple relocations for the same
>>>     address must use only one type)."
>>>
>>>     Does LLVM emit rel/rela relocations with ARM ?
>>>
>>>
>>> I hope not. Ew.
>>
>> The only reason this is Ew is that llvm has decided not to prepare for
>> it. There shouldn't be a single rel/rela type for a given output. The
>> one that works for the given situation for the given section should be
>> used. The mechanism for handling both should not be complicated.
>>
>> There are rules about relocation order and multiple relocations can
>> operate on a single point in the binary. There is no reason a section
>> of .rel relocations cannot operate alongside a section of .rela
>> relocations.
>>
> Is there a reason behind this, if the compiler doesit ? anything on
> performance ? It makes tools that operate on the object files bit rusty.

History. In the beginning there was REL, thus there is a lot of binaries with REL. 
Then came along RELA and people didn't want things to break so they allowed
both in the same binaries. Besides, RELA is bigger than REL which keeps its
addend in the instruction or data point. Size might not be an issue for you, but
it is for some.

Personally, I don't care which is used and where. I just assume I have to deal 
with both. It does create somewhat redundant code, but that's life.

Jack

> I would have preferred using RELA all over.

> Thanks

> Shankar Easwaran