Optimal struct layout template?

Don nospam at nospam.com
Tue Dec 16 01:09:41 PST 2008

Andrei Alexandrescu wrote:
> Sergey Gromov wrote:
>> Sun, 14 Dec 2008 13:17:45 -0800, Andrei Alexandrescu wrote:
>>> BCS wrote:
>>>> Reply to dsimcha,
>>>>> According to the spec, data stored in structs is guaranteed to be laid
>>>>> out in the order specified in the source code.  While this is
>>>>> necessary in some low-level code, I have a use case where I need to
>>>>> pack structs as efficiently as possible.  These structs are to be
>>>>> stored in an array, and for efficiency reasons I want to store them
>>>>> directly rather than storing pointers, so using classes is out of the
>>>>> question.  Does anyone know how to write a template that, given a
>>>>> tuple of types, will reorder the tuple so that it will produce the
>>>>> optimal struct layout?  I don't care, at least for now, if it assumes
>>>>> x86-32/DMD instead of handling the more general case.
>>>> would "align 1" work?
>>> That could make things excruciatingly slow. What's really needed is 
>>> (in first approximation) to sort the members in decreasing order of 
>>> size. Odd-sized arrays of char foil this plan to some extent, which 
>>> is where the fun begins.
>> I would say in decreasing order of field/array element alignment
>> requirement.
> Sounds indeed right. Even simpler than I thought!

Close, but doesn't work in all cases. You sometimes need to insert 
elements of smaller size.

Example 1:
Given real, double, double, short, int:
real (10 bytes, wants 16 byte alignment) --(only on Windows, 12 byte 
size on Linux32, 16-byte size on Linux64)
double (8 bytes, wants 8 byte alignment)
short (2 bytes, wants 2 byte alignment)
int (4 bytes, wants 4 byte alignment)

There are only two solutions:
{ real, short, int, double, double }
{ double, double, real, short, int }

Example 2:
struct Foo { double, int } // 12 byte size, wants 8-byte alignment.

given Foo, Foo, int, int:
solution is
{ Foo, int, Foo, int }

> Andrei

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