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Re: [PATCH] -mcmodel=large -fpic TLS GD and LD support gcc + binutils (PR target/58067)


On Wed, Aug 14, 2013 at 12:03 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
> On Tue, Aug 13, 2013 at 9:42 PM, Jakub Jelinek <jakub@redhat.com> wrote:
>
>> We right now ICE with -mcmodel=large -fpic on x86_64 on TLS GD and LD
>> sequences, because obviously we can't call __tls_get_addr@plt there from code
>> potentially more than 2GB away from the PLT slot.
>>
>> The attached patches add support for that in gcc and also teaches linker
>> about those, because otherwise the linker will fail if you try to link such
>> -mcmodel=large -fpic code into binaries or PIEs.
>>
>> To make transitions possible, we emit always
>> leaq foo@tlsgd(%rip), %rdi
>> movabsq $__tls_get_addr@pltoff, %rax
>> addq $rbx, %rax
>> call *%rax
>> resp.
>> leaq foo@tlsld(%rip), %rdi
>> movabsq $__tls_get_addr@pltoff, %rax
>> addq $rbx, %rax
>> call *%rax
>> sequences (22 bytes, 6 bytes longer than what we do for TLSGD for normal
>> libraries).
>>
>> Bootstrapped/regtested on x86_64-linux and i686-linux, attached is also the
>> sources I've used to test all the 3 different transitions.
>>
>> Ok for trunk and 4.8 branch (and binutils trunk)?
>
> The implementation for x86 is technically OK, but I wonder if these
> sequences should be documented in some authoritative document about
> TLS relocations. The "ELF Handling For Thread-Local Storage" document
> [1] doesn't mention various code models fo x86_64, so I was not able
> to cross-check the implementaton vs. documentation.
>
> [1] http://www.akkadia.org/drepper/tls.pdf
>

I agree.  We need to document the TLS code sequences
for PIC/non-PIC medium/large models first.


-- 
H.J.


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