Back-End Compiler Design: Instruction Selection u0026 Code Generation Quiz Quiz

Assess your understanding of back-end compiler design with a focus on instruction selection and code generation techniques. This quiz covers core concepts, including pattern matching, target-dependent code, tree-based approaches, and optimization essentials within compiler back-ends.

1. Role of Instruction Selection

   In compiler back-ends, what is the primary role of the instruction selection phase when translating an intermediate representation (IR) into target code?

   1. To map IR operations to specific machine instructions optimized for the target architecture
   2. To rearrange the IR to improve high-level readability for programmers
   3. To perform lexical analysis on the IR statements
   4. To perform garbage collection before generating code

   Explanation: The main purpose of instruction selection is to translate IR operations into appropriate target machine instructions, ensuring the generated code is efficient and compatible with the hardware. Option B is incorrect as readability is not a concern at this stage. Option C confuses instruction selection with the earlier lexical analysis phase. Option D represents a process unrelated to code generation.

2. Tree Pattern Matching

   Which common data structure is often used in instruction selection algorithms to match patterns between intermediate code and machine instructions?

   1. Graphs
   2. Syntax trees
   3. Linked lists
   4. Hash tables

   Explanation: Syntax trees are widely used in instruction selection as they represent the hierarchical structure of expressions, allowing efficient matching with machine instruction patterns. Graphs are more complex and typically reserved for control flow representation. Hash tables and linked lists serve different data handling purposes and are not central to this matching process.

3. Target-Dependent Code Generation

   Why does code generation in the back-end of a compiler need to be target-dependent?

   1. Because target-independent code is always slower
   2. Because IR cannot be optimized
   3. Because each hardware architecture has unique instruction sets and calling conventions
   4. Because the syntax of source code changes with the target machine

   Explanation: Target-dependent code generation is necessary due to differences in instruction sets, registers, and conventions between architectures. Option B is incorrect; slowdowns aren’t the sole reason. Option C is wrong as source code syntax does not depend on target hardware. Option D is irrelevant as optimization is possible at various stages.

4. Peephole Optimization Placement

   At which stage is peephole optimization typically applied in the back-end of a compiler, and what does it commonly achieve?

   1. During syntax analysis, to improve parsing accuracy
   2. Before code generation, to speed up lexical analysis
   3. In the front-end, to optimize high-level code structure
   4. After instruction selection, to replace short instruction sequences with more efficient ones

   Explanation: Peephole optimization is performed after instruction selection, scanning for and replacing inefficient instruction sequences with better choices. Option B is incorrect as lexical analysis occurs much earlier. Option C is unrelated to syntax analysis. Option D confuses front-end optimization with back-end code improvements.

5. Linear Scan vs. Tree-Covering

   Which characteristic best distinguishes the linear scan approach from the tree-covering approach in instruction selection?

   1. Both approaches always produce identical output
   2. Linear scan is used only for high-level languages, never for machine code
   3. Tree-covering uses hash tables, while linear scan uses only recursion
   4. Linear scan processes code sequentially, while tree-covering matches patterns in a hierarchical tree

   Explanation: The linear scan method processes operations in the order they appear, whereas tree-covering leverages hierarchical trees to match patterns optimally. Option B is false as the approaches may yield different outputs. Option C inaccurately describes the use of hash tables and recursion. Option D incorrectly claims linear scan is restricted to high-level languages.