What are the techniques used for code optimization in a compiler?

Code optimization in a compiler is achieved through techniques like loop optimization, constant folding, dead code elimination, and strength reduction.

Loop optimization is a technique where the compiler makes changes to the loop structure to reduce the overhead of loop control. This can involve unrolling the loop (replacing a loop with multiple copies of the loop body), loop fusion (combining two or more loops that iterate over the same range into one), and loop inversion (changing a while loop into a do-while loop).

Constant folding is another technique where the compiler evaluates constant expressions at compile time rather than runtime. For example, if the code contains an expression like '3*4', the compiler will replace it with '12' during the compilation process. This reduces the amount of computation required at runtime, thereby improving the performance of the program.

Dead code elimination is a technique where the compiler removes code that does not affect the program's output. This includes code that is never executed (like code after a return statement in a function), code whose results are not used, and code that computes the same result multiple times. By eliminating such code, the compiler can reduce the size of the compiled program and improve its speed.

Strength reduction is a technique where the compiler replaces a more expensive operation with a less expensive one. For example, the compiler might replace multiplication with addition, or replace division with multiplication by the reciprocal. This can significantly improve the performance of the program, especially in cases where the operation is performed many times.

In addition to these techniques, compilers also use other methods like function inlining (replacing a function call with the body of the function), peephole optimization (optimizing small sections of code), and data-flow analysis (analyzing the flow of data values in the program to optimize the code). These techniques help to further improve the efficiency and performance of the compiled program.