. Algebraic transformations and re-ordering
- Remove/simplify operations like
. Multiplication by 1
. Multiplication by 0
. Addition with 0
- Reorder instructions based on
. Commutative properties of operators
. For example x+y is same as y+x (always?)
Instruction selection
- Addressing mode selection
- Opcode selection
- Peephole optimization
Some of the different optimization methods are :
1) Constant Folding - replacing y= 5+7 with y=12 or y=x*0 with y=0
2) Dead Code Elimination - e.g.,
If (false)
a = 1;
else
a = 2;
with
a = 2;
3) Peephole Optimization - a machine-dependent optimization that makes a pass
through low-level assembly-like instruction sequences of the program( called
a peephole), and replacing them with a faster (usually shorter) sequences by
removing redundant register loads and stores if possible.
4) Flow of Control Optimizations
5) Strength Reduction - replacing more expensive expressions with cheaper ones
- like pow(x,2) with x*x
6) Common Sub expression elimination - like a = b*c, f= b*c*d with
temp = b*c, a= temp, f= temp*d;
