840c1b85ef
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>
42 lines
1.3 KiB
Markdown
42 lines
1.3 KiB
Markdown
# SSA Loop Fix - Simple Approach
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## Problem
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Loop variables are not updated correctly in SSA form:
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```mir
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bb1: ; loop header
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%4 = icmp Le %0, %3 ; Always uses initial value %0!
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br %4, label bb2, label bb3
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bb2: ; loop body
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%8 = %0 Add %7 ; Calculate i + 1
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br label bb1 ; But %0 is never updated!
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```
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## Simple Fix Approach
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### Step 1: Track loop-modified variables
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Before entering the loop body, save the current variable_map.
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After the loop body, compare to find which variables were modified.
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### Step 2: Insert phi nodes for modified variables
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For each modified variable:
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1. Create a phi node at the loop header
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2. Inputs: (entry_block, initial_value), (loop_body, updated_value)
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3. Update variable_map to use the phi result
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### Step 3: Fix variable references
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The key issue: we need to use phi results in the condition evaluation.
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### Minimal Implementation Plan
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1. Add a mechanism to insert instructions at the beginning of a block
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2. Track which variables are modified in loops
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3. Create phi nodes after loop body is built
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4. Update variable references to use phi results
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### Alternative: Simpler non-SSA approach
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If SSA is too complex, we could:
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1. Use explicit Load/Store instructions
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2. Maintain variable storage locations
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3. Update variables in-place
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But this would require VM changes too. |