优化技术

当我们完成计算机语言编译后我们能够得到工作机器的代码,但是这一结果是不稳定的,在代码的生成过程中会发生遗落,因此我们需要对输出结果进行优化,本章节就此介绍了我们对编译结果如何进行优化,介绍了优化的几种方法。
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1.Chap 10: Optimization Prof. Steven A. Demurjian Computer Science & Engineering Department The University of Connecticut 371 Fairfield Way, Unit 2155 Storrs, CT 06269-3155 steve@engr.uconn.edu http://www.engr.uconn.edu/~steve (860) 486 - 4818 Material for course thanks to: Laurent Michel Aggelos Kiayias Robert LeBarre

2.Overview Motivation and Background Code Level Optimization Common Sub-expression elimination Copy Propagation Dead-code elimination Peephole optimization Load/Store elimination Unreachable code Flow of Control Optimization Algebraic simplification Strength Reduction Concluding Remarks/Looking Ahead

3.Motivation What we achieved We have working machine code What is missing Code generation does not see the “big” picture We can generate poor instruction sequences What we need A simple way to locally improve the code quality Goal: Transition from “Lousy” Intermediate Code to More Effective and Efficient Code Response Time, Performance (Algorithms), Memory Usage Measured in terms of Number of Variables Saved, Operands Saved, Memory Accesses, etc.

4.Where can Optimation Occur? Software Engineer can: Profile Program Change Algorithm Data Transform/Improve Loops Front End LA, Parse, Int. Code Code Generator Int. Code Target Program Source Program Compiler Can: Improve Loops/Proc Calls Calculate Addresses Use Registers Selected Instructions Perform Peephole Opt. All are Optimizations 1 st is User Controlled and Defined At Intermediate Code Level by Compiler At Assembly Level for Target Architecture (to take advantage of different machine features)

5.Code Level Optimization First Look at Optimization Section 9.4 in 1 st Edition Introduce and Discuss Basic Blocks Requirements for Optimization Section 10.1 in 1 st Edition Basic Blocks, Flow Graphs Indepth Examination of Optimization Section 10.2 in 1 st Edition Function Preserving Transformations Loop Optimizations

6.First Look at Optimization Optimization Applied to 3 Address Coding (3AC) Version of Source Program - Examples: A + B[i] * c t1 = b[i] t2 = t1 * a t3 = t2 * c

7.First Look at Optimization Once Code has been Generated in 3AC, an Algorithm can be Applied to: Identify each Basic Block which Represents a set of Three Address Statements where Execution Enters at Top and Leaves at Bottom No Branches within Code Represent the Control Flow Dependencies Among and Between Basic Blocks Defines what is Termed a “Flow Graph” Let’s see an Example

8.First Look at Optimization Steps 1 to 12 from two Slides Back Represented as: Optimization Works with Basic Blocks and Flow Graph to Perform Transformations that: Generate Equivalent Flow Graph w/Improved Perf.

9.First Look at Optimization Optimization will Perform Transformations on Basic Blocks/Flow Graph Resulting Graph(s) Passed Through to Final Code Generation to Obtain More Optimal Code Two Fold Goal of Optimization Reduce Time Reduce Space Optimization Used to Come at a Cost: In “Old Days” Turning on Optimizer Could Double the Compilation Time From 2 hours to 4 hours Is this an Issue Today?

10.First Look at Optimization Two Types of Transformations Structure Preserving Inherent Structure and Implicit Functionality of Basic Blocks is Unchanged Algebraic Elimination of Useless Expressions x = x + 0 or y = y * 1 Replace Expensive Operators Change x = y ** 2 to x = y * y Why? We’ll Focus on Both …

11.Structure Preserving Transformations Common Sub-Expression Elimination How can Following Code be Improved? a = b + c b = a – d c = b + c d = a – d What Must Make Sure Doesn’t happen? Dead-Code Elimination If x is not Used in Block, Can it be Removed? x = y + z What are the Possible Ramifications if so? d = b

12.Structure Preserving Transformations Renaming Temporary Variables Consider the code t = b + c Can be Changed to u = b + c May Reduce the Number of temporaries Make Change from all t’s to all u’s Interchange of Statements Consider and Change to: t1 = b + c t2 = x + y t2 = x + y t1 = b + c This can Occur as Long as: x and y not t1 b and c not t2 What Do you have to Check?

13.Requirements for Optimization Identify Frequently Executed Portions of Code and Make them Perform Better Rule-of-Thumb - Most Programs spend 80% of their Time in 20% of Code – Is this True? We Focus on Loops since Every Gain in Space or Time is Multiplied by Loop Iterations Reduce Loop’s Code and Improve Performance What Other Programming Technique Should be a Major Concern for Optimization?

14.Requirements for Optimization Criteria for Transformations Preserve Meaning of Code Don’t Change Output, Introduce Errors, etc. Speed up Programs by Measurable Amount (on Average for Entire Code) Must be Work the Effort Stick to Meaningful, Useful Transformations Provide Different Versions of Compiler Non-Optimizing Optimizing Extra Optimization on Demand

15.Requirements for Optimization Beware that Some Optimization Directives are Ignored! In C, Define variable as “register int I;” While a Feature of Language, cc States that these Instructions are Ignored and Compiler Controls Use of Registers

16.The Overall Optimization Process Advantages Intermediate Code has Explicit Operations and Their Identification Promotes Optimization Intermediate Code is Relatively Machine Independent Therefore, Optimization Doesn’t Impact Final Code Generation

17.Example Source Code

18.Generated Three Address Coding

19.Flow Graph of Basic Blocks

20.Indepth Examination of Optimization Code-Transformation Techniques: Local – within a “Basic Block” Global – between “Basic Blocks” Data Flow Dependencies Determined by Inspection what do i, a, and v refer to? Dependent in Another Basic Block Scoping is Very Critical

21.Indepth Examination of Optimization Function Preserving Transformations Common Subexpressions Copy Propagation Deal Code Elimination Loop Optimizations Code Motion Induction Variables Strength Reduction

22.Common Sub-Expressions E is a Common Sub-Expression if E as Previously Computed Value of E Unchanged since Previous Computation What Can be Saved in B5? t6 and t7 same computation t8 and t10 same computation Save: Remove 2 temp variables Remove 2 multiplications Remove 4 variable accesses Remove 2 assignments t6 := 4 * i x := a[t6] t7 := 4 * i t8 := 4 * j t9 := a[t8] a[t7] := t9 t10 := 4 * j a[t10]:= x Goto B2 t6 := 4 * i x := a[t6] t8 := 4 * j t9 := a[t8] a[t6] := t9 a[t8]:= x Goto B2

23.Common Sub-Expressions What about B6? t11 and t12 t13 and t15 Similar Savings as in B5 t11 := 4 * i x := a[t11] t12 := 4 * i t13 := 4 * n t14 := a[t13] a[t12]:= t14 t15 := 4 * n a[t15]:= x t11 := 4 * i x := a[t11] t13 := 4 * n t14 := a[t13] a[t11]:= t14 a[t13]:= x

24.Common Sub-Expressions What else Can be Accomplished? Where is Variable j Determined? In B3 – and when drop through B3 to B4 and into B5, no change occurs to j! What Does B5 Become? Are we done? No t9 same as t5! Again savings in access, variables, operations, etc. t6 := 4 * i x := a[t6] t8 := 4 * j t9 := a[t8] a[t6] := t9 a[t8]:= x Goto B2 j := j - 1 t4 := 4 * j t5 := a[t4] if t5>4 goto B3 B4 t6 := 4 * i x := a[t6] t9 := a[t4] a[t6] := t9 a[t4]:= x Goto B2 t6 := 4 * i x := a[t6] a[t6] := t5 a[t4]:= x Goto B2

25.Common Sub-Expressions Are we done yet? Where is “i” defined? Any Values we can Leverage? Yes! t2 = 4*i Defined in B2 and is unchanged as it arrives at B5 t3 = a[t2] in B3 and B2 and also unchanged as it arrives Result at Left Saves: From 9 statements down to 4 4 Multiplications are Gone 4 addr/array offsets are only 2 t6 := 4 * i x := a[t6] a[t6] := t5 a[t4]:= x Goto B2 x := t3 a[t2] := t5 a[t4]:= x Goto B2

26.Common Sub-Expressions B6 is Similarly Changed …. t11 := 4 * i x := a[t11] t13 := 4 * n t14 := a[t13] a[t11]:= t14 a[t13]:= x x := t3 t14 := a[t1] a[t2]:= t14 a[t1]:= x

27.Resulting Flow Diagram

28.Copy Propagation Introduce a Common Copy Statement to Replace an Arithmetic Calculation with Assignment Regardless of the Path Chosen, the use of an Assignment Saves Time and Space a:= d + e b:= d + e c:= d + e a:= d + e a:= t b:= d + e a:= t c:= t

29.Copy Propagation In our Example for B5 and B6 Below: Since x is t3, we can replace the use of x on right hand side as below: We’ll come back to this shortly! x := t3 t14 := a[t1] a[t2]:= t14 a[t1]:= x x := t3 a[t2] := t5 a[t4]:= x Goto B2 x := t3 t14 := a[t1] a[t2] := t14 a[t1] := t3 x := t3 a[t2] := t5 a[t4] := t3 Goto B2