Hi everyone,

I made some updates and cleaned things up with the "extra optimisation feature" I started to experiment with a while ago. Well, I've managed to find a better showcase for the feature (although it might need refactoring to speed the compiler up):
https://gitlab.com/freepascal.org/fpc/source/-/merge_requests/74

Under x86_64, I use the feature to leave 'hints' for the compiler on "mov %reg32,%reg32" instructions when a search has determined that the source register has its upper 32 bits set to zero.  This permits optimisations where the full 64 bits of the destination register are used later on because it knows that replacing it with the source register won't leave the upper 32 bits undefined.  This has a cascade effect and can cause the MOV to be deleted completely.

https://gitlab.com/freepascal.org/fpc/source/-/merge_requests/74

The compiler itself benefits from the feature a lot!  Excluding the 3 files with source changes, 38 units in the RTL and compiler combined show improvements, and as a result, the net growth of the x86_64-win64 compiler is only 2,048 bytes under -O4.

Gareth aka. Kit