this post was submitted on 21 Jan 2024
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Strict aliasing exists not for optimization, but for type alignment. You may need more space on stack to save uint32_t than uint8_t[5] because former has 32-bit alignment.
Either way, this is a rule that you as a human are required to follow, and if you fail the compiler is allowed to do anything, including killing your cat.
It's not a rule that the compiler enforces by failing to build code with undefined behavior.
That is a fundamental, and extremely important, difference between C and rust.
Also, C compilers do make optimization decisions by assuming that you as a human programmer have followed these strict aliasing rules.
https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
Has a few examples where code runs "properly" without optimizations but "improperly" with optimizations.
I put "improperly" in quotes because the C spec says that a compiler can do whatever it wants if you as a human invoke undefined behavior. Safe rust does not have undefined behavior, because if you write code which would invoke UB, rustc will refuse to build it.