Why do x64 projects use a default packing alignment of 16?

Question

If you compile the following code in a x64 project in VS2012 without any /Zp flags:

#pragma pack(show)

then the compiler will spit out:

value of pragma pack(show) == 16

If the project uses Win32 then, the compiler will spit out:

value of pragma pack(show) == 8

What I don't understand is that the largest natural alignment of any type (ie: long long and pointer) in Win64 is 8. So why not just make the default alignment 8 for x64?

Somewhat related to that, why would anyone ever use /Zp16?

EDIT:

Here's an example to show what I'm talking about. Even though pointers have a natural alignment of 8 bytes for x64, Zp1 can force them to a 1 byte boundary.

struct A
{
    char a;
    char* b;
}

// Zp16
// Offset of a == 0
// Offset of b == 8

// Zp1
// Offset of a == 0
// Offset of b == 1

Now if we take an example that uses SSE:

struct A
{
    char a;
    char* b;
    __m128 c; // uses declspec(align(16)) in xmmintrinsic.h
}

// Zp16
// Offset of a == 0
// Offset of b == 8
// Offset of c == 16

// Zp1
// Offset of a == 0
// Offset of b == 1
// Offset of c == 16

If __m128 were truly a builtin type, then I'd expect the offset to be 9 with Zp1. But since it uses __declspec(align(16)) in its definition in xmmintrinsic.h, that trumps any Zp settings.

So here's my question worded a little differently: is there a type for 'c' that has a natural alignment of 16B but will have an offset of 9 in the previous example?

Answer 1

The MSDN page here includes the following relevant information about your question "why not make the default alignment 8 for x64?":

Writing applications that use the latest processor instructions introduces some new constraints and issues. In particular, many new instructions require that data must be aligned to 16-byte boundaries. Additionally, by aligning frequently used data to the cache line size of a specific processor, you improve cache performance. For example, if you define a structure whose size is less than 32 bytes, you may want to align it to 32 bytes to ensure that objects of that structure type are efficiently cached.

Answer 2

Why do x64 projects use a default packing alignment of 16?

On x64 the floating point is performed in the SSE unit. You state that the largest type has alignment 8. But that is not correct. Some of the SSE intrinsic types, for example __m128, have alignment of 16.