问题
I think I understand memory alignment, but what confuses me is that the address of a pointer on some systems is going to be in virtual memory, right? So most of the checking/ensuring of alignment I have seen seem to just use the pointer address. Is it not possible that the physical memory address will not be aligned? Isn't that problematic for things like SSE?
回答1:
The physical address will be aligned because virtual memory only maps aligned pages to physical memory (and the pages are typically 4KB).
So unless you need alignment > page size, the physical memory will be aligned as per your requirements.
In the specific case of SSE, everything works fine because you only need 16 byte alignment.
回答2:
I am not aware of any actual system in which an aligned virtual memory address can result in a misaligned physical memory address.
Typically, all alignments on a given platform will be powers of two. For example, on x86 32-bit integers have a natural alignment of 4 bytes (2^2). The page size - which defines how fine a block you can map in physical memory - is generally a large power of two. On x86, the smallest allowable page size is 4096 bytes (2^12). The largest datatype that might need alignment on x86 is 128 bits (for XMM registers and CMPXCHG16B) 32 bytes (for AVX) - 2^5. Since 2^12 is divisible by 2^5, you'll find that everything aligns right at the start of a page, and since pages are aligned both in virtual and physical memory, a virtual-aligned address will always be physical-aligned.
On a more practical level, allowing aligned virtual addresses to map to unaligned physical addresses not only would make it really hard to generate code, it would also make the CPU architecture more complex than simply allowing any alignment (since now we have odd-sized pages and other weirdness...)
Note that you may have reason to ask for larger alignments than a page from time to time. Typically, for user space coding, it doesn't matter if this is aligned in physical RAM (for that matter, if you're requesting multiple pages, it's unlikely to be even contiguous!). Problems here only arise if you're writing a device driver and need a large, aligned, contiguous block for DMA. But even then usually the device isn't a stickler about larger-than-page-size alignment.
来源：https://stackoverflow.com/questions/8191533/virtual-memory-and-alignment-how-do-they-factor-together