Code Optimization

(最適化)

Language Theory and Compilers

Code generation is machine dependent
The resulting code can be represented in assembly language
There are no machine instructions for conditionals, if/for/... statements
⇒ we have to use conditional jump instructions
jump uses comparison with 0 as a condition
In C, jump can be expressed with goto
Restricted C is an intermediate representation between (full) C and assembly language

Make program execution faster
Reduce code size
Additional considerations:
- Maintain program functionality (meaning)
- Speed of compilation
- Ease of debugging
- Properties of target CPU/machine

Peephole optimization: Local exchange of instructions,...
Control flow analysis
- Split program into basic blocks:
  - No jumps from outside
  - No jumps to other locations
  → linear control flow inside block
- Create a graph where:
  - Basic blocks are nodes
  - Jumps from (the end of) one block to (the start of) another are directed edges
- Example: Control flow analysis for Homework 2
Data flow analysis
Use control flow graph to analyse which variable assignments affect which variable usages

Repeatedly try to use different techniques to continuously optimize the code further

Constant folding
24 * 60 * 60 ⇒ 86400
Constant propagation
a = 3; b = a * 4; ⇒ a = 3; b = 3 * 4;
Move common code out of loops
while (a<500) { a += b*c*d; }
⇒ e = b*c*d; while (a<500) { a+= e; }
Dead code elimination: Eliminate code that will never be executed
Reuse of register values
STORE a, R1 LOAD R1, a
⇒
STORE a, R1

Make execution faster even if the amout of code may increase

Function inlining:

double square(double x) { return x*x; }
square(a);

⇒a*a

Highly dependent on machine type

Exchange of instructions (different execution time for different instructions)
Example: x*2 ⇒ x+x or x<<1
Instruction reordering (example: instructions such as LOAD may take longer, but run in parallel)

Expression: 5 * a

Before optimization (LOAD (memory access) takes time):

        CONST   R1, 5
        LOAD    R2, a
        MUL     R3, R1, R2

After optimization (during LOAD, CONST can also be executed):

        LOAD    R2, a
        CONST   R1, 5
        MUL     R3, R1, R2

without optimization (gcc -O0 -S code.c)

with optimization (gcc -O1 -S code.c)

much more optimized (gcc -O3 -S code.c, with parallel processing instructions)

(assembly language for Intel PCs (CISC))

Optimization options for gcc: English, Japanese

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