Transcript Pipeline Timing

No Pipeline
add r3, r1, r2
add r6,r4, r5
add r9, r7, r8
add r12, r10,r11
add r15, r13,r14
time
1
2
3
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
4
IF
Assuming 1 cycle for IF/ID/EX/MEM/WB,
instr
Total # cycles for n instructions: n*5
For n = 5 => (25 cycles)
ID
EX
MEM
WB
5-stage Pipeline
add r3, r1, r2
add r6,r4, r5
add r9, r7, r8
add r12, r10,r11
add r15, r13,r14
time
1
2
3
4
5
instr
IF
ID
EX
MEM
IF
ID
EX
MEM
IF
ID
EX
IF
WB
WB
MEM
WB
ID
EX
MEM
IF
ID
EX
WB
MEM
WB
Assuming 1 cycle for IF/ID/EX/MEM/WB,
Total # cycles for n instructions = 5 + (n-1) = n + 4
For (n = 5) => (9 cycles)
Pipeline Forwarding
add r3, r1, r2
add r5,r4, r3
time
add r7, r6, r5
1
2
3
4
5
instr
IF
ID
EX
MEM
IF
ID
EX
MEM
IF
ID
EX
IF
add r10, r7,r8
WB
add r12, r10,r9
WB
MEM
WB
ID
EX
MEM
IF
ID
EX
Forwarding: To communicate
deps between stage of earlier
inst to following insts
WB
MEM
WB
Assuming 1 cycle for IF/ID/EX/MEM/WB,
Total # cycles for n instructions = 5 + (n-1) = n + 4
For (n = 5) => (9 cycles)
Superscalar with Issue Width = 2
time
add r3, r1, r2
add r6,r4, r5
add r9, r7, r8
add r12, r10,r11
add r15, r13,r14
1
IF
ID
EX
MEM
WB
2
IF
ID
EX
MEM
WB
3
IF
ID
EX
MEM
WB
4
IF
ID
EX
MEM
WB
IF
ID
EX
5
MEM
WB
instr
Assuming 1 cycle for IF/ID/EX/MEM/WB,
Total # cycles for n instructions = 5 + ceil[(n-2)/2]
For (n = 5) => (7 cycles)
SIMD Pipeline
Assume 5-element vector
v1 = [r1, r4, r7,r10,r13]
v2 = [ r2, r5, r8, r11, r14]
time
1
IF
ID
EX
MEM
WB
addv v3, v1, v2
EX
Single vector instr takes 5 cycles
(5 scalar insts
in 7 cycles on superscalar)
EX
EX
addv v6, v5, v4
addv v9, v8, v7
EX
2
IF
ID
EX
MEM
WB
(Total of 15 scalar adds in 7 cycles) vs
(4 scalar adds in 7 cycles)
EX
EX
EX
instr
3
EX
IF
ID
EX
EX
MEM
WB
Fewer instructions operate on more
elements in parallel
What if you had dependencies between vector insts ?
addv v3, v1, v2
addv v5, v4, v3
addv v7, v6, v5
Vector SIMD Pipeline: Chaining
time
1
IF
ID
EX
MEM
WB
EX
MEM
EX
EX
EX
EX
2
IF
ID
WB
EX
EX
Vector SIMD Pipeline (Chaining): Composition
of SIMD with Pipeline Forwarding
EX
instr
3
EX
IF
ID
EX
EX
MEM
WB
Vector SIMD Pipeline: Chaining
time
1
IF
ID
IF
2
EX
MEM
WB
EX
ID/EX
EX
MEM
IF/EX
ID/EX
EX
EX
IF/EX
ID/EX
EX
EX
EX
EX
EX
EX
EX
EX
EX
WB
MEM
EX
EX
3
WB
MEM
WB