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Comment by Dwedit | Hacker News Reader

We need an '__unaligned' modifier for pointers to specify that the pointer will be used for unaligned reads and writes.

If you've got control over the type of the pointee and not just the pointer, __attribute__((packed)) can work for this.

    struct foo {                         int x; };
    struct bar { __attribute__((packed)) int x; };

    _Static_assert(_Alignof(struct foo) == 4, "");
    _Static_assert(_Alignof(struct bar) == 1, "");

Yeah, I end up using __attribute__((packed)) for this at work. For tortured reasons, part of our codebase allocates memory with only 8-byte alignment, but the buffer is cast to a type that would have 16-byte alignment without __attribute__((packed)). As a result Clang wants to generate VMOVDQAs that require 16-byte alignment, unless you use packed, in which case it generates VMOVDQU.

Why not just use memcpy()?

That is the solution they recommend.

Because, in some hardware, unaligned read is ok. and you want to take advantage of the hardware feature?

Both gcc and clang will optimize a memcpy to an unaligned load/store where possible.

If the compiler is so smart, I guess it could insert a memcpy when needed?

The standard, you may say.. I would argue it's the standard need to be changed. The modern reading of the standard is not useful as a low-level language and is unsafe as a high-level language.

> If the compiler is so smart, I guess it could insert a memcpy when needed?

If I'm reading your comment and the blog post correctly, the compiler would need a memory like access on every multibyte pointer argument where the compiler cannot otherwise prove alignment. Is that correct?

Internally, the compiler could represent it however it wants. LLVM IR's load/store instructions just have an "align" property, which is usually sizeof(the type), but can be set to 1 to mimic memcpy (and indeed llvm/clang immediately translate a memcpy to such - https://godbolt.org/z/7T46a6aqT).

Though it seems that, independent of this, it assumes that an int* in general will be 4-byte-aligned, so e.g. https://godbolt.org/z/aWTEd4s3K still has an "align 4" despite using memcpy. So one must also cast to a char* before using memcpy() to actually have it work. yay for more footguns!

> memory like

I meant memcpy()-like

Right, I agree that it would be nice to have some way to request unaligned load/store to be permitted, alike -fwrapv for signed int wrapping. But nevertheless the UB behavior is a reasonable option that's beneficial for other things.

Arrays?

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