[bonzini]

Constant propagation and also tracking of nonzero bits has been a huge performance improvement in QEMU. The latter helps a lot with x86's automated zero extension, because if you know the high 32 bits are zero you can use a 64-bit move instead of an AND.

But QEMU only does one basic block at a time without jumps so it's relatively easy to implement.

(I tested "7z b" and QEMU user mode emulation is about 30% for x86-on-x86, I'm curious to see how it fares for x86-on-RISCV but I don't have a board to test on).