[libcxx-commits] [libcxxabi] [libcxx] [libcxxabi] Dont throw in make exception ptr if __cxa_init_primary_exception is available (PR #65534)

[via libcxx-commits]

itrofimow wrote:

Benchmarks to back the performance claims up:
```
itrofimow at itrofimow-nix:~/work/llvm-project/ex_ptr_bench$ taskset -c 8-15 ~/work/gbench/tools/compare.py benchmarks ./main_libcxxabi ./patch_libcxxabi 
RUNNING: ./main_libcxxabi --benchmark_out=/tmp/tmpi96tlrpa
2023-10-15T20:57:14+03:00
Running ./main_libcxxabi
Run on (16 X 1280.96 MHz CPU s)
CPU Caches:
  L1 Data 48 KiB (x8)
  L1 Instruction 32 KiB (x8)
  L2 Unified 1280 KiB (x8)
  L3 Unified 24576 KiB (x1)
Load Average: 0.36, 1.30, 1.61
--------------------------------------------------------------------------
Benchmark                                Time             CPU   Iterations
--------------------------------------------------------------------------
bm_make_exception_ptr/threads:1        745 ns          745 ns       950011
bm_make_exception_ptr/threads:2        816 ns         1633 ns       455610
bm_make_exception_ptr/threads:4        863 ns         3208 ns       227044
bm_make_exception_ptr/threads:8       1120 ns         8064 ns        87896
RUNNING: ./patch_libcxxabi --benchmark_out=/tmp/tmpvd2nklsl
2023-10-15T20:57:16+03:00
Running ./patch_libcxxabi
Run on (16 X 1259.78 MHz CPU s)
CPU Caches:
  L1 Data 48 KiB (x8)
  L1 Instruction 32 KiB (x8)
  L2 Unified 1280 KiB (x8)
  L3 Unified 24576 KiB (x1)
Load Average: 0.36, 1.30, 1.61
--------------------------------------------------------------------------
Benchmark                                Time             CPU   Iterations
--------------------------------------------------------------------------
bm_make_exception_ptr/threads:1       25.6 ns         25.6 ns     27321613
bm_make_exception_ptr/threads:2       12.7 ns         25.4 ns     27881074
bm_make_exception_ptr/threads:4       6.84 ns         27.4 ns     27500684
bm_make_exception_ptr/threads:8       3.56 ns         28.5 ns     25787520
Comparing ./main_libcxxabi to ./patch_libcxxabi
Benchmark                                         Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------
bm_make_exception_ptr/threads:1                -0.9656         -0.9656           745            26           745            26
bm_make_exception_ptr/threads:2                -0.9844         -0.9844           816            13          1633            25
bm_make_exception_ptr/threads:4                -0.9921         -0.9915           863             7          3208            27
bm_make_exception_ptr/threads:8                -0.9968         -0.9965          1120             4          8064            28
OVERALL_GEOMEAN                                -0.9892         -0.9887             0             0             0             0
```

It can be seen that with the patch not only the performance is x30(!) in single-threaded case, but it also scales linearly with the number of threads (CPU / Time == number of threads), when on current thunk it's not the case: i'm using not the most recent gcc's unwinder, so contention on the lock in `dl_iterate_phdr` takes place. 

The effect is the same with `libcxxrt`, although it seems to be a bit slower overall.

I've addressed review comments and the build is green. so re-requesting the review.

https://github.com/llvm/llvm-project/pull/65534