“My Little Optimizer: Undefined Behavior is Magic"
summary
Modern C++ compilers treat undefined behavior as permission to change code: LLVM turns a load-from-null into unreachable and removes the preceding store, collapses uninitialized values via undef folding, and uses the NSW flag for signed overflow to simplify arithmetic and loop bounds. Strict-aliasing and lifetime markers let stores be eliminated, malloc’s noalias on malloc enables load-forwarding, and atomics can be optimized under the standard’s ordering rules. Use UBSan, sanitizers and fuzzing to find these bugs before optimizations hide them.
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chapters (9)
- 0:00Introduction to UB
- 3:21Representing UB in LLVM
- 10:00Control flow and lifetimes
- 14:25Null pointers and aliasing
- 16:43Signed vs unsigned arithmetic
- 25:57Memory models and realloc
- 29:44Atomic and thread consistency
- 33:47Mitigation and standards
- 39:10Q&A session
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