Automated field re-ordering in C structs to avoid

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

Have a look at #pragma pack. This changes how the compiler aligns elements in the structure. You can use it to force them to be closely packed together without spaces.

See more details here

Answer 2

Thinking about how I'd go about making such a tool... I think I'd start with the debugging info.

Getting the size of each structure from the source is a pain. It overlaps a lot of work that the compiler already does. I'm not familiar enough with ELF to say exactly how to extract the structure size info from a debug binary, but I know that the info exists because debuggers can display it. Perhaps objdump or something else in the binutils package can get this for you trivially (for platforms that use ELF, at least).

After you've got the info, the rest is pretty straightforward. Order the members from largest to smallest, trying to keep as much as the ordering of the original struct as possible. With perl or python it'd even be easy to crossreference it with the rest of the source and maybe even preserve comments or #ifdefs depending on how cleanly they were used. The biggest pain would be changing all initializations of the struct in the entire codebase. Yikes.

Here's the thing. It sounds really nice, but I don't know of any such existing tool that does this, and by the time you write your own... I think you'll have been able to manually reorder most of the structs in your program.