Fix linker failure when building opcache statically #18939
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This fixes the following linker error:
TLS transition from R_X86_64_TLSGD to R_X86_64_GOTTPOFF against
`_tsrm_ls_cache' at 0x12fc3 in section `.text' failed"
The error arises from how we obtain information about the _tsrm_ls_cache TLS
variable for use in JIT'ed code:
Normally, TLS variables are resolved via linker relocations [1], which of course
can not be used in JIT'ed code. Therefore we emit the relocation in AOT code and
use the result in JIT.
Specifically we use a fragment of the "General Dynamic" code sequence described
in [1]. Using the full code sequence would give us the address of the variable
in the current thread. Therefore we only use a fragment that gives us the
variable's TLS index and offset.
When Opcache is statically linked into the binary, linkers attempt to relax
(rewrite) this code sequence into a more efficient one. However, this fails
because they will not recognize the code sequence.
We now take a different approach:
* Emit the exact full code sequence expected by linkers
* Extract the TLS index/offset or TCB offset by inspecting the ASM code, rather
than executing it (execution would give us the thread-local address).
* 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.
[1] https://www.akkadia.org/drepper/tls.pdf
TimWolla
reviewed
Jun 25, 2025
TimWolla
reviewed
Jun 25, 2025
Co-authored-by: Tim Düsterhus <[email protected]>
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Spun of #18660
RFC: https://wiki.php.net/rfc/make_opcache_required
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 #15074:
Here I take a different approach:
One 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.
This is tested on Linux (glibc, musl), FreeBSD, MacOS, Windows; lld, gold, bdf; clang; gcc; VS; x86, x86_64, aarch64 (not MacOS/Apple Silicon, as JIT+ZTS is not supported on this combo yet, and not Windows/aarch64 for the same reason), with various combinations of static/shared/dl(). The PR includes these tests. Other OSes fallback to the slower mechanism.
[1] https://www.akkadia.org/drepper/tls.pdf