[llvm-dev] PointerIntPair

[Stefanos Baziotis via llvm-dev]

> Does every target guarantee at least 4-byte alignment of stack/heap
objects?

The short answer is no, for example, check here:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf
paragraph 6.2.8, point 1
There are "informal" rules though e.g., malloc is assumed to return at
least 4-byte aligned pointers.

> I presume that compilers impose alignments anyway, but are they
consistent enough to rely on?

It depends on what you mean "rely on". If you have less alignment from one
compiler to another, your code won't compile because `static_assert`'s are
going to fire [1]. So, you can rely on
that in the sense that you won't get a runtime error. However, the way I
understand it, you can't rely on not even getting a compile error across
compilers.

In practice, however, you'll almost never get an error because the
alignments are for the most part consistent.

[1] Here's the actual place in the code:
https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/ADT/PointerIntPair.h#L148
If you follow what this is based on (i.e. the traits), you eventually end
up here for the default case:
https://llvm.org/doxygen/PointerLikeTypeTraits_8h_source.html#l00056

Best,
Stefanos

Στις Πέμ, 31 Δεκ 2020 στις 9:13 μ.μ., ο/η Paul C. Anagnostopoulos via
llvm-dev <llvm-dev at lists.llvm.org> έγραψε:

> The bits are orthogonal, so I think an enum makes more sense. I take it
> from your response that you think a PointerIntPair is a fine thing to use.
>
> There are no alignment requirements on an X86, for example. I presume that
> compilers impose alignments anyway, but are they consistent enough to rely
> on?
>
> At 12/31/2020 01:51 PM, Craig Topper wrote:
> >A PointerIntPair can be placed in the pointer part of another
> PointerIntPair. So you can nest them to have 2 booleans without creating an
> enum. This works because PointerIntPair always puts the bit in the highest
> bit possible leaving the low bits free to be used by another int.
> >
> >The number of bits available in a pointer depends on the alignment
> requirement of the type the pointer points to. If it’s a class/struct it
> depends on the largest alignment required by its fields. The alignof
> operator is used to check the alignment.
>
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