hakorune/docs/archive/proposals/phase-195-pattern4.md

Phase 195: Pattern 4 (Loop with Continue) Implementation Plan

Status: Historical

Status: Deferred (not yet implemented)

Overview

Pattern 4 handles loops with continue statements that skip to the next iteration. This is the most complex loop pattern due to additional control flow requirements.

Why Deferred?

1. Continue semantics require additional PHI and control flow analysis

   • Continue creates an additional edge to the loop header
   • Requires phi nodes for both continue and normal paths
   • More complex than break (which exits the loop)

2. Pattern 3 covers most practical cases

   • Pattern 1: Simple while loops
   • Pattern 2: Loops with break
   • Pattern 3: Loops with if + PHI (most common complex pattern)
   • Pattern 4: Loops with continue (less common in practice)

3. Lower priority than break/if patterns

   • Break patterns (Pattern 2) are more common
   • If + PHI patterns (Pattern 3) handle complex control flow
   • Continue can often be refactored using if statements

Example Use Case

local i = 0
local sum = 0
loop(i < 10) {
  i = i + 1
  if (i % 2 == 0) {
    continue  // Skip even numbers
  }
  sum = sum + i
}
// sum = 25 (1+3+5+7+9)

Implementation Requirements

When implemented, Pattern 4 lowering will need to:

1. Detect continue statements in the loop body
2. Generate PHI nodes for continue target (loop header)
3. Handle carrier variables (i, sum) across continue boundaries
4. Generate exit PHI nodes for final values after loop

Control Flow Diagram

       header
         |
         v
       body
         |
    /----+----\
   /           \
  v             v
continue      normal
  |             |
  \-----+-------/
        |
        v
      latch
        |
    /---+---\
   /         \
  v           v
loop        exit

Workaround

Until Pattern 4 is implemented, use Pattern 3 (if + PHI) instead:

local i = 0
local sum = 0
loop(i < 10) {
  i = i + 1
  if (not (i % 2 == 0)) {  // Invert condition
    sum = sum + i
  }
}

Migration Path

1. Pattern 1: Simple while loops (no break/continue)
2. Pattern 2: Loops with break
3. Pattern 3: Loops with if + PHI
4. Pattern 4: (FUTURE) Loops with continue statements

Related Files

• src/mir/builder/control_flow/joinir/patterns/pattern4_with_continue.rs - Stub implementation
• src/mir/join_ir/lowering/loop_with_continue_minimal.rs - Lowering logic (TODO)

Timeline

• Phase 195+: Implementation planned but deferred
• Priority: Lower than Pattern 1-3
• Complexity: High (additional control flow edges)