llvm dev - Jun 2012 - [LLVMdev] [Patch, RFC] Re: Adding support for explicitly specified TLS models (PR9788)

>> 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.
> Joerg

Cheers,
Rafael

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.

Joerg

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

> 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.
Yes, the case that is interesting is where both can. Why should the
compiler wait for the linker?
> Joerg
Cheers,
Rafael