We use linker relocations to fetch the TLS index and offset of _tsrm_ls_cache.
When building Opcache statically, linkers may attempt to optimize that into a
more efficient code sequence (relaxing from "General Dynamic" to "Local Exec"
model [1]). Unfortunately, linkers will fail, rather than ignore our
relocations, when they don't recognize the exact code sequence they are
expecting.
This results in errors as reported by GH-15074:
TLS transition from R_X86_64_TLSGD to R_X86_64_GOTTPOFF against
`_tsrm_ls_cache' at 0x12fc3 in section `.text' failed"
Here I take a different approach:
* Emit the exact full code sequence expected by linkers
* Extract the TLS index/offset by inspecting the linked ASM code, rather than
executing it (execution would give us the thread-local address).
* We detect when the code was relaxed, in which case we can extract the TCB
offset instead.
* This is done in a conservative way so that if the linker did something we
didn't expect, we fallback to a safer (but slower) mechanism.
One additional benefit of that is we are now able to use the Local Exec model in
more cases, in JIT'ed code. This makes non-glibc builds faster in these cases.
Closes GH-18939.
Related RFC: https://wiki.php.net/rfc/make_opcache_required.
[1] https://www.akkadia.org/drepper/tls.pdf
