Re: aarch64 TLS optimizations?

[Szabolcs Nagy <Szabolcs dot Nagy at arm dot com>]

On 17/05/2019 14:51, Tom Horsley wrote:
> I'm trying (for reason too complex to go into) to
> locate the TLS offset of the tcache_shutting_down
> variable from malloc in the ubuntu provided
> glibc on aarch64 ubuntu 18.04.
> 
> Various "normal" TLS variables appear to operate
> much like x86_64 with a GOT table entry where the
> TLS offset of the variable gets stashed.

this is more of a glibc question than a gcc one
(i.e. libc-help list would be better).

tls in glibc uses the initial-exec tls access model,
(tls object is at a fixed offset from tp across threads),
that requires a GOT entry for the offset which is set
up via a R_*_TPREL dynamic reloc at startup time.

(note: if a symbol is internal to the module its TPREL
reloc is not tied to a symbol, it only has an addend
for the offset within the module)

> But in the ubuntu glibc there is no GOT entry for
> that variable, and disassembly of the code shows
> that it seems to "just know" the offset to use.

i see adrp+ldr sequences that access GOT entries.

e.g. in the objdump of libc.so.6:

00000000000771d0 <__libc_malloc@@GLIBC_2.17>:
...
   77400:       f00006c0        adrp    x0, 152000 <sys_sigabbrev@@GLIBC_2.17+0x278>
   77404:       f9470c00        ldr     x0, [x0, #3608]
   77408:       d53bd041        mrs     x1, tpidr_el0

you can verify that 0x152000 + 3608 == 0x152e18 is
indeed a GOT entry (falls into .got) and there is a

0000000000152e18 R_AARCH64_TLS_TPREL64  *ABS*+0x0000000000000010

dynamic relocation for that entry as expected.
(but i don't know which symbol this entry is for,
only that the symbol must be a local tls sym)

> Is there some kind of magic TLS optimization that
> can happen for certain variables on aarch64? I'm trying
> to understand how it could know the offset like
> it appears to do in the code.

there is no magic.