Unity builds have been largely supplanted by LTO. They still have uses for build time improvements in one-off builds, as LTO on a non-incremental build is usually slower than the equivalent unity build.
GCC version: 11.3 target: Cortex-A9 Qt version: 5.15
I think we tested single core and quad core, also possibly a newer GCC version, but I'm not sure. Just wanted to add my two cents.
And programs full of pointer-chasing are quite pessimized; highly-OO code is a common example, which includes almost all GUIs, even in C++.
In any case even with whole program optimization, O would expect that effectively devirtualizing an heavily object oriented application to be very hard.