hakorune/docs/development/current/main/phase213-if-sum-implementation.md

Phase 213: Pattern 3 If-Sum AST-based Lowerer

Overview

Phase 213 implements an AST-based JoinIR lowerer for "simple if-sum" patterns in Pattern 3 (Loop with If-Else PHI).

Design Decision: Approach B

Dual-mode architecture:

1. if-sum mode: AST-based lowering for simple patterns (e.g., phase212_if_sum_min.hako)
2. legacy mode: Hardcoded PoC lowering for existing tests (e.g., loop_if_phi.hako)

This approach:

• Minimizes risk by keeping legacy code path intact
• Only generalizes for detected if-sum patterns
• Enables incremental migration

Implementation

Files Modified/Created

1. loop_update_summary.rs

   • Added is_simple_if_sum_pattern() method
   • Detects: 1 CounterLike + 1-2 AccumulationLike carriers

2. pattern_pipeline.rs

   • Added is_if_sum_pattern() to PatternPipelineContext
   • Added extract_if_statement() helper

3. pattern3_with_if_phi.rs

   • Dual-mode dispatch: ctx.is_if_sum_pattern() → branch
   • lower_pattern3_if_sum(): calls AST-based lowerer
   • lower_pattern3_legacy(): existing hardcoded logic

4. loop_with_if_phi_if_sum.rs (NEW)

   • AST-based if-sum lowerer (~420 lines)
   • Extracts from AST:
     • Loop condition (i < len)
     • If condition (i > 0)
     • Then update (sum = sum + 1)
     • Counter update (i = i + 1)
   • Generates JoinIR with dynamic values

Pattern Detection

pub fn is_simple_if_sum_pattern(&self) -> bool {
    if self.counter_count() != 1 { return false; }
    if self.accumulation_count() < 1 { return false; }
    if self.accumulation_count() > 2 { return false; }
    true
}

AST Extraction

The lowerer extracts pattern components from AST:

• extract_loop_condition(): i < 3 → (var="i", op=Lt, limit=3)
• extract_if_condition(): i > 0 → (var="i", op=Gt, value=0)
• extract_then_update(): sum = sum + 1 → (var="sum", addend=1)
• extract_counter_update(): i = i + 1 → (var="i", step=1)

Testing Results

AST Extraction: Working ✅

[joinir/pattern3/if-sum] Loop condition: i Lt 3
[joinir/pattern3/if-sum] If condition: i Gt 0
[joinir/pattern3/if-sum] Then update: sum += 1
[joinir/pattern3/if-sum] Counter update: i += 1

Known Issue: Pattern 3 Pipeline

Both if-sum mode and legacy mode return RC=0 instead of expected values.

This is a pre-existing issue in the JoinIR → MIR conversion pipeline (Phase 33-21/177):

• Loop back branch targets bb5 instead of loop header bb4
• PHI nodes not properly updated on loop back

This issue is NOT specific to Phase 213 - the legacy loop_if_phi.hako has the same problem.

Future Work

1. Phase 214+: Fix Pattern 3 JoinIR → MIR pipeline

   • Investigate loop back branch target
   • Ensure PHI updates are correctly wired

2. Phase 213-B (optional): Support variable limits

   • Currently only integer literals supported
   • i < len where len is a variable → Phase 214+

File Locations

• Lowerer: src/mir/join_ir/lowering/loop_with_if_phi_if_sum.rs
• Dispatcher: src/mir/builder/control_flow/joinir/patterns/pattern3_with_if_phi.rs
• Pattern detection: src/mir/join_ir/lowering/loop_update_summary.rs
• Pipeline context: src/mir/builder/control_flow/joinir/patterns/pattern_pipeline.rs