hakorune/docs/development/current/main/mir-nested-if-loop-bug.md

MIR Nested-If-in-Loop Bug (Critical)

Date: 2025-12-04 Severity: Critical Status: Workaround Applied, Root Fix Needed

🐛 Problem Summary

Symptom: Infinite loop when using nested if-else statements inside loop() blocks.

Error Message:

[ERROR] VM error: vm step budget exceeded (max_steps=1000000, steps=1000001)

📊 Root Cause Analysis

MIR Lowering Bug

The MIR builder generates incorrect control flow for nested if-else statements inside loops:

// ❌ Causes infinite loop
loop(condition) {
    if expr1 {
        if expr2 {
            // code
        } else {
            break
        }
    } else {
        break
    }
}

Control Flow Issue

Generated MIR:

bb4: loop header (PHI)
bb6: unconditional jump to bb4
bb11: unconditional jump to bb6

Jump chain: bb11 → bb6 → bb4 → ... (infinite)

Problem: The PHI node in bb4 never gets updated because the execution gets stuck in the bb11→bb6→bb4 jump chain.

🔬 Reproduction Case

Minimal Test Case

static box Main {
  main() {
    local i = 0

    loop(i < 3) {
      local x = 1

      if x == 1 {
        if x == 1 {
          i = i + 1
        } else {
          break
        }
      } else {
        break
      }
    }

    return 0
  }
}

Result: vm step budget exceeded at bb6

MIR Dump Analysis

bb6:
    1: br label bb4

bb11:
    1: br label bb6

Infinite unconditional jump chain with no PHI update.

✅ Workaround

Strategy: Flatten Nested Ifs

Before (infinite loop):

loop(p < s.length()) {
    local ch = s.substring(p, p+1)
    if ch >= "0" && ch <= "9" {
        num_str = num_str + ch
        p = p + 1
    } else {
        break
    }
}

After (fixed):

local parsing_done = 0
loop(p < s.length()) {
    if parsing_done == 1 { break }

    local ch = s.substring(p, p+1)
    local digits = "0123456789"
    local digit_pos = digits.indexOf(ch)

    if digit_pos >= 0 {
        num_str = num_str + ch
        p = p + 1
    } else {
        parsing_done = 1
    }
}

Patterns to Avoid

1. Nested if-else in loop:

   loop(cond) {
       if a {
           if b { ... } else { break }
       } else { break }
   }
   

2. && operator in loop condition:

   loop(cond) {
       if x >= "0" && x <= "9" { ... }
   }
   

Safe Patterns

1. Flatten with flags:

   local done = 0
   loop(cond) {
       if done == 1 { break }
       // single-level if statements
   }
   

2. Use indexOf instead of range check:

   local digits = "0123456789"
   if digits.indexOf(ch) >= 0 { ... }
   

📋 Affected Code

Fixed Files

1. tools/hako_shared/json_parser.hako:

   • _parse_number(): Used indexOf() workaround
   • _parse_string(): Flattened escape sequence check
   • _unescape_string(): Flattened ch == "\\" && i + 1 < s.length()

2. Converted while → loop() (10 occurrences):

   • All while loops converted to loop() syntax per language spec

Commit

• Commit: 608693af
• Title: fix(json_parser): Fix infinite loop by working around MIR nested-if bug
• Files Changed: 1 file, +45/-23 lines

🎯 Root Fix Needed

MIR Builder Issue

Location: src/mir/builder/ (control flow lowering)

Problem: When lowering nested if-else inside loops, the builder creates:

• Unreachable PHI nodes
• Unconditional jump chains
• Missing latch block updates

Solution Required:

1. Fix control flow graph generation for nested conditionals
2. Ensure PHI nodes are properly connected
3. Add test cases for nested if-else in loops

Test Coverage

Add comprehensive tests for:

• Nested if-else in loops (2+ levels)
• && and || operators in loop conditions
• break and continue in nested contexts

📚 Related Issues

• Phase 173 Task 3: JsonParserBox bug fix (completed with workaround)
• CLAUDE.md: while → loop() syntax migration
• Loop Builder: src/mir/loop_builder.rs (potential fix location)

🔗 References

• Test Case: test_nested_if_loop.hako (reproduces bug)
• JSON Parser: tools/hako_shared/json_parser.hako (workaround applied)
• CURRENT_TASK.md: Phase 173 tracking

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Status: Workaround deployed, root fix tracked for future MIR lowering improvements.