hakorune/docs/development/current/main/phase171-1-boundary-analysis.md

Phase 171-1: Boundary Coverage Analysis

Date: 2025-12-07 Status: Analysis Complete

Current Boundary Coverage Table

Component	Currently in Boundary?	How Mapped?	File Location
Loop variable (i)	✅ Yes	join_inputs[0] → host_inputs[0]	inline_boundary.rs:115-124
Carriers (sum, count)	✅ Yes	exit_bindings (Phase 190+)	inline_boundary.rs:150-167
Exit values	✅ Yes	exit_bindings.join_exit_value	exit_binding.rs:87-116
Condition inputs (start, end, len)	❌ NO	MISSING	N/A

---

Detailed Analysis

1. Loop Variable (i) - ✅ Properly Mapped

Mapping Flow:

HOST: variable_map["i"] = ValueId(5)
  ↓ join_inputs = [ValueId(0)]  (JoinIR local ID)
  ↓ host_inputs = [ValueId(5)]  (HOST ID)
JoinIR: main() parameter = ValueId(0)
  ↓ merge_joinir_mir_blocks() injects:
MIR: ValueId(100) = Copy ValueId(5)  // Boundary Copy instruction

Evidence:

• inline_boundary.rs:175-189 - new_inputs_only() constructor
• merge/mod.rs:104-106 - Boundary reconnection call
• Works correctly in all existing JoinIR tests

---

2. Carriers (sum, count) - ✅ Properly Mapped (Phase 190+)

Mapping Flow:

HOST: variable_map["sum"] = ValueId(10)
  ↓ exit_bindings = [
      LoopExitBinding {
        carrier_name: "sum",
        join_exit_value: ValueId(18),  // k_exit param in JoinIR
        host_slot: ValueId(10)          // HOST variable
      }
    ]
JoinIR: k_exit(sum_exit) - parameter = ValueId(18)
  ↓ merge_joinir_mir_blocks() remaps:
  ↓ remapper.set_value(ValueId(18), ValueId(200))
MIR: variable_map["sum"] = ValueId(200)  // Reconnected

Evidence:

• inline_boundary.rs:259-311 - new_with_exit_bindings() constructor
• exit_binding.rs:87-116 - Exit binding builder
• merge/mod.rs:188-267 - reconnect_boundary() implementation
• Works correctly in Pattern 3/4 tests

---

3. Condition Inputs (start, end, len) - ❌ NOT MAPPED

Current Broken Flow:

HOST: variable_map["start"] = ValueId(33)
  ↓ condition_to_joinir() reads from variable_map DIRECTLY
  ↓ lower_value_expression() returns ValueId(33)
JoinIR: Uses ValueId(33) in Compare instruction
  ↓ NO BOUNDARY REGISTRATION
  ↓ NO REMAPPING
MIR: ValueId(33) undefined → RUNTIME ERROR

Evidence (from Phase 170):

[ssa-undef-debug] fn=TrimTest.trim/1 bb=BasicBlockId(12)
                 inst_idx=0 used=ValueId(33)

Root Cause:

• condition_to_joinir.rs:183-189 - Reads from builder.variable_map directly
• condition_to_joinir.rs:236-239 - Returns HOST ValueId unchanged
• NO registration in JoinInlineBoundary
• NO remapping in merge_joinir_mir_blocks()

---

Why This is a Problem

Example: loop(start < end)

What SHOULD happen:

// HOST preparation
let start_host = ValueId(33);
let end_host = ValueId(34);

// JoinIR lowerer
let start_joinir = ValueId(0);  // Local param
let end_joinir = ValueId(1);    // Local param

// Boundary
JoinInlineBoundary {
  join_inputs: [ValueId(0), ValueId(1)],      // start, end in JoinIR
  host_inputs: [ValueId(33), ValueId(34)],    // start, end in HOST
  // ...
}

// Merge
// Injects: ValueId(100) = Copy ValueId(33)  // start
// Injects: ValueId(101) = Copy ValueId(34)  // end
// Remaps all JoinIR ValueId(0) → ValueId(100), ValueId(1) → ValueId(101)

What CURRENTLY happens:

// JoinIR lowerer
let start = builder.variable_map.get("start");  // Returns ValueId(33) - HOST ID!
let end = builder.variable_map.get("end");      // Returns ValueId(34) - HOST ID!

// JoinIR uses HOST ValueIds directly
Compare { lhs: ValueId(33), rhs: ValueId(34) }  // WRONG - uses HOST IDs

// No boundary registration → No remapping → UNDEFINED VALUE ERROR

---

Comparison: Loop Variable vs Condition Inputs

Aspect	Loop Variable (i)	Condition Inputs (start, end)
Who allocates ValueId?	JoinIR lowerer (alloc_value())	HOST (builder.variable_map)
Boundary registration?	✅ Yes (join_inputs[0])	❌ NO
Remapping?	✅ Yes (via boundary Copy)	❌ NO
Result?	✅ Works	❌ Undefined ValueId error

---

Root Cause Summary

The core problem is two-faced ValueId resolution in condition_to_joinir():

1. Loop variable (i):

   • Allocated by JoinIR lowerer: i_param = alloc_value() → ValueId(0)
   • Used in condition: i < end
   • Properly registered in boundary ✅

2. Condition variables (start, end):

   • Read from HOST: builder.variable_map.get("start") → ValueId(33)
   • Used in condition: start < end
   • NOT registered in boundary ❌

---

Files Involved

Boundary Definition

• src/mir/join_ir/lowering/inline_boundary.rs (340 lines)
  • JoinInlineBoundary struct
  • join_inputs, host_inputs fields
  • Missing: Condition inputs field

Boundary Builder

• src/mir/builder/control_flow/joinir/patterns/exit_binding.rs (401 lines)
  • LoopExitBinding struct
  • ExitBindingBuilder - builds exit bindings
  • Missing: Condition input builder

Merge Implementation

• src/mir/builder/control_flow/joinir/merge/mod.rs (268 lines)
  • merge_joinir_mir_blocks() - main merge coordinator
  • reconnect_boundary() - updates variable_map with exit values
  • Missing: Condition input Copy injection

Condition Lowering

• src/mir/join_ir/lowering/condition_to_joinir.rs (443 lines)
  • lower_condition_to_joinir() - AST → JoinIR conversion
  • lower_value_expression() - reads from builder.variable_map
  • Problem: Returns HOST ValueIds directly

Loop Lowerers

• src/mir/join_ir/lowering/loop_with_break_minimal.rs (295 lines)
  • lower_loop_with_break_minimal() - Pattern 2 lowerer
  • Calls lower_condition_to_joinir() at line 138-144
  • Missing: Extract condition variables, register in boundary

---

Next Steps (Phase 171-2)

We need to design a "box" for condition inputs. Three options:

Option A: Extend JoinInlineBoundary with condition_inputs field Option B: Create new LoopInputBinding structure Option C: Extend LoopExitBinding to include condition inputs

Proceed to Phase 171-2 for design decision.

---

References

• Phase 170 Analysis: phase170-valueid-boundary-analysis.md
• Phase 170 Completion: phase170-completion-report.md
• JoinIR Design: docs/development/current/main/phase33-10-if-joinir-design.md