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1 .Introduction to LLVM (II) Bojian Zheng CSCD70 Spring 2018 bojian@cs.toronto.edu 1
2 .Makefile Error: Optimize.mk In the Optimize.mk file provided in the first assignment, you might need to add ./ in front of the optimizer target FunctionInfo.so . Thanks a lot to Chengyu (Tyrone) Xiong for pointing this out. 2
3 .Review Keywords: Intermediate Representation (IR) Optimization Pass Analysis & Transformation 3
4 .Review Keywords: Program Structure Iterators Downcasting LLVM Pass Interface 4
6 .Insert/Remove/Move/Replace Instructions Three Options Instruction class methods. Ask parent ( BasicBlock ) to do this. Make use of BasicBlockUtils . 6
7 .Attention! Iterator Hazard As you do transformations, iterators might be invalidated . Demo on std ::vector < unsigned > ::iterator Thanks a lot to Qiongsi Wu for bringing this up. 7
8 .Attention! Reference Updates Original Code %2 = add %1, 0 %3 = mul %2, 2 Transformed Code %2 = add %1, 0 %3 = mul ??? , 2 8
9 .Questions? Keywords: Iterator Hazard References Update (More Later On) 9
10 .LLVM Instruction: The User-Use- Usee Design Pattern 10
11 .LLVM Class Hierarchies 11 Value Instruction User
12 .Value ( Usee ) The Value class is the most important base class in LLVM. It has a type (integer, floating point, …): getType () It might or might not have a name: hasName () , getName () Keeps track of a list of User s that are using this Value . 12
13 .Instruction (User) An User keeps track of a list of Values that it is using as Operands : User user = … for (auto iter = user.op_begin (); iter != user.op_end (); ++ iter ) {Value * operand = * iter ; …} An Instruction is a User . 13
14 .But wait, … Is Instruction (User) a Value ( Usee ) ? DO NOT interpret this statement as “the result of Instruction is assigned to ”, instead, think this way – “ is the Value Representation of Instruction ”. Therefore, whenever we use the Value , we mean to use the Instruction . 14
15 .To Conclude Suppose we have an instruction: Instruction inst = … What is this instruction using? for (auto iter = inst. op _begin (); iter != inst. op _end (); ++ iter ) {…} What is using this instruction? for (auto iter = inst. user _begin (); iter != inst. user _end (); ++ iter ) {…} 15
16 .Questions? Keywords: User-Use- Usee Design Pattern 16
18 .Optimizer Manager What is this doing? void Analysis:: getAnalysisUsage ( AnalysisUsage & AU) const { AU.setPreservesAll (); } Very frequently, when writing a pass, we want the followings: What information does this pass require? Will this information still be preserved after this pass? 18
19 .Questions Keywords: Require Preserve 19
20 .Code Download Links https://github.com/ArmageddonKnight/CSCD70-Tutorial-Demo Visitor Design Pattern serves as an alternative to Dynamic Casting. You can find an example on this in the repository. 20
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