Comment by JJJollyjim

Comment by JJJollyjim 8 days ago

It is in fact documented that you can't do this:

"Currently the default global allocator is unspecified. Libraries, however, like cdylibs and staticlibs are guaranteed to use the System by default.", however:

"[std::alloc::System] is based on malloc on Unix platforms and HeapAlloc on Windows, plus related functions. However, it is not valid to mix use of the backing system allocator with System, as this implementation may include extra work, such as to serve alignment requests greater than the alignment provided directly by the backing system allocator."

https://doc.rust-lang.org/std/alloc/index.html https://doc.rust-lang.org/std/alloc/struct.System.html

astrange 8 days ago

> such as to serve alignment requests greater than the alignment provided directly by the backing system allocator

Surely the system allocator provides memalign() or similar? Does Windows not have one of those?

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jsheard 8 days ago

Kind of but not really, the Windows heap doesn't support arbitrary alignments natively, so the aligned malloc function it provides is implemented as a wrapper around the native heap which over-allocates and aligns within the allocated block. The pointer you get back isn't the start of the allocation in the native heap, so you can't pass it to the standard free() without everything exploding.
I don't think fixing that mess was ever a priority for Microsoft because it's mainly an issue for C, and their focus has long been on C++ instead. C++ new/delete knows the alignment of the type being allocated for so it can dispatch to the appropriate path automatically with no overhead in the common <=16 byte case.

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jcelerier 8 days ago

On macOS, aligned_alloc works starting from macOS 10.15.

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