llvm dev - Apr 2013 - [LLVMdev] How to prevent LLVM from emitting R_X86_64_32 ELF relocations?

On 04/26/2013 19:50, Jiong Wang wrote:
>
>
>   why do you want to prevent R_X86_64_32 generation? for 32bit dwarf, 
> I think generation of R_X86_64_32 is reasonable.
>
>   you can check http://dwarfstd.org/doc/DWARF4.pdf. 
Because R_X86_64_32 elements are 4-byte addresses and can't be relocated 
for the 64-bit address space over 32-bit limit.

Dwarf2 actually allows for both 32bit and 64-bit relocations. The 
document, mentioned by you, explains this in section 7.5.1.1 on page 
143. If the first DWORD of .debug_info section is 0xffffffff, then this 
is 64-bit format.

But LLVM for some reason always chooses DWARF 32-bit format. This is why 
I asked the question. Resulting relocable 64-bit objects can't be loaded 
into addresses that are over 32-bit limit.

Yuri

On 04/27/2013 11:02 AM, Yuri wrote:
> On 04/26/2013 19:50, Jiong Wang wrote:
>>
>>
>>   why do you want to prevent R_X86_64_32 generation? for 32bit dwarf, 
>> I think generation of R_X86_64_32 is reasonable.
>>
>>   you can check http://dwarfstd.org/doc/DWARF4.pdf. 
>
> Because R_X86_64_32 elements are 4-byte addresses and can't be 
> relocated for the 64-bit address space over 32-bit limit.
>
> Dwarf2 actually allows for both 32bit and 64-bit relocations. The 
> document, mentioned by you, explains this in section 7.5.1.1 on page 
> 143. If the first DWORD of .debug_info section is 0xffffffff, then 
> this is 64-bit format.
>
> But LLVM for some reason always chooses DWARF 32-bit format. This is 
> why I asked the question. Resulting relocable 64-bit objects can't be 
> loaded into addresses that are over 32-bit limit.
there is no support for Dwarf 64 yet. From what I known, there is no way 
you can get rid of that.

looks like there are some discussions on this already, you can search 
bugzilla.

http://llvm.org/bugs/show_bug.cgi?id=14969
http://llvm.org/bugs/show_bug.cgi?id=15173


>
> Yuri

-- 
Regards,
Jiong. Wang
Tilera Corporation.

I think that you may have confused 2 separate issues here; the size of the
DWARF tables with the size of addresses on the target system.

The 32-bit DWARF vs. 64-bit DWARF is all about the size of the tables in the
DWARF sections (and hence also the size of the references between entries in
the tables).

So a 32-bit DWARF section can correctly describe code/data addresses that
are 64-bit, and any references to variables/functions will require 64-bit
relocation entries.

However in this case the DWARF sections are effectively limited in size, and
references between one DWARF entry and another will use 32-bit relocation
entries.   It is these 32-bit relocations entries that I think you are
seeing in your image.

You only need to go to 64-bit DWARF when your debugging information becomes
too large to fit in 32-bit DWARF tables.

Keith

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Yuri
Sent: 27 April 2013 04:02
To: Jiong Wang
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] How to prevent LLVM from emitting R_X86_64_32 ELF
relocations?

On 04/26/2013 19:50, Jiong Wang wrote:
>
>
>   why do you want to prevent R_X86_64_32 generation? for 32bit dwarf, 
> I think generation of R_X86_64_32 is reasonable.
>
>   you can check http://dwarfstd.org/doc/DWARF4.pdf. 
Because R_X86_64_32 elements are 4-byte addresses and can't be relocated 
for the 64-bit address space over 32-bit limit.

Dwarf2 actually allows for both 32bit and 64-bit relocations. The 
document, mentioned by you, explains this in section 7.5.1.1 on page 
143. If the first DWORD of .debug_info section is 0xffffffff, then this 
is 64-bit format.

But LLVM for some reason always chooses DWARF 32-bit format. This is why 
I asked the question. Resulting relocable 64-bit objects can't be loaded 
into addresses that are over 32-bit limit.

Yuri
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On 04/29/2013 09:12, Keith Walker wrote:
> You only need to go to 64-bit DWARF when your debugging information becomes
> too large to fit in 32-bit DWARF tables.
I am not sure if this is true.
Currently R_X86_64_32 EL relocations are issued for DWARF-32 debug info 
sections. This is because the size of address in DWARF-32 is only 
32-bits, according to the above mentioned specification. Such 
relocations can't be resolved (without overflow) when the base address 
is 64-bit and higher than 4GB threshold.

So I have to always load such ELFs into the space within the first 4GB, 
otherwise relocation resolver will fail with overflow.

Yuri