GCC 8.3.0 vs. 9.0.1

Hi,

(Apologies if this isn't the right place to post this)

A few days ago a blog post appeared on phoronix.com[1] comparing GCC 8.3.0 against 9.0.1 on Intel cascadelake processors.
A notable difference was seen in the PostgreSQL benchmark (v10.3, pgbench, read/write, more detail below), both when compiling with -march=native and -march=skylake:

GCC version | -march= | TPS
8.3.0 | skylake | 5667
9.0.1 | skylake | 11684 (2.06x speed up)
8.3.0 | native | 8075
9.0.1 | native | 11274 (1.40x speed up)

I'm interested to know the devs' take on this is - does GCC 9 contain some new feature(s) that are particularly well suited to compiling and optimising Postgres? Or was GCC 8 particularly bad?

The test script seems to be this one[2], and goes something like this:

- Postgres 10.3 is configure using --without-readline and --without-zlib (after patching it so that it can run as root). The remaining compiler options seem to be (implicitly?) "-fno-strict-aliasing -fwrapv -O3 -lpgcommon -lpq -lpthread -lrt -lcrypt -ldl -lm", plus the -march setting under test.

- initdb is run with --encoding=SQL_ASCII --locale=C

- the db is started with "pg_ctl start -o '-c autovacuum=false'"

- createdb pgbench

- pgbench -i -s <system memory in MB * 0.003> pgbench

- pgbench -j <number of cores> -c <number of cores * 4> -T 60 pgbench

Cheers,
Steven.

[1] https://www.phoronix.com/scan.php?page=news_item&px=Intel-Cascade-Lake-GCC9
[2] https://openbenchmarking.org/innhold/b53a0ca6dcfdc9b8597a7b144fae2110fa6af1fb

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