Very slow symbol lookup speed with gdb

[user202729]

To reproduce:

• assume ld is symlinked to mold somewhere

• compile gcc

  git clone --depth 1 --single-branch git://gcc.gnu.org/git/gcc.git
  cd gcc
  mkdir build
  cd build
  ../configure --enable-languages=c,c++ --disable-multilib --disable-bootstrap \
    --disable-libstdcxx \
    CFLAGS="-g3 -gdwarf-2 -Og" \
    CPPFLAGS="-g3 -gdwarf-2 -Og" \
    CXXFLAGS="-g3 -gdwarf-2 -Og"
  make

• try to debug cc1plus as follows

  time gdb ./gcc/cc1plus -ex 'b gimple-range-gori.cc:1510' -ex 'q'

• see that it takes ≈ 9 seconds

• now unlink ld (it should point to GNU ld)

• recompile

  rm ./gcc/cc1plus
  ( cd gcc; time make cc1plus )

  (will take 15 second to link, unlike mold which only take 1 second)

• rerun

  time gdb ./gcc/cc1plus -ex 'b gimple-range-gori.cc:1510' -ex 'q'

  now it only take 3 second

Any idea why? (could also be gdb's fault though)

Searching on gdb's bug database, there is https://sourceware.org/bugzilla/show_bug.cgi?id=30008 which talks about gdb being unable to read gdb_index generated by mold (the issue is still open). i

[rui314]

What is your mold version? I tried that on Ubuntu 20.04 and Ubuntu 24.04 in Docker, and I couldn't see the difference (e.g. 2.287s vs. 0m2.169s.)

[user202729]

@rui314 Latest master (at the time of reporting).

Any idea how might I debug it? (I try running gdb under perf record to make a flamegraph maybe?)

---

My g++ version is 11.4.0 and gdb version is 12.1, I'm using Ubuntu 22.4. But you tried it on both surrounding Ubuntu versions and can't reproduce it...

[user202729]

Strangely enough, the issue is no longer reproducible for me.

Guess I'll close it for now.