[PATCH 0/5] perf symbol: Speed up demangling of long Rust symbols

[Michal Pluta <michalpl2003@gmail.com>]

dso__demangle_sym() demangles Rust v0 symbols itself instead of using
a helper like the other demanglers. When a demangled name doesn't fit
the buffer, it grows the buffer by a fixed 32 bytes and reformats the
whole name from scratch. The number of attempts grows with the name's
length, so the total work is quadratic. Rust programs with deeply
nested generics can have very long demangled names, and I noticed this
as real time spent in perf report on perf.data recorded from launching
Bevy and Typst.

Patch 3 is the actual optimisation, doubling the size of the buffer
instead.  Patch 5 moves the Rust-specific logic into its own function,
matching how the other demanglers are called.  I think the cleanup is
worthwhile but I understand it comes down to personal taste and the
patch can be skipped if maintainers would rather keep
dso__demangle_sym() as is.

I used an LLM to review my patches and find the issues with the
demangle tests and the unterminated buffer bug.

Testing:
- 'perf test' passes, including the demangle Java, OCaml and Rust
  suites (patch 3 adds a case for a symbol that exceeds the buffer
  limit).
- 'perf report' output is unchanged before and after this series on
  every Rust binary I tried it on.
- Built and tested on x86-64 with a 7.0 kernel.

The series is based on perf-tools-next, commit 0ae6fc78c5ce ("perf
timechart: Remove the unused use_old_power_events variable").

Michal Pluta (5):
  perf test demangle: Fail when demangling fails
  perf symbol: Don't return an unterminated Rust demangle buffer
  perf symbol: Grow the Rust demangle buffer geometrically
  perf symbol: Shrink the demangled Rust buffer to fit
  perf symbol: Move Rust demangling into its own function

 tools/perf/tests/demangle-java-test.c    |  1 +
 tools/perf/tests/demangle-rust-v0-test.c | 13 +++++
 tools/perf/util/symbol.c                 | 60 +++++++++++++++---------
 3 files changed, 52 insertions(+), 22 deletions(-)


base-commit: 0ae6fc78c5ce0dfd18d8712a50f0fd4602eff103
-- 
2.43.0