Control Hazard

Transcript

1. 51
   Control Hazards
   • Control hazards
   • Must fetch post-branch insns before branch outcome is known
   • Since we don’t know whether or not we’re branching until the insn
   reaches the X stage (ALU)
   • Default: assume “branch not taken” (at fetch, can’t tell it’s a branch)
   PC
   Insn
   Mem
   Register
   File
   s1 s2 d
   +
   4
   <<
   2
   F/D D/X
   X/M
   PC
   A
   B
   IR
   O
   B
   IR
   PC
   IR
   S
   X
   

   View Slide

2. 52
   Branch Recovery
   • Branch recovery: what to do when branch is taken
   • Flush insns currently in F/D and D/X (they’re wrong)
   • Replace with NOPs
   + Haven’t yet written to RegFile or DMem
   PC
   Insn
   Mem
   Register
   File
   s1 s2 d
   +
   4 <<
   2
   PC
   F/D D/X
   X/M
   nop
   nop
   PC
   A
   B
   IR
   O
   B
   IR
   PC
   IR
   S
   X
   

   View Slide

3. 56
   Fast Branch
   • Fast branch: can decide at D instead of X
   • Duplicate comparison logic only, not the whole ALU
   + New taken branch penalty is now 1 stall instead of 2
   – Additional insns (slt) for more complex tests, must bypass to D too
   • 25% of branches have complex tests that require extra insn
   • CPI = 1 + 0.20*0.75*1(branch) + 0.20*0.25*1(extra insn) = 1.2
   PC
   Insn
   Mem
   Register
   File
   s1 s2 d
   +
   4
   <<
   2
   F/D
   D/X X/M
   S
   X
   <>
   0
   O
   B
   IR
   A
   B
   IR
   PC
   IR
   S
   X
   

   View Slide