1. ECE562/468 Advanced Computer Architecture Prof. Honggang Wang
Ch2. Instruction-Level Parallelism & Its Exploitation 2. Dynamic Scheduling
ECE Department
University of Massachusetts Dartmouth 285 Old Westport Rd. North Dartmouth, MA
Slides based on the PowerPoint Presentations created by David Patterson as part of the Instructor Resources for the textbook by Hennessy & Patterson
Updated by Honggang Wang.
CS252 S05

2. Administrative Issues (02/25/2016)
Xing - Fall09
Administrative Issues (02/25/2016)
Project proposal is due Thursday, March.1
5-10 mins project proposal presentation on March. 1
Project proposal guideline can be found on my teaching website
Background Review Lecture

3. Review of Last Lecture ILP: Concepts and Challenges
Compiler techniques to increase ILP
Loop Unrolling
Branch Prediction
Static Branch Prediction
Dynamic Branch Prediction
Overcoming Data Hazards with Dynamic Scheduling

4. Outline Dynamic Scheduling Speculation Memory Aliases Exceptions
Tomasulo Algorithm
Speculation
Speculative Tomasulo Example
Memory Aliases
Exceptions
Increasing instruction bandwidth
Register Renaming vs. Reorder Buffer
Value Prediction
The idea,
div.D F0, f2, F4
Add.d F10, F0, F8
SUB.D F12, F8, F14
Depndence,the sub instrcution cannot execute because the depdence of ADD.D on DIV.D cause the pipeline to stall.
Instruciton issued in program order, out of order execution, which implies out-of-order completion.
Tracking instruction dependenceis to allow the execution as soon as operands are available and renaming registers to avoid WAR and WAW.
RAW only are avoid by executing an instrcution only when its operands are available.
WAR and WAW hazards, which arise from name dependencies, are eliminated by register renaming
Compiler cannot handle register renaming across branches,
Reservation station which buffers the operands of instructions waiting to issue, the basic idea is that a that a resevation station fetched and buffers an operand as soon as it is avaiable, elimating the need to get the operand from a regsiter, in addition, pending instrcution desinate the reservation station that will provide their input. Finally, when successive writes to a register overlap in execution, only the last one is actually used to update the register.
When successive writes to a register overalp in execution, Only the last one is actually used to update the regsiter, As instruction are issued, the register specifiers for pending operands are renamed to the names of the reservation station, which provides register renaming.
There are more resevation stations than name dependences than real register.
The use of resvation station, rhan centrilized register fiel, leads two imporant properties, first hazard detection and execution controal are distributed: the information held in the reservation stations at each functional unit determine when an instruction can beign execution at the unit.
Results are passed directly to functional units from the reservation stations where they are buffered.
Common data bus, all the units waiting for a operand can be loaded simutaneously.

5. Advantages of Dynamic Scheduling
Dynamic scheduling - hardware rearranges the instruction execution to reduce stalls while maintaining data flow and exception behavior
It handles cases when dependences unknown at compile time
it allows the processor to tolerate unpredictable delays such as cache misses, by executing other code while waiting for the miss to resolve
It allows code that compiled for one pipeline to run efficiently on a different pipeline
It simplifies the compiler
Hardware speculation, a technique with significant performance advantages, builds on dynamic scheduling
The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory locations. A cache miss refers to a failed attempt to read or write a piece of data in the cache, which results in a main memory access with much longer latency
Reduce the delay.
Daddu R2, R3, R4
Beqz R2, L1
LW R1, 0(R2)
Swith beqz and lw cause memory protection exception.

6. HW Schemes: Instruction Parallelism
Key idea: Allow instructions behind stall to proceed DIVD F0,F2,F4 ADDD F10,F0,F8 SUBD F12,F8,F14
Enables out-of-order execution and allows out-of-order completion (e.g., SUBD)
In a dynamically scheduled pipeline, all instructions still pass through issue stage in order (in-order issue)
Will distinguish when an instruction begins execution and when it completes execution; between 2 times, the instruction is in execution
Note: Dynamic execution creates WAR and WAW hazards and makes exceptions harder
Expalin what is WAR, WAW, switching the ordering cuase the hazard.
Cause the pipleline to stall; yet sub.d is not data depdence on anything in the pipeline. Not keep the program order
Five stage pipeline, both structural and data hazards could be checked. Out of –order executrion ,
need check data hazard and structural hazard
DIV.D F0, f2, f4
ADD.D F6, F0, F8
SUB.D F8,F10,F14
MUL.D F6, F10, F8

7. Dynamic Scheduling Step 1
Simple pipeline had 1 stage to check both structural and data hazards: Instruction Decode (ID), also called Instruction Issue
Split the ID pipe stage of simple 5-stage pipeline into 2 stages:
Issue—Decode instructions, check for structural hazards
Read operands—Wait until no data hazards, then read operands

8. A Dynamic Algorithm: Tomasulo’s
For IBM 360/91 (before caches!)
 Long memory latency
Goal: High Performance without special compilers
Small number of floating point registers (4 in 360) prevented interesting compiler scheduling of operations
This led Tomasulo to try to figure out how to get more effective registers — renaming in hardware!
Why Study 1966 Computer?
The descendants of this have flourished!
Alpha 21264, Pentium 4, AMD Opteron, Power 5, …

9. Tomasulo Algorithm
Control & buffers distributed with Function Units (FU)
FU buffers called “reservation stations”; have pending operands
Registers in instructions replaced by values or pointers to reservation stations(RS); called register renaming ;
Renaming avoids WAR, WAW hazards
More reservation stations than registers, so can do optimizations compilers can’t
Results to FU from RS, not through registers, over Common Data Bus that broadcasts results to all FUs
Avoids RAW hazards by executing an instruction only when its operands are available
Load and Stores treated as FUs with RSs as well
The Tomasulo algorithm is a hardware algorithm developed in 1967 by Robert Tomasulo from IBM. It allows sequential instructions that would normally be stalled due to certain dependencies to execute non-sequentially (out-of-order execution)
Algorithm:
The basical idea is that a reservation station feteches and buffers an operand as soon as it is available, eliminating the need to get the operand from a register.
Register rename is provided by reservation stations.
Better than centralized register files.
Hazard detection and execution control are distributed.,
Results are directly passed to function units from reservation stations where they are buffered, rather than going through the registers. This is done by CDB.
not going trhorugh the register,

10. Tomasulo Organization
FP Registers
From Mem
FP Op
Queue
Load Buffers
Load1
Load2
Load3
Load4
Load5
Load6
Store
Buffers
Add1
Add2
Add3
Mult1
Mult2
Reservation
Stations
To Mem
FP adders
FP multipliers
Common Data Bus (CDB)

11. Reservation Station Components
Op: Operation to perform in the unit (e.g., + or –)
Vj, Vk: Value of Source operands
Store buffers has V field, result to be stored
Qj, Qk: Reservation stations producing source registers (value to be written)
Store buffers only have Qi for RS producing result
Busy: Indicates reservation station or FU is busy
Register result status—Indicates which functional unit will write each register, if one exists. Blank when no pending instructions that will write that register.

12. Three Stages of Tomasulo Algorithm
1. Issue—get instruction from FP Op Queue
If reservation station free (no structural hazard), control issues instr & sends operands (renames registers).
2. Execute—operate on operands (EX)
When both operands ready then execute; if not ready, watch Common Data Bus for result
3. Write result—finish execution (WB)
Write on Common Data Bus to all awaiting units; mark reservation station available
Common data bus: data + source (“come from” bus)
64 bits of data + 4 bits of Functional Unit source address
Write if matches expected Functional Unit (produces result)
Does the broadcast
Example speed: 2 clocks for +,-; 10 for * ; 40 clks for /

13. Tomasulo Example Instruction stream 3 Load/Buffers FU count
down
3 FP Adder R.S.
2 FP Mult R.S.
Clock cycle counter

14. Tomasulo Example Cycle 1

15. Tomasulo Example Cycle 2
Note: Can have multiple loads outstanding

16. Tomasulo Example Cycle 3
Note: registers names are removed (“renamed”) in Reservation Stations; MULT issued
Load1 completing; what is waiting for Load1?

17. Tomasulo Example Cycle 4
Load2 completing; what is waiting for Load2?

18. Tomasulo Example Cycle 5
Timer starts down for Add1, Mult1

19. Tomasulo Example Cycle 6
There are three situations in which a data hazard can occur:
read after write (RAW), a true dependency
write after read (WAR)
write after write (WAW)
consider two instructions i and j, with i occurring before j in program order.
[edit] Read After Write (RAW)
(j tries to read a source before i writes to it) A read after write (RAW) data hazard refers to a situation where an instruction refers to a result that has not yet been calculated or retrieved. This can occur because even though an instruction is executed after a previous instruction, the previous instruction has not been completely processed through the pipeline.
[edit] Example
For example:
i1. R2 <- R1 + R3 i2. R4 <- R2 + R3
The first instruction is calculating a value to be saved in register 2, and the second is going to use this value to compute a result for register 4. However, in a pipeline, when we fetch the operands for the 2nd operation, the results from the first will not yet have been saved, and hence we have a data dependency.
We say that there is a data dependency with instruction 2, as it is dependent on the completion of instruction 1.
[edit] Write After Read (WAR)
(j tries to write a destination before it is read by i) A write after read (WAR) data hazard represents a problem with concurrent execution.
i1. R4 <- R1 + R3 i2. R3 <- R1 + R2
If we are in a situation that there is a chance that i2 may be completed before i1 (i.e. with concurrent execution) we must ensure that we do not store the result of register 3 before i1 has had a chance to fetch the operands.
[edit] Write After Write (WAW)
(j tries to write an operand before it is written by i) A write after write (WAW) data hazard may occur in a concurrent execution environment.
i1. R2 <- R1 + R2 i2. R2 <- R4 + R7
We must delay the WB (Write Back) of i2 until the execution of i1.
Issue ADDD here despite name dependency on F6?

20. Tomasulo Example Cycle 7
Add1 (SUBD) completing; what is waiting for it?

21. Tomasulo Example Cycle 8

22. Tomasulo Example Cycle 9

23. Tomasulo Example Cycle 10
Add2 (ADDD) completing; what is waiting for it?

24. Tomasulo Example Cycle 11
Write result of ADDD here?
All quick instructions complete in this cycle!

25. Tomasulo Example Cycle 12

26. Tomasulo Example Cycle 13

27. Tomasulo Example Cycle 14

28. Tomasulo Example Cycle 15
Mult1 (MULTD) completing; what is waiting for it?

29. Tomasulo Example Cycle 16
Just waiting for Mult2 (DIVD) to complete

30. Faster than light computation (skip a couple of cycles)

31. Tomasulo Example Cycle 55

32. Tomasulo Example Cycle 56
Mult2 (DIVD) is completing; what is waiting for it?

33. Tomasulo Example Cycle 57
Once again: In-order issue, out-of-order execution and out-of-order completion.

34. Why can Tomasulo overlap iterations of loops?
Reservation stations: renaming to larger set of registers + buffering source operands
Prevents registers as bottleneck
Avoids WAR hazards (by buffering old values of registers) and avoids WAW hazards
Allows loop unrolling in HW – “dynamic loop unrolling” (Register Renaming: Multiple iterations use different physical destinations for registers)
Not limited to basic blocks
Other perspective: Tomasulo building data flow dependency graph on the fly

35. Tomasulo’s scheme offers 2 major advantages
Distribution of the hazard detection logic
distributed reservation stations and the CDB
If multiple instructions waiting on single result, & each instruction has other operand, then instructions can be released simultaneously by broadcast on CDB
If a centralized register file were used, the units would have to read their results from the registers when register buses are available
Elimination of stalls for WAW and WAR hazards

36. Tomasulo Drawbacks Complexity Performance limited by Common Data Bus
delays of 360/91, MIPS 10000, Alpha 21264, IBM PPC 620 in CA:AQA 2/e, but not in silicon!
Performance limited by Common Data Bus
Each CDB must go to multiple functional units high capacitance, high wiring density
Number of functional units that can complete per cycle limited to one!
Multiple CDBs  more FU logic for parallel assoc stores
Non-precise interrupts!
We will address this later

37. Next Topics Things To Do Project proposal is due Tuesday, March 1
Xing - Fall09
Next Topics
Ch2. Dynamic Scheduling
Things To Do
Project proposal is due Tuesday, March 1
5-10 mins project proposal presentation on March. 1
Check out the class website about
lecture notes
reading assignments