Speculation and Predication in Compilers

Beatriz Aguilar Gallo

SPECULATION AND

PREDICATION

1.  Introduction

2.  Introduction to Predication

3.  Introduction to Speculation

4.  Intel Itanium (IA-64), EPIC Architecture

5.  Conclusions

OUTLINE

!  ILP measured the number of instructions that can be performed during a single clock cycle

1. INTRODUCTION INTRODUCTION | INTRODUCTION TO PREDICATION | INTRODUCTION TO SPECULATION | INTEL ITANIUM (IA-64), EPIC ARCHITECTURE | CONCLUSIONS


!  Limited by: - Dependences - Branches




!  Two techniques to exploit ILP !  Predication !  Speculation




!  Two techniques to exploit ILP !  Predication !  Speculation

!  EPIC (Explicitly Parallel Instruction Computing) !  Intel Itanium processor


2. INTRODUCTION TO PREDICATION INTRODUCTION | INTRODUCTION TO PREDICATION | INTRODUCTION TO SPECULATION | INTEL ITANIUM (IA-64), EPIC ARCHITECTURE | CONCLUSIONS

!  Control dependences limit the amount of parallelism that can be exploited



!  Predication " An instruction refers to a condition, which is evaluated as part of the instruction execution.




!  If the condition is true, the instruction is executed normally





!  if the condition is false, the execution continues as if the instruction were a no-op






!  Effects of predication






!  Effects of predication !  Eliminate branches " execute both path of the flow






!  Effects of predication !  Eliminate branches " execute both path of the flow !  Converts control dependences into data dependences " If-conversion






!  Effects of predication !  Eliminate branches " execute both path of the flow !  Converts control dependences into data dependences " If-conversion


!  If-conversion " Transforms conditional branches into

! Operations that defines predicates

! Operations guarded by predicates, corresponding to alternative control paths





!  if the condition is false, the execution continues as if the instruction was a no-op

!  Effects of predication

!  Eliminate branches " execute both path of the flow

!  Converts control dependences into data dependences " If-conversion

!  Necessary include predicated instructions to the ISA


! Predication example


! Predication complications



!  Increase the fetch utilization




! When Annul an operation?





!  Early in the pipeline during instruction issue " the value of controlling condition must be known early





!  Early in the pipeline during instruction issue " the value of controlling condition must be known early

!  Later in the pipeline before commit any result or raise an exception " annulled instruction will consume FU and will have negative impact in performance


Predication is useful for


Predication in useful for ! Implementing short alternative control flows


Predication is useful for ! Implementing short alternative control flows ! Eliminating some unpredictable branches


Predication is useful for ! Implementing short alternative control flows ! Eliminating some unpredictable branches ! Reducing the overhead of global code scheduling




Predication usefulness limited by



Predication usefulness limited by ! Predicated instructions annulled take processor

resources



Predication usefulness limited by ! Predicated instructions annulled take processor resources

! Most useful when the predicate can be evaluated early





! Conditional instructions can be limited with complex alternative sequences





! Conditional instructions can be limited with complex alternative sequences

! Speed penalty compared with unconditional instructions

3. INTRODUCTION TO SPECULATION INTRODUCTION | INTRODUCTION TO PREDICATION | INTRODUCTION TO SPECULATION | INTEL ITANIUM (IA-64), EPIC ARCHITECTURE | CONCLUSIONS

!  Idea of speculation " refers to the early execution of memory operations in order to hide their latency



!  Two techniques to apply

! Hardware Speculation ! Software Speculation






! Two techniques to apply

! Hardware Speculation " hardware tables



! Hardware Speculation " hardware tables ! Advantages 1.) It is responsive to changes in program behavior



! Hardware Speculation " hardware tables

! Advantages 1.) It is responsive to changes in program behavior 2.) Can apply the optimization to selective instances



! Hardware Speculation " hardware tables ! Advantages 1.) It is responsive to changes in program behavior

2.) Can apply the optimization to selective instances

! Disadvantage " requires that the optimization can be applied as the instructions travel down the pipeline







! Software Speculation " directly into instructions



! Software Speculation " directly into instructions

! Advantages 1.) The program can be transformed assuming the speculations are correct



! Software Speculation " directly into instructions ! Advantages 1.) The program can be transformed assuming the speculations are correct

2.) The optimizations need not be applied on each dynamic instance as it travels down the pipeline





! Disadvantage 1.) Latency associated





! Disadvantage 1.) Latency associated 2.) Necessary a recompilation


! Two types of speculation

! Control Speculation

! Data Speculation











! Break control dependences





! How to break a control dependence?





! How to break a control dependence? Making an operation’s execution independent of the branch






! Problem "






! Problem " correct handling of exceptions


! Control Speculation Example:











! Breaks data flow dependences between memory operations





! How to break a data dependence?





! How to break a data dependence? Guessing that the two memory operations will access different locations.


! Data Speculation Example:


! Control + Data Speculation Example:


! How to deal with exceptions ?



!  Ignore exceptions for speculative instructions. “Fast mode” in several processors.




!  Checks are introduced to determine when an exception should occur.





!  Poison bits " attached to the result registers written by speculated instructions when the instructions cause exceptions.





!  Poison bits " attached to the result registers written by speculated instructions when the instructions cause exceptions.

!  Hardware buffers the instruction result until it is certain that the instruction is no longer speculative.


!  Explictily Parallel Instruction Computing" indicated parallelism of programs explicitly in the instruction sequence

4. INTEL ITANIUM (IA 64), EPIC ARCHITECTURE INTRODUCTION | INTRODUCTION TO PREDICATION | INTRODUCTION TO SPECULATION | INTEL ITANIUM (IA-64), EPIC ARCHITECTURE | CONCLUSIONS


!  Designed for high performance scientific and enterprise computing




!  Itanium support predication and speculation




!  Itanium support predication and speculation " is able to:





!  Increase throughput






!  Reduce the need for explicit transfers and associated pipeline stalls






!  Reduce the need for explicit transfers and associated pipeline stalls PREDICATION







!  Reduce the latency associated with loads from memory,







!  Reduce the latency associated with loads from memory SPECULATION


!  Predication in IA-64


!  Predication in IA-64 !  Every instruction in the IA-64 architecture can be predicated


!  Predication in IA-64 !  Every instruction in the IA-64 architecture can be predicated !  An instruction is predicated by specifying a predicate register


!  Predication in IA-64 !  Every instruction in the IA-64 architecture can be predicated !  An instruction is predicated by specifying a predicate register !  Lower overhead in if-conversion



!  Speculation in IA-64



!  Speculation in IA-64 !  Separate support for control speculation " deferring with exceptions


! It is possible use predication + speculation in IA-64?



YES, IMPACT project



YES, IMPACT project


!  Performance: Predication, Control Speculation, Data Speculation, Predication + Speculation


!  Itanium Nowadays


!  Itanium Nowadays !  Intel Itanium 9560 serie, Poulson (2012)


!  Itanium Nowadays !  Intel Itanium 9560 serie, Poulson (2012) !  Next " Kittson 32 nm instead of 22 nm



! Will it be the last Itanium?



! Will it be the last Itanium? ! Maybe…




! Why?




! Why? !  High performance chip 15-core Xeon E7 v2 chip





!  No compatibility problems






!  Out of order processors






!  Out of order processors !  Advantages:







!  Instructions from different basic blocks to execute at the same time







!  Instructions from different basic blocks to execute at the same time !  Dynamic scheduled





!  No compatibility problems !  Out of order processors

!  Advantages: !  Instructions from different basic blocks to execute at the same time !  Dynamic scheduled !  No changes in the ISA


!  Predication and speculation good techniques to ILP

5. CONCLUSIONS INTRODUCTION | INTRODUCTION TO PREDICATION | INTRODUCTION TO SPECULATION | INTEL ITANIUM (IA-64), EPIC ARCHITECTURE | CONCLUSIONS


!  Itanium was good to include these features in the past but nowadays the appearance of new processors that support out of order execution and are more compatibles with the applications is causing that the used of Itanium decrease.



!  Itanium was good to include these features in the past but nowadays the appearance of new processors that support out of order execution and are more compatibles with the applications is causing that the used of Itanium decrease.

!  Not always it is possible predict how the market will evolve


Beatriz Aguilar Gallo

SPECULATION AND

PREDICATION