Much slower compilation time with boost-1.81 than boost-1.80 + much bigger .text section

From: Dominique Pellé (dominique.pelle_at_[hidden])
Date: 2023-04-17 13:32:14

Hi

1) Compiling the attached "brands.cpp" file which uses
boost::container::flat_set is very slow with clang when using
boost-1.81.0. It was much faster with boost-1.80.0 somehow.

2) Furthermore, the size of the .text section is much bigger
when using boost-1.81.0 than with boost-1.80.0 when using clang.

The source file is a simplification of an actual file in my project.
Note that the code is in anonymous namespace (removing the
anonymous namespace makes the compilation time much faster,
presumably because there are more optimizations in the
anonymous namespace).

Benchmarks with clang-16 + boost-1.81.0 (on Linux Ubuntu-22.04):
```
$ time clang++-16 -isystem boost-1.81 -O2 -DNDEBUG \
     -std=c++14 -o brands.o -c brands.cpp

real    1m46.638s
user    1m46.090s
sys     0m0.520s

$ size brands.o
   text    data     bss     dec     hex filename
2403932   20032      48 2424012  24fccc brands.o
```

Compare with the older boost-1.80.0:
```
$ time clang++-16 -isystem boost-1.80 -O2 -DNDEBUG \
     -std=c++14 -o brands.o -c brands.cpp

real    0m2.163s
user    0m2.122s
sys     0m0.041s

$ size brands.o
   text    data     bss     dec     hex filename
 101280   20032      48  121360   1da10 brands.o
```

In other words:
- file is 81 times slower to compile with boost-1.81 than with boost-1.80
- the .text section is 23 times bigger with boost-1.81 than with boost-1.80

This may be a clang compiler issue, but since the performance
regression is introduced from boost-1.80.0 to boost-1.81.0
I'm, reporting it here (I may report it to clang later depending on
how the discussion here goes).

It's even worse with older clang compilers. The table below shows
the compilation time of brands.cpp file and size of the .text section
for with various compilers using boost-1.80 and boost-1.81:

```
compiler   boost   compilation time    size .text section
--------   -----   ----------------    ------------------
clang-16   1.81         106.638 sec       2,403,932 bytes
clang-16   1.80           2.164 sec         101,280 bytes

clang-15   1.81         137.508 sec       2,861,332 bytes
clang-15   1.80           2.527 sec         107,944 bytes

clang-14   1.81         153.504 sec       3,033,487 bytes
clang-14   1.80           2.294 sec         111,599 bytes

clang-5.0  1.81         276.031 sec       2,720,187 bytes
clang-5.0  1.80           2.328 sec         126,811 bytes

gcc-12.1   1.81           2.821 sec         139,833 bytes
gcc-12.1   1.80           2.505 sec         140,217 bytes

gcc-7.5    1.81           1.943 sec         121,497 bytes
gcc-7.5    1.80           1.707 sec         122,050 bytes
```

Regards
Dominique

PS: full versions of compiler used in above table (--version option):

- Ubuntu clang version 16.0.2
(++20230414073413+b5aa566a7e53-1~exp1~20230414073428.73)
- Ubuntu clang version 15.0.7
- Ubuntu clang version 14.0.0-1ubuntu1
- clang version 5.0.2 (tags/RELEASE_502/final)
- gcc-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0
- gcc-7 (Ubuntu 7.5.0-6ubuntu2) 7.5.0