[llvm-dev] Relocations used for PPC32 in non-PIC mode

[Hal Finkel via llvm-dev]

Hi, Vit,

I believe that we may use PIC by default, but it should be possible to 
disable it. Please feel free to submit a patch for review.

Thanks,
Hal

On 10/04/2017 12:04 PM, vit9696 via llvm-dev wrote:
> Hal,
>
> I very well understand that LDD may not be in a good state for PPC32, 
> and it would definitely need some improvements sooner or later. In 
> fact I even submitted a patch adding a relocation to ldd just a few 
> hours ago.
>
> However, this particular case is not related to LDD, it is a design 
> issue and furthermore a regression in LLVM itself. I checked gcc, and 
> neither does it try to use PLT and R_PPC_PLTREL24 relocations unless I 
> pass -fpic:
>
> $ gcc -c t.c -o t.o
> $ gcc -c s.c -o s.o
> $ ld.lld t.o s.o -o t
>
> vs
>
> $ gcc -c -fpic t.c -o t.o
> $ gcc -c -fpic s.c -o s.o
> $ ld.lld t.o s.o -o t
> error: t.c:(function _start): unrecognized reloc 18
>
> The behaviour is simply incorrect, and in fact it may even be 
> dangerous for embedded software with custom bootloaders that is 
> migrating to clang compilers. It is not (just) for LDD we should fix 
> this, but also for compatibility with gcc, and because currently 
> -fno-pic switch simply does not affect this part, making its undesired 
> behaviour unconditionally enforced.
>
> Vit
>
>> 4 окт. 2017 г., в 19:28, Hal Finkel <hfinkel at anl.gov 
>> <mailto:hfinkel at anl.gov>> написал(а):
>>
>> Hi, Vit,
>>
>> I'm not under the impression that ppc support in lld is anywhere near 
>> ready for use. I had it linking hello world on ppc64 some time ago, 
>> and it's not clear that ppc is even that far. I'd not adjust anything 
>> in LLVM based on ppc support in lld right now.
>>
>>  -Hal
>>
>> On 10/04/2017 11:03 AM, vit9696 via llvm-dev wrote:
>>> Hello,
>>>
>>> I am currently facing an issue at linking stage when compiling basic C code for an embedded PPC32 platform and linking with LLD. For external symbol linkage LLVM appears to use PLT which results in generating a R_PPC_PLTREL24 relocation, that is not support by LDD. Therefore even such a basic example cannot be built:
>>>
>>> /* s.c */
>>> int f() { return 0; }
>>>
>>> /* t.c */
>>> int f();
>>> int _start() { return f(); }
>>>
>>> $ clang -c -target ppc32-gnu-linux-eabi t.c -o t.o
>>> $ clang -c -target ppc32-gnu-linux-eabi s.c -o s.o
>>> $ ld.lld t.o s.o -o t
>>> ld.lld: error: t.c:(function _start): unrecognized reloc 18
>>>
>>> When I tried ld.bfd the objects linked fine, so I initially thought that it was a lld issue. Yet, after I checked the source code, I started to feel that PLT use on PPC32 is not even desired, and it is likely just an overlooked regression. If you check the original code, PLT was only meant to be used with PIC code:https://reviews.llvm.org/rL213427#C65598OL3360. Note, that there are two places covering the PPCII::MO_PLT_OR_STUB assignment, which are both guarded with getRelocationModel check.
>>>
>>> Now let's look athttps://reviews.llvm.org/rL273499#C65598OL4294  andhttps://reviews.llvm.org/rL273595#C65598OL4316. For some unknown reason these two commits silently removed -fno-pic handling, and effectively enforced PLT use. As a result they broke the compatibility with linkers not implementing PLT-relative relocations.
>>>
>>> While it might be good for LLD to support R_PPC_PLTREL24, in my opinion there still is no reason for LLVM to emit this in non-PIC mode. If my understanding is correct, I can submit a patch that will replace
>>>
>>> bool UsePlt = !Local && Subtarget.isTargetELF() && !isPPC64;
>>>
>>> with
>>>
>>> bool UsePlt = !Local && Subtarget.isTargetELF() && !isPPC64 &&
>>>                DAG.getTarget().getRelocationModel() == Reloc::PIC_;
>>>
>>> Vit
>>>
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>> -- 
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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