``Cancelling'' branches

Cancelling (or ``nullifying'') branches are a variation on delayed branches.

Idea:

• Get compiler to predict whether branch is probably taken or not
• Have two variants of delayed branch -
  • BEQZL ``branch likely'': predicted taken, ie delayed branch, cancel if not taken
  • BEQZU ``branch unlikely'': predicted not-taken, ie delayed branch, cancel if taken
• Squash the delay slot's updates if mispredicted

Unfortunately, compilers are not very good at predicting branches. Profile-based branch prediction improves performance substantially.