----- On Apr 7, 2016, at 8:25 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:
> On Thu, Apr 07, 2016 at 02:03:53PM +0200, Florian Weimer wrote:
>> > struct tlabi {
>> > union {
>> > __u8[64] __foo;
>> > struct {
>> > /* fields go here */
>> > };
>> > };
>> > } __aligned__(64);
>>
>> That's not really “fixed size” as far as an ABI is concerned, due to the
>> possibility of future extensions.
>
> sizeof(struct tlabi) is always the same, right? How is that not fixed?
>
>> > People objected against the fixed size scheme, but it being possible to
>> > get a fixed TCB offset and reduce indirections is a big win IMO.
>>
>> It's a difficult trade-off. It's not an indirection as such, it's avoid
>> loading the dynamic TLS offset.
>
> What we _want_ is being able to use %[gf]s:offset and have it work (I
> forever forget which segment register userspace TLS uses).
>
>> Let me repeat that the ELF TLS GNU ABI has very limited support for
>> static offsets at present, and it is difficult to make them available
>> more widely without code generation at run time (in the form of text
>> relocations, but still).
>
> Do you have a pointer to something I can read? Because I'm clearly not
> understanding the full issue here.
For what is is worth, here are a couple of objdump snippet of my
test program without and with -fPIC:
* Compiled with -O2, *without* -fPIC, x86-64:
__thread __attribute__((weak)) volatile struct thread_local_abi __thread_local_abi;
static
int32_t read_cpu_id(void)
{
if (unlikely(!(__thread_local_abi.features & TLABI_FEATURE_CPU_ID)))
40064e: 64 8b 04 25 c0 ff ff mov %fs:0xffffffffffffffc0,%eax
400655: ff
400656: a8 01 test $0x1,%al
400658: 74 71 je 4006cb <main+0xab>
return sched_getcpu();
return __thread_local_abi.cpu_id;
40065a: 64 8b 14 25 c4 ff ff mov %fs:0xffffffffffffffc4,%edx
400661: ff
}
* Compiled with -O2, with -fPIC, x86_64:
__thread __attribute__((weak)) volatile struct thread_local_abi __thread_local_abi;
4006de: 64 48 8b 04 25 00 00 mov %fs:0x0,%rax
4006e5: 00 00
static
int32_t read_cpu_id(void)
{
if (unlikely(!(__thread_local_abi.features & TLABI_FEATURE_CPU_ID)))
4006e7: 48 8d 80 c0 ff ff ff lea -0x40(%rax),%rax
4006ee: 8b 10 mov (%rax),%edx
4006f0: 83 e2 01 and $0x1,%edx
4006f3: 0f 84 80 00 00 00 je 400779 <main+0xc9>
return sched_getcpu();
return __thread_local_abi.cpu_id;
4006f9: 8b 50 04 mov 0x4(%rax),%edx
}
So with -fPIC (libraries), TLS adds an extra indirection. However,
it just needs to load the base address once, and can then access
both "features" and "cpu_id" fields as offsets from that base.
For executables compiled without -fPIC, there is no indirection.
This justifies the following paragraph in the proposed man page:
The symbol __thread_local_abi is recommended to be used across
libraries and applications wishing to register a the thread-local
ABI structure for tlabi_nr 0. The attribute "weak" is recommended
when declaring this variable in libraries. Applications can
choose to define their own version of this symbol without the weak
attribute as a performance improvement.
Thoughts ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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