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

[Hans Wennborg]

On Tue, Jun 12, 2012 at 7:05 PM, Joerg Sonnenberger
<joerg at britannica.bec.de> wrote:
> On Tue, Jun 12, 2012 at 12:36:28PM -0400, Rafael Espíndola wrote:
>> >> 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).
>> >
>> > codegen can be more efficient. E.g. less or no calls to __tls_get_addr
>> > needed.
>>
>> My point also :-) What I was asking for a rationale on was *not* doing
>> the optimization in the compiler.
>
> Ah, sorry. Because the compiler might not know it yet. Consider linking
> code compiled with -fPIC into the main binary, either using PIE or not.

Right, but the issue we're discussing here, is why GCC chooses to not
optimize a variable like this:

  static __thread int __attribute((tls_model("global-dynamic"))) x;

even if it could (e.g. when compiled without -fpic).

We're trying to figure out if there's a good reason for this, and if
we want to mimic that behaviour in LLVM, or if we should just take the
tls_model attribute as a starting point, and choose a better model if
we can.

 - Hans