The UB Taxonomy That Matters

The load-bearing categories

• Signed arithmetic overflow (INT_MAX + 1): the compiler may assume it never happens, so overflow-guard code written after the add can be deleted as dead.
• Out-of-bounds access: an OOB loop index licenses the compiler to assume the loop bound is large — bounds checks vanish.
• Uninitialized reads: an indeterminate value is not 'random garbage' — reading one is UB; the compiler may propagate anything.
• Shift abuse: shifting by >= width, or negative left operands, is UB; 1u << 32 on a 32-bit int is not 'wrap to 0'.
• Lifetime violations: use-after-free, use-after-scope, use-before-init.

What UB is for

It is not malice: every category marks a place where the abstract machine stops constraining the implementation, letting optimizers reason about the other 99% of the program. The cost is that a bug in the 1% can invalidate proofs about everything.

The professional response

You do not memorize the 200 rules; you know the load-bearing dozen above, and you make every one detectable: warning flags, checked arithmetic at boundaries, initialized-everywhere discipline, and the sanitizer runs of module 16.