tree 684b25125a0059941567a697bc460bdca65283bd
parent f76487cc4c0cf0b47c579492f33e3e6ee1dea030
author Adrian Vogelsgesang <avogelsgesang@tableau.com> 1658955358 -0700
committer Copybara-Service <copybara-worker@google.com> 1660087266 -0700

[libunwind] Use `_dl_find_object` if available

As shown in P2544R0 [1] and the accompanying benchmark [2], the
current unwinding logic does not scale for multi-threaded programs.
This is because `dl_iterate_phdr` takes a global lock.

glibc 2.35 added `_dl_find_object` which directly returns the unwind
info for a given target address. `_dl_find_object` is fully lock-free
and hence allows parallel exception unwinding on multiple threads.

With this commit, libunwind now takes advantage of `_dl_find_object`.
Thereby, this commit improves libunwind's performance on benchmark [2]
for unwinding exception on 20 threads from 1103ms to 78ms.
(measured on Intel Xeon Silver 4114 with 20 physical cores)

[1] https://isocpp.org/files/papers/P2544R0.html
[2] https://github.com/neumannt/exceptionperformance

Detailed performance numbers from the benchmark:

Before:
> Testing unwinding performance: sqrt computation with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 34 35 34 35 35 36
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 16 32 33 34 35 36
> failure rate 0.1%: 16 32 34 36 35 36
> failure rate 1%: 20 40 40 43 90 113
> failure rate 10%: 59 92 140 304 880 1103
> [...]
>
> Testing invocation overhead: recursive fib with occasional errors
>
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 19 32 37 38 39 36
> failure rate 0.1%: 22 32 40 40 39 34
> failure rate 1%: 20 28 38 39 48 40
> failure rate 10%: 25 39 44 50 92 113

After:
> Testing unwinding performance: sqrt computation with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 19 30 35 38 39 35
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 32 35 33 34 34 36
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 16 35 33 37 35 35
> failure rate 0.1%: 16 32 36 33 34 37
> failure rate 1%: 21 37 39 40 40 41
> failure rate 10%: 72 75 76 80 80 78
> [...]
>
> Testing invocation overhead: recursive fib with occasional errors
>
> testing baseline using 1 2 4 8 16 20 threads
> failure rate 0%: 18 35 37 34 38 37
> testing exceptions using 1 2 4 8 16 20 threads
> failure rate 0%: 19 33 40 40 41 39
> failure rate 0.1%: 21 33 39 38 39 38
> failure rate 1%: 20 36 39 40 41 40
> failure rate 10%: 25 45 41 42 44 43

Differential Revision: https://reviews.llvm.org/D130668

GitOrigin-RevId: df9a23e2feda18f0308b6d4dbd591ebe6b605aa4