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Moe Charm 840c1b85ef feat: Fix VM SSA loop execution with proper phi node handling

Fixed infinite loop issue in VM by addressing phi node caching problem.
The phi node was caching the initial value and returning it for all
subsequent iterations, preventing loop variable updates.

Changes:
- Created vm_phi.rs module to separate loop execution logic (similar to mir/loop_builder.rs)
- Disabled phi node caching to ensure correct value selection each iteration
- Added LoopExecutor to track block transitions and handle phi nodes properly
- Fixed VM to correctly track previous_block for phi input selection

The VM now correctly executes SSA-form loops with proper variable updates:
- Loop counter increments correctly
- Phi nodes select the right input based on control flow
- Test case now completes successfully (i=1,2,3,4)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

2025-08-18 23:36:40 +09:00

1.3 KiB

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SSA Loop Fix - Simple Approach

Problem

Loop variables are not updated correctly in SSA form:

bb1:  ; loop header
    %4 = icmp Le %0, %3  ; Always uses initial value %0!
    br %4, label bb2, label bb3

bb2:  ; loop body
    %8 = %0 Add %7       ; Calculate i + 1
    br label bb1         ; But %0 is never updated!

Simple Fix Approach

Step 1: Track loop-modified variables

Before entering the loop body, save the current variable_map. After the loop body, compare to find which variables were modified.

Step 2: Insert phi nodes for modified variables

For each modified variable:

Create a phi node at the loop header
Inputs: (entry_block, initial_value), (loop_body, updated_value)
Update variable_map to use the phi result

Step 3: Fix variable references

The key issue: we need to use phi results in the condition evaluation.

Minimal Implementation Plan

Add a mechanism to insert instructions at the beginning of a block
Track which variables are modified in loops
Create phi nodes after loop body is built
Update variable references to use phi results

Alternative: Simpler non-SSA approach

If SSA is too complex, we could:

Use explicit Load/Store instructions
Maintain variable storage locations
Update variables in-place

But this would require VM changes too.

1.3 KiB Raw Blame History