Control Flow Code Generation

Topic Overview

Branches and Labels in Codegen

If statements, loops, and short-circuit logic require the code generator to emit labels and conditional jumps. GCC's expand_expr and LLVM's CreateCondBr split control flow into basic blocks before machine lowering.

If-Else Pattern

Evaluate condition to a temp. Emit conditional jump to else label. Generate then-branch. Unconditional jump to end. Else label. Generate else-branch. End label.

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• Short-circuit &&: jump past right operand if left is false
• While loops: head label, test, body, jump to head
• Phi destruction may insert moves at block ends
• Fall-through layout reduces extra jumps

Connecting to real compilers

Production compilers embed this step inside a longer pipeline. GCC flows through cpp, cc1, assembly, and ld; Clang uses the driver, Sema, LLVM IR passes, and a target backend. LLVM bitcode, JVM bytecode, and WASM are other familiar IRs at the same layer. The exercise isolates one pass so you can test it alone before chaining it to the next stage.

Why for this exercise

You will implement control flow code generation for if, else, and loops. This exercise requires emitting labels, conditional branches, and merge points so execution follows the AST's control structure.

Your Task

Generate code for control flow using jumping code: a condition is never turned into a 0/1 value. Instead it compiles directly into conditional jumps, with &&, || and ! handled purely by where the jumps go. This is how real compilers emit if, loops, break and continue.

Input

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&& binds tighter than ||. A bare expr used as a condition means expr != 0. break/continue only appear inside loops.

Translation scheme (follow it exactly)

Expressions translate as in Translate an AST into Your IR: operands first, then a new temporary tN per operator (t = a OP b). Labels L1, L2, … and temporaries are numbered in allocation order.

Conditions. jump(c, want, L) emits code that jumps to L if c evaluates to want, and falls through otherwise:

Statements (labels are allocated when the statement starts):

Output

The TAC, then temps=T labels=L.

Example

Input:

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Output:

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No temporaries: the condition exists only as control flow.

Requirements

1. Parse into an AST first (the if rule needs to know whether an else exists before allocating labels).
2. One recursive jump(cond, want, label) function, and a stack of loop labels for break/continue.

Hidden tests cover for loops with continue and break, || combined with !, nested loops (break leaves only the inner one), arithmetic inside relations, and bare-expression conditions.