logoalt Hacker News

stevenhuang • yesterday at 7:22 AM • 2 replies • view on HN

Byte and int has different alignment requirements. It is UB the moment you make such a ptr.

Great way to demonstrate the point of the article.

Replies

gritzko • yesterday at 7:40 AM

That better be marked "historical". At least, Lemire says:

On recent Intel and 64-bit ARM processors, data alignment does not make processing a lot faster. It is a micro-optimization. Data alignment for speed is a myth. // https://lemire.me/blog/2012/05/31/data-alignment-for-speed-m...

(while in the olden days, a program may crash on unaligned access, esp on RISC)

➕ show 1 reply
dmitrygr • yesterday at 7:50 AM

Without memcpy there is no guarantee that that line produces an invalid pointer

I don’t see what spec part would prohibit that cast from validly compiling to

   BIC r3, r0, #3
Spec only guaranteed round-trip through char* of properly aligned for type pointers. This doesn’t break that.