GCC 9.3 seems to be able to compile rtree.c with -O2:
- No new regressions in ATF.
- System survives over a night, at least, under heavy loads.
On the other hand, unfortunately, GCC 9.3 still miscompiles tcache.c
with -O2 or -O1. For example, even ``gcc -g hello.c'' fails with ICE
if tcache.c is compiled with -O[12] in libc.
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