Re: [PATCH v3 2/3] This patch adds support to debug thread local variables defined in shared libraries in AIX.

[Simon Marchi <simon.marchi@polymtl.ca>]

On 9/8/26 9:32 AM, Aditya Vidyadhar Kamath wrote:
> From: Aditya Vidyadhar Kamath <aditya.kamath1@ibm.com>
> 
> Sample debug output of this patch is as below
> Thread 3 hit Breakpoint 1, thread_runner (arg=0x2) at tls_main.c:36
> 36        volatile int bp_here = 0; (void)bp_here;
> 
> $3 = 20
> $4 = 40
> thread 2: my_tls_var=20  lib_tls_var=40
> [Thread 1 (tid 101646715) (id 1) exited]
> [Thread 515 (tid 88015327) (id 3) exited]
> [Inferior 1 (process 21758254) exited normally]
> 
> where lib_tls_var=40 is a variable from a thread library.
> 
> Module-id resolution for shared libraries uses two strategies:
> 
> 1. R_TLSML (local-dynamic): there is exactly one R_TLSML reloc per
>    module, and the loader always writes that module's own id into the
>    corresponding TOC slot.  Read it from the inferior and return it.
> 
> 2. R_TLSM (global-dynamic): each R_TLSM relocation in any loaded module's .loader
>    section references the library that exports the named TLS symbol.
>    Scan R_TLSM relocs across all loaded objfiles for each, resolve the
>    loader symbol name and check whether the library of interest exports
>    it.  If so, read the TOC slot value, that is the library's module-id.
>    Matching is done by symbol name, not by symbol value, so two
>    libraries that both define a TLS variable at within-module offset 0
>    are distinguished correctly.
> 
> The module-id is cached in a per-objfile structure so the .loader scan
> is paid only once per shared library.
> 
> Module-id 0 is a valid AIX loader assignment in initial-exec model and
> is not treated as "not yet allocated".  The main executable is
> distinguished from a module-id-0 library by returning the out-of-band
> sentinel XCOFF_MODID_MAIN_EXE (UINT64_MAX) instead of 0.
> 
> the address computation is done using 3 cases, they are:
> 
> - XCOFF_MODID_MAIN_EXE: local-exec where the XCOFF symbol value is the
>   signed TP-relative offset directly; address = tp + offset.
> 
> - module-id 0 is initial-execed shared library where the AIX loader merges all
>   initial-exec modules into one contiguous TLS segment and adjusts each
>   variable's TP-relative offset accordingly.  The link-time XCOFF symbol
>   value does not necessarily equal the runtime offset.  The patch scans
>   the library's R_TLS slots and matches by l_value (unique within one
>   module) to find the slot the loader filled with the actual runtime
>   TP-relative offset, then computes address = tp + runtime_offset.
> 
> - module-id n > 0 is global-dynamic where we iterate the per-thread thread vector:
>   tls_base = thread_vector[n]; address = tls_base + offset.

Please reword the commit subject to something like:

  Add support for TLS variables in shared libraries on AIX

Simon