GoKawiilCompiler optimizations expose hidden UB risk in C/C++ type punning
A developer traced a hard-to-find bug to type punning via pointer casts, which behaves correctly at -O0 but breaks under -O2 optimization. The root cause is that pointer casts violate strict aliasing rules and constitute undefined behavior in C, even though they compile and run correctly on most platforms, and C++ treats type distinctions even more strictly than C.
Many developers rely on pointer casts for serialization, networking, and hardware-level code because they 'just work,' but this reliance is fragile since compilers can silently miscompile such code once optimizations are enabled. Using safer alternatives like unions or memcpy avoids this class of latent bugs that only surface under specific build configurations.
- Pointer casts for type punning are undefined behavior under strict aliasing rules in C, despite appearing to work.
- Unions and memcpy are the defined-behavior-safe ways to perform type punning in C.
- C and C++ handle type aliasing differently, with C++ giving compilers more license to assume types never overlap, making optimization-related bugs more likely.
C Programming Language book (K&R) — If you're wrestling with type punning, strict aliasing, and undefined behavior, it pays to go back to the source material on C semantics. This classic reference clarifies exactly the kind of low-level memory model questions raised in the article. A great desk companion for anyone debugging optimizer-sensitive C/C++ code.
See C Programming Language book (K&R) on Amazon → Affiliate link — we may earn a commission on purchases, at no extra cost to you. Product picked by AI based on this article; it is not a tested recommendation.Source: news.ycombinator.com — Steve Schnepp, 2026-09-22
Published there as: “Type Punning in C and C++”
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