On Saturday, 1 October 2022 at 01:37:00 UTC, Steven Schveighoffer
wrote:
On 9/30/22 11:57 AM, Quirin Schroll wrote:
Also, is the alignment of any type guaranteed to be a power of
2?
In practice, it's not necessarily a power of 2, but it's *at
least* 16 bytes.
**Types** always require some power of 2 alignment (on any
sensible platform, anyway), and it is usually *less* than 16
bytes - typically `size_t.sizeof`.
The fact that the current GC implementation apparently has a
minimum block size of 16 bytes, and that minimum size blocks are
always size-aligned, is not guaranteed by the public API and
*should not be* when requesting memory for something that the
type system says only requires an alignment of `void.alignof ==
1`.
D and C both have formal ways to communicate alignment
requirements to the allocator; people should use them and not
constrain all future D GC development to conform to undocumented
details of the current implementation.
In general there are very few types (maybe vectors?) that need
alignment more than 16 bytes.
256 bit SIMD (AVX/AVX2) and 512 bit SIMD (AVX512) `__vector`s
should be `.sizeof` aligned (32 and 64 bytes, respectively).
Memory used for inter-thread communication (such as mutexes) may
perform significantly better if cache line aligned (typically 64
bytes, but CPU dependent).
I don't know any other examples off the top of my head.
The list of bit sizes is currently here:
I'm pretty sure those are in **bytes** not **bits**.
https://github.com/dlang/dmd/blob/82870e890f6f0e0dca3e8f0032a7819416319124/druntime/src/core/internal/gc/impl/conservative/gc.d#L1392-L1414
That's not a list of alignments, it is block sizes for some GC
memory pools. The alignment of each block depends on the
alignment of its pool, not just its size.
It's not immediately obvious from the context, but I suspect the
pools are actually page aligned, which would mean that the non
power of 2 sized blocks are **not** consistently aligned to their
own sizes.
Regardless, it's not part of the public API, so it could change
without warning.
