Advantages of Multipass Compiler Over Single Pass Compiler

Programmers, write computer programs that make certain tasks easier for users. This program code is written in High-Level Programming languages like C, C++, etc. Computer device doesn't understand this language or the program written by a programmer, so the translator that translates the High-Level Program code into Machine Readable Instructions is known as Compiler. 

Types of Compiler

There are 2 types of compilers that are described below.

• Single Pass Compiler
• Multiple Pass Compiler 

In this article, we will go deep into the understanding of Single Pass and Multiple Pass compilers and explore the advantages of multipass compilers over single-pass compilers.

Single Pass Compiler

Single Pass Compiler is the most basic compiler in which source code or program code goes through all the stages of compilation. Here, the code is read only once and translation is also done in a single go. This compiler translates each part into its final machine program. Single Pass Compiler is also called the Narrow Compiler.

Initially, the compiler scans the program code during Lexical Analysis and divides it into the same categorical tokens. After, with the help of the Parse Tree, the syntax analysis is been performed, which assures the code with proper grammar. Finally, the machine code is generated which is been analyzed by the Computer. There is no Intermediate Code Generation stage in Single-Pass Compiler. This compiler requires less memory and generates machine code faster.

Multi-Pass Compiler

In Multi-Pass Compiler, the program code goes via multiple passes during the process of compilation. It also generated intermediate optimized code for the completion of each step. Later, it translates the source program into 1 or more intermediate steps between program code and machine code. Multi-Pass compiler makes the program code perform parsing, analyzing, code generation, etc processes multiple times. Here, each pass takes the result of the previous pass as the input and then generates the intermediate code or output.

From the above diagram, we can see that in the first pass, the compiler analyses the source/program code, generates the token, and saves the result for the next step. In Pass 2, the compiler read the result created by Pass 1, builds the parse tree, and performs the syntactical analysis. Output is also generated here for the next step.

In Pass 3, the compiler reads the output of Pass 2 and checks the grammar using the tree. Output is created and given as the input to Pass 4. This passes continues till the final output is been generated.

Advantages of Multipass Compiler Over Single Pass Compiler

There are many advantages of Multi pass compiler over a Single Pass Compiler which makes it the best tool for the translation of High-Level program code into Machine-Readable Instructions.Let's see some of the advantages:

• Optimizations: As compared to the Single Pass Compiler, Multi Pass compilers perform more advanced optimizations on the source program, they can properly analyze the code throughout the multiple passes and apply suitable optimization techniques that require general understanding and efficient code structure. This optimization leads to efficient code generation and also enhances the overall performance.

• Reusability: Multi-Pass Compiler is machine independent, as this includes more than 1 pass that is in a proper modular structure, and also the code generated in each pass is created separately, these passes can also be reused for various other machines, and the efforts for recompilation is been saved using the Multi-Pass compiler.

• Complex Languages Support: Multi-Pass Compiler has the capability to manage complex programming languages more deliberately as compared to Single Pass compiler. Some of the languages have features that need multiple passe to properly scan and generate code, so this can be fulfilled by Multi Pass Compiler.

• Error Detection Capability: No doubt, Multi Pass Compiler has good error detection and reporting as compared to Single Pass Compiler. As the Multi-Pass Compiler takes multiple passes on Source Code, easily error catching and inconsistencies in the program code can be done, which is quite difficult in Single Pass Compiler.

• Cross-Referencing Support: In most languages, functions, variables or types are to be declared before they are being used. So to handle this cross-referencing, Multi Compiler is a good option as compared to Single Pass Compiler because due to multiple passes, a Multi-Pass Compiler can easily resolve these dependencies more easily and efficiently, by assuring a decrement in the chances of errors while execution. 
• Global analysis: Multipass compilers have the advantage of global analysis since they consider the entire program in multiple passes. This enables to make better decisions based on the complete context of the program.
• Memory efficiency: Multipass compilers can be more memory - efficient than single pass compilers. Single pass compilers processes code sequentially and may need to store intermediate data, while multipass compilers can release data after each pass.
• Code generation: Multipass compilers generate the code that suits the target architecture. They can optimize the generated code for specific hardware, and hence increasing the performance and portability.
• Code size reduction: Multipass compilers uses code reduction techniques like dead code elimination and code compression, that helps in generating smaller executable files./
• Loop optimization: Multipass compilers can apply loop optimizations, such as loop unrolling and loop-invariant code motion, more effectively. These optimizations improve the performance of code within loops.
• Efficient register allocation: Multipass compilers can perform advanced register allocation and optimization techniques. This helps in efficient use of registers for variables and operations.