Register allocator: Stale content after spill operations
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评估
调研方向
Start in src/reg-alloc.c at the TODO about outdated register content after store operations, then reproduce the issue with the provided test_spill_bug pattern under register pressure. Trace spill, store, and later value access paths. Done means spilled values are invalidated or versioned so subsequent uses load fresh memory values and the reproducer returns the correct result.
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描述
Description
The register allocator has a critical bug where register content becomes stale after spill operations, potentially causing incorrect program behavior.
Current Behavior
- When registers are spilled to memory, the allocator doesn't properly track that the register content is outdated
- This can lead to using stale values from registers instead of loading fresh values from memory
Expected Behavior
After a spill operation, the register allocator should mark the register content as invalid and ensure fresh loads from memory when the value is needed again.
Location in Code
- Primary location:
src/reg-alloc.c - Comment in code: "TODO: Outdated register content after store operations needs proper handling to prevent using stale values"
Impact
- CRITICAL: Can cause incorrect program execution
- Affects correctness of compiled programs
- May produce subtle bugs that are hard to debug
Test Case
// This pattern can trigger the stale register bug
int test_spill_bug(int a, int b, int c, int d, int e, int f, int g, int h) {
// Force register pressure to cause spills
int t1 = a + b;
int t2 = c + d;
int t3 = e + f;
int t4 = g + h;
int t5 = t1 * t2;
int t6 = t3 * t4;
// Modify spilled values
t1 = t1 + 100;
t3 = t3 + 200;
// Use potentially stale register values
return t1 + t2 + t3 + t4 + t5 + t6;
}
Proposed Solution
- Track register validity state after spills
- Force reload from memory when accessing previously spilled values
- Add register content versioning or invalidation mechanism
Severity
HIGH - This is a correctness issue that can cause wrong program behavior
- 主要语言
- C
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