bug#38322: GCC optimize levels makes huge impact on performance

[Balázs Vinarz]

Hello there!

Today I was working on two bigger, plain text, csv-like database files
(file1: ~175k lines and 15MB, file2: ~ 168k lines and 14MB). I just
searched for lines, using grep -f $file2 $file1. I was so surprised
when I realized the search was running for minutes already without a
single line at the standard output. I decided to have a try with
custom compiled binaries, because in my mind the size optimized
binaries are the fastest.
In the end grep (3.1) was running for:
- 4m50s if I used the one was coming from Ubuntu,
- 4m29s in case of custom recompiled with GCC7.4 and CFLAGS="O2" and
- 3m17s in case of custom recompiled with GCC7.4 and CFLAGS="Os".
I repeated the runs multiple times, I would say it's accurate. The
files were located on tmpfs.
Binary sizes are: 215K for Ubuntu, 184K for O2 and 150K for Os.
CPU: Intel I5-8350U
OS: Ubuntu 18.04.3 LTS
Would you mind change the default optimize level on the make
configuration? Did somebody ever measured the benefits using different
GCC optimalization levels?
I know that this is a special use case, but the improvement is huge.
I'm looking forward for your feedback.

Best regards