[LLVMdev] [Patch, RFC] Re: Adding support for explicitly specified TLS models (PR9788)

[Hans Wennborg]

On Tue, Jun 12, 2012 at 3:41 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> I thought it was a good idea to make the user's choice explicit in the
>> IR. If we combined the default and globaldynamic modes, LLVM wouldn't
>> be able to tell the difference.
>>
>> It may or may not be important to be able to tell the difference, but
>> it would be unfortunate if we'd have to go and change the IR format
>> later because we limited ourselves here.
>>
>> Also, my patch does make a difference between the default and
>> globaldynamic. If user specifies globaldynamic, LLVM will use that
>> model, even if some other model would be better (it even adds support
>> for doing globadynamic in non-PIC code). GCC does the same.
>
> Do you know what is the rationale for that? The static linker will
> optimize it anyway (but not do as good a job as the compiler could).

I managed to dig out the original thread for GCC:
http://gcc.gnu.org/ml/gcc-patches/2002-09/msg00668.html

It doesn't give a rationale for the case we're discussing, though :/

My intuition still tells me that it would be good to separate the
default and globaldynamic cases to

1) Respect the user's request: if the user went through the trouble of
specifying __attribute__((tls_model("globaldynamic"))), we should
assume there's a reason and give him what he wants, even if we think
the linker is going to optimize it
2) To match GCC's behavior.

I don't have any more compelling reasons than those, and I don't feel
super strongly about this, so I'm willing to give in if others
disagree with me :)

Thanks,
Hans