http://forum.rvspace.org/t/safe-cflags-gcc-12-2-1/1638/29
with -mabi=lp64d the performance is equal to no flags.
CFLAGS="-O2 -pipe -fomit-frame-pointer --param l1-cache-size=32 --param l2-cache-size=2048 -mabi=lp64d -march=rv64imafdc_zicsr_zba_zbb -mcpu=sifive-u74 -mtune=sifive-7-series" make -j4
zstd -T4 -b3 ~/silesia.tar
3#silesia.tar : 211948544 -> 66543158 (x3.185), 22.8 MB/s, 43.4 MB/s