42 lines
1.3 KiB
Markdown
42 lines
1.3 KiB
Markdown
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# 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.
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