[llvm-dev] PIC preferred too strongly, even at CodeModel::Large?

[Lang Hames via llvm-dev]

Hi All,

Ah... I think it might have been possible to use ELF on OS X with MCJIT at
> some point, but it's not really supported in any case.


ELF on Darwin in the JIT may work, especially for simple cases, but
definitely isn't supported.

- Lang.


On Fri, Jul 29, 2016 at 10:44 AM, Eli Friedman via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Fri, Jul 29, 2016 at 9:57 AM, Ramkumar Ramachandra <artagnon at gmail.com>
> wrote:
>
>> Eli Friedman wrote:
>> > On Thu, Jul 28, 2016 at 6:13 PM, Ramkumar Ramachandra via llvm-dev
>> > <llvm-dev at lists.llvm.org> wrote:
>> >> We were just debugging a sporadic crash the other day, when we noticed
>> >> that RIP-relative addressing was being used in a JumpTable, even when
>> >> code and data were well over 4G apart. This is confusing, because we
>> >> picked CodeModel::Large, and expected this to be taken care of. Isn't
>> >> that what gcc would do given a Large CodeModel?
>> >
>> >
>> > This sounds like a bug, but I can't reproduce it.  Testcase?
>>
>> I've attached an example with a standard switch instruction, compiled
>> with `llc -code-model=large`. It produces:
>>
>> movslq (%rax,%rdi,4), %rsi
>> addq %rax, %rsi
>> jmpq *%rsi
>>
>
> Ah; I guess I actually should have spotted that.
>
> See
> https://github.com/llvm-mirror/llvm/blob/f79c57a412cf8ba35884c1d4e011e07baad334d9/lib/CodeGen/SelectionDAG/TargetLowering.cpp#L281
> .  I think you can just force it to use EK_BlockAddress for the large code
> model.
>
>
>> >> Second, is it okay to
>> >> silently fold into Reloc::PIC_ in this case and leave the user with
>> >> sporadic crashes?
>> >
>> >
>> > Large code model and PIC should be compatible.
>>
>> Technically, yes. My understanding is that, instead of a cheap
>> implicit $rip offset, you have to materialize the value in a register
>> and do the `add`. I don't think LLVM is doing it correctly.
>>
>> >> Finally, can we bypass this limitation by simply
>> >> appending "-elf" to our Target Triple, forcing ELF generation on all
>> >> three platforms?
>> >
>> >
>> > What exactly are you planning to do with an ELF object file on OS X?
>>
>> I forgot to mention: we're JIT'ting. In any case, isOSDarwin() isn't
>> influenced by an extra "-elf" to the target triple.
>>
>
> Ah... I think it might have been possible to use ELF on OS X with MCJIT at
> some point, but it's not really supported in any case.
>
> -Eli
>
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