WAR hazard

Modified bypass

• An executing instruction must broadcast results to the issue queue
  • Waiting instructions compare their source register numbers with the destination register number of the bypassed value
  • Also, now it needs to make sure that it is consuming the right value in program order to avoid WAR
  
• Need to tag every instruction with its last producer
• Can we simplify this?

WAR and WAW

• These are really false dependencies
  • Arises due to register allocation by the compiler
• Thus far we have assumed that ROB has space to hold the destination values: needs wide ROB entries
• These values are written back to the register file when the instructions retire or commit in-order from ROB
• Also, bypass becomes complicated
• Better way to solve it: rename the destination registers