What is the difference between "minimum alignment" and "preferred alignment"?

Question

Recently I observed that on Clang 9.0 alignof and __alignof are returning different values for unsigned long long and the same has been discussed at https://reviews.llvm.org/D54814:

Starting in Clang 8.0 and GCC 8.0, alignof and __alignof return different values in same cases. Specifically alignof and _Alignof return the minimum alignment for a type, where as __alignof returns the preferred alignment.

I know about type alignment but never came across "minimum alignment" and "preferred alignment".

Could somebody please help me understand what exactly these are and what is the difference? Thanks.

Answer 1

The minimal alignment is (on a given platform) the one which won't give crashes. On x86-64 it is one byte. On PowerPC or Sparc or RISC-V it is probably 4 or 8 bytes.

The preferred alignment is the one which is usual, e.g. because of processor bus or CPU caches. On x86-64 for unsigned long long it probably is 8 bytes. Any less aligned access has a performance penalty.

Details are target processor and ABI specific (for example, see this). Think of cross-compilers.

The semantics of C or of C++ is not perfectly defined and not fully formalized. Look into the C++ draft standard: it is written in English, not formalized. But see also Frama-C (it has an experimental front end for C++) and CompCert. Read about undefined behavior.