Spectre V1 diagnostic / mitigation

[Florian Weimer]

* Richard Biener:

> On Wed, 19 Dec 2018, Florian Weimer wrote:
>
>> * Richard Biener:
>> 
>> > The cost is probably target dependent - on x86 it's simply a $fs based
>> > load/store.
>> 
>> Do you need to reserve something in the TCB for this?
>
> No idea.  But I figured using TLS with the patch only works when
> optimizing and not with -O0.  Huh.  Anyway, it should be equivalent
> to what presence of
>
> __thread void *_SV1MASK = (void *)-1l;
>
> requires (plus I make that symbol GNU_UNIQUE).  I see this
> allocates -1 in the .tdata section marked TLS.

Oh.  That's going to be substantially worse for PIC, even with the
initial-exec model, especially on architectures which do not have
arbitrary PC-relative loads.  Which is why I'm asking about the TCB
reservation.

Thanks,
Florian