Automated field re-ordering in C structs to avoid padding

Question

I've spent a few minutes manually re-ordering fields in a struct in order to reduce padding effects[1], which feels like a few minutes too much. My gut feeling says that my time could probably be better spent writing up a Perl script or whatnot to do this kind of optimization for me.

My question is whether this too is redundant; is there already some tool that I'm not aware of, or some compiler feature that I should be able to turn on[2] to pack structs?

The issue is even more complicated by the fact that this needs to be consistently optimized across a few different architectures, so whatever tool used needs to be able to account for different struct alignments and pointer sizes as well.

EDIT: A quick clarification -- what I want to do is re-order the field in the source code in order to avoid padding, not "pack" the struct as is compiling without padding.

EDIT #2: Another complication: depending on the configuration, sizes of some data types may also change. The obvious ones are pointers and pointer-diffs for different architectures, but also floating-point types (16, 32 or 64-bit depending on the 'exactness'), checksums (8 or 16-bit depending on 'speed') and some other non-obvious stuff.

[1] The struct in question is instantiated thousands of times on an embedded device, so each 4-byte reduction of the struct could mean the difference between a go and no-go for this project.

[2] Available compilers are GCC 3.* and 4.* , Visual Studio, TCC, ARM ADS 1.2, RVCT 3.* and a few others more obscure.

Answer 1

If every single word you can squeeze out of the storage is critical, then I have to recommend optimizing the struct by hand. A tool could arrange the members optimally for you, but it doesn't know, for example, that this value here that you're storing in 16 bits actually never goes above 1024, so you could steal the upper 6 bits for this value over here...

So a human will almost certainly beat a robot on this job.

[Edit] But it seems like you really don't want to hand-optimize your structs for each architecture. Maybe you really have a great many architectures to support?

I do think this problem isn't amenable to a general solution, but you might be able to encode your domain knowledge into a custom Perl/Python/something script that generates the struct definition for each architecture.

Also, if all your members have sizes that are powers of two, then you will get optimal packing simply by sorting members by size (largest first.) In that case, you can just use good old-fashioned macro-based struct-building - something like this:

#define MYSTRUCT_POINTERS      \
    Something*  m_pSomeThing;  \
    OtherThing* m_pOtherThing; 

#define MYSTRUCT_FLOATS        \
    FLOAT m_aFloat;            \
    FLOAT m_bFloat;

#if 64_BIT_POINTERS && 64_BIT_FLOATS
    #define MYSTRUCT_64_BIT_MEMBERS MYSTRUCT_POINTERS MYSTRUCT_FLOATS
#else if 64_BIT_POINTERS
    #define MYSTRUCT_64_BIT_MEMBERS MYSTRUCT_POINTERS
#else if 64_BIT_FLOATS
    #define MYSTRUCT_64_BIT_MEMBERS MYSTRUCT_FLOATS
#else
    #define MYSTRUCT_64_BIT_MEMBERS
#endif

// blah blah blah

struct MyStruct
{
    MYSTRUCT_64_BIT_MEMBERS
    MYSTRUCT_32_BIT_MEMBERS
    MYSTRUCT_16_BIT_MEMBERS
    MYSTRUCT_8_BIT_MEMBERS
};