How C/C++ Compiler Generate Assembly Code For Switch Statement

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Structure of a Switch Statement in C/C++

A switch statement allows multi-way branching based on the value of an expression. It provides a more organized way to handle multiple conditions compared to using a series of if-else statements. A switch statement generally consists of a switch keyword followed by an expression in parentheses, and a block containing case labels and an optional default label. Here's an example:

Assembly Code Generation for Switch Statements

When a C/C++ program is compiled, the switch statement is translated into assembly code. The generated assembly code can differ based on the compiler used, the optimization level, and the target architecture. However, we can broadly discuss three common ways that compilers translate switch statements into assembly: jump tables, jump tables with case (index) tables, and conditional branches.

1. Jump table:

A jump table is an array of pointers to different code branches. The compiler calculates the index based on the input value and performs an indirect jump to the corresponding code block. Jump tables are efficient for dense and contiguous values in the case labels, as they allow constant-time lookup. The order of the case labels does not matter, as long as they are contiguous in general so that the compiler can calculate the offset of each case from the base address of the jump table:

2. Jump table with an additional case (index) table:

Under certain circumstances, compilers may generate an additional table, often called the "case table" or "index table," which stores the index of each case in the jump table. This approach is used when the case values are not dense but still have some regularity or pattern, making it more efficient than a series of conditional branches. The compiler calculates the index in the case table based on the input value and then performs a lookup in the jump table using the retrieved index.

Note below is just an simple example to illustrate the concept, if you copy and compile it and investigate the assembly, chances are the compiler will choose to use conditional branch approach instead!

3. Conditional branches:

If the case labels are sparse and do not exhibit a regular pattern, the compiler may just generate a series of conditional branches. Each branch tests the input value against the case label, and if it matches, the code jumps to the corresponding block. This method resembles a chain of if-else statements and can be slower for a large number of cases.