[COD-3196] fix(callgrind): represent os threads under --separate-threads=yes

[GuillaumeLagrange]

Make callgrind per-thread dumps (--separate-threads=yes) behave like OS
threads: a thread spawns, runs, ends, and its costs stay attributed to it —
under the part being dumped, kept separate from other threads that reused its
slot, and tagged with its name.

Why it misbehaved

Two independent causes:

1. Slot reuse. The valgrind core recycles ThreadId slots on exit.
   Callgrind keyed all per-thread state — and BBCC identity — on that slot with
   no thread-exit hook, so a new OS thread landing on a recycled slot silently
   appended its costs to the dead thread's containers.
2. Mid-run dumps only flushed the caller. CALLGRIND_DUMP_STATS dumped only
   the current thread; since dumping is destructive, every other thread's whole
   history was deferred to the termination dump, showing up once under the final
   part.

What changed

• Each thread_info gets a monotonic serial, independent of the recycled
  slot; used for the thread: header and -NN file suffix.
• BBCC identity re-keyed on the serial (was the raw tid). This is the real
  fix for slot reuse — the per-BB LRU cache and hash lookup compared the tid, so
  a reused slot otherwise re-merged or tripped the clone_bbcc assertion.
• A pre_thread_ll_exit hook unwinds the exiting thread, snapshots its name,
  and retires it (freed once flushed). Gated on separate_threads — the =no
  path is byte-for-byte untouched.
• Every thread is flushed at each part dump (live + retired), so each
  thread's delta lands under the part being dumped. Zero-delta threads are
  skipped uniformly (incl. termination), which cleanly absorbs a thread that
  lived entirely inside an instrumentation-off window.
• CALLGRIND_ZERO_STATS zeros all threads under separate_threads.
• New VG_(get_thread_name) tool API surfaces the core's thread name, emitted
  as a desc: Thread name: line for the backend (COD-3197) to parse.

The retire path is safe on an already-unwound/empty state, so it survives the
STOP/START_INSTRUMENTATION the codspeed integrations wrap benchmarks in.

Review notes

• callgrind/threads.c (retire + retired-list iteration) and callgrind/dump.c
  (per-part flush, retired-thread install, zero-delta skip) are the core of the
  change; bbcc.c is the serial re-keying.
• The main thread is itself retired on its own exit before finalisation, so the
  termination dump also iterates the retired list.

Validation

Two new regression tests: thread-serial (sequential named workers with a dump
between them → two distinct serials, name emitted, costs under the dumped part)
and thread-instr (instrumentation toggling + an off-window thread → no crash,
no spurious section). Full callgrind make check (25 tests, incl. the existing
--separate-threads=yes threads/threads-use) and cachegrind suite (17,
guards the shared core change) pass. Manually re-ran the COD-3188 repros
(sequential, parallel, dump-while-running) — topology as expected.

Draft: opening for early review; will un-draft once the backend side (COD-3197)
is ready to consume the format.

Refs COD-3196

[greptile-apps]

Greptile Summary

This PR fixes Callgrind per-thread dumps for recycled thread slots and mid-run dumps. The main changes are:

• Tracks a stable OS thread ID on each Callgrind thread record.
• Re-keys BBCC ownership checks on the stable thread ID.
• Retires exited thread state so it can be flushed after exit.
• Flushes live and retired threads for each dump part in separate-thread mode.
• Drops retired thread state when stats are zeroed.
• Adds tool APIs for reading live thread names and LWP IDs.

Confidence Score: 5/5

This looks safe to merge.

• The retired-thread zero boundary is now handled by resetting live threads and dropping exited thread state.
• No blocking issues were found in the changed code.

Important Files Changed

Filename	Overview
callgrind/main.c	Updates stats zeroing so separate-thread mode resets live threads and clears retired thread state.
callgrind/threads.c	Adds stable thread identity, retired-thread tracking, exit handling, and cleanup helpers.
callgrind/dump.c	Flushes live and retired thread sections per dump part and emits thread metadata.
callgrind/bbcc.c	Uses stable OS thread IDs for BBCC lookup, creation, and clone checks.
callgrind/global.h	Extends thread metadata and declares the new thread lifecycle helpers.
coregrind/m_threadstate.c	Adds guarded core accessors for thread names and LWP IDs.
include/pub_tool_threadstate.h	Exposes thread name and LWP ID accessors to Valgrind tools.

_{Reviews (8): Last reviewed commit: "fix(callgrind): represent os threads und..." | Re-trigger Greptile}

[codspeed-hq]

Merging this PR will not alter performance

✅ 84 untouched benchmarks
⏩ 60 skipped benchmarks¹

---

_{Comparing cod-3196-fix-valgrind-separate-threadsyes-behavior-to-represent-os (8a969c4) with master (448b7d0)²}

Footnotes

1. 60 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on master (8a969c4) during the generation of this report, so 448b7d0 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[art049]

I'll review as well

[not-matthias]

LGTM