Program Representations - Advanced Compiler - Lecture Slides , Slides of Computer Science

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Program Representations

Representing programs

• Goals

Option 1: high-level syntax based IR

• Represent source-level structures and expressions directly
• Example: Abstract Syntax Tree

Option 2: low-level IR

• Translate input programs into low-level primitive chunks, often close to the target machine
• Examples: assembly code, virtual machine code (e.g. stack machines), three- address code, register- transfer language (RTL) - Standard RTL instrs:

Comparison

Comparison

• Advantages of high-level rep
  • analysis can exploit high-level knowledge of constructs
  • easy to map to source code (debugging, profiling)
• Advantages of low-level rep
  • can do low-level, machine specific reasoning
  • can be language-independent
• Can mix multiple reps in the same compiler

Control dependencies

• Option 1: high-level representation
  • control implicit in semantics of AST nodes
• Option 2: control flow graph (CFG)
  • nodes are individual instructions
  • edges represent control flow between instructions
• Options 2b: CFG with basic blocks
  • basic block: sequence of instructions that don’t have any branches, and that have a single entry point
  • BB can make analysis more efficient: compute flow functions for an entire BB before start of analysis

Control dependencies

• CFG does not capture loops very well
• Some fancier options include:
  • the Control Dependence Graph
  • the Program Dependence Graph
• More on this later. Let’s first look at data dependencies

Def/use chains

• Directly captures dataflow
  • works well for things like constant prop
• But...
• Ignores control flow
  • misses some opt opportunities since conservatively considers all paths
  • not executable by itself (for example, need to keep CFG around)
  • not appropriate for code motion transformations
• Must update after each transformation
• Space consuming

SSA

• Static Single Assignment
  • invariant: each use of a variable has only one def

SSA

• Create a new variable for each def
• Insert φ pseudo-assignments at merge points
• Adjust uses to refer to appropriate new names
• Question: how can one figure out where to insert φ nodes using a liveness analysis and a reaching defns analysis.

Converting back from SSA

• Semantics of x 3 := φ(x 1 , x 2 )
  • set x 3 to xi if execution came from ith predecessor
• How to implement φ nodes?

Common Sub-expression Elim

• Want to compute when an expression is available in a var
• Domain:

Flow functions

X := Y op Z

in

out

FX := Y op Z (in) =

X := Y

in

out

FX := Y(in) =