On Mon, Dec 03, 2012 at 07:57:33PM +0100, Behan Webster wrote: > However, in order to approximate what gcc is doing in code, obviously > some math is required. The thought was that macros would hide the > worst of it, trying not to obfuscate what was actually being done. Why hide? The thing that is done here is setting up pointers and keep this struct as a container. It is never used again, just freed. Therefore I would suggest to remove and do something different. > One of the project members came up with this alternative. How about > something like this? Less math, though more string pasting. When > compiled, the unused variables get optimized away. Otherwise the > memory packing is identical to using VLAIS in gcc. > > #define vla_struct(structname) size_t structname##__##next = 0 > #define vla_struct_size(structname) structname##__##next > > #define vla_item(structname, type, name, n) \ > type * structname##_##name; \ > size_t structname##_##name##__##pad = \ > (structname##__##next & (__alignof__(type)-1)); \ > size_t structname##_##name##__##offset = \ > structname##__##next + structname##_##name##__##pad; \ > size_t structname##_##name##__##sz = n * sizeof(type); \ > structname##__##next = structname##__##next + \ > structname##_##name##__##pad + > structname##_##name##__##sz; > > #define vla_ptr(ptr,structname,name) structname##_##name = \ > (typeof(structname##_##name))&ptr[structname##_##name##__##offset] > > > Then you can do something like this that looks vaguely struct-like: > > vla_struct(foo); > vla_item(foo, char, vara, 1); > vla_item(foo, short, varb, 10); > vla_item(foo, int, varc, 5); > vla_item(foo, long, vard, 3); > size_t total = vla_struct_size(foo); > char buffer[total]; > > vla_ptr(buffer, foo, varc); > foo_varc = 1; I prefer to try to rewritte the code in the gadget in a different manner before using macro magic. I guess most people around here think that extensive usage of macros equals giving a gun to a chimpanzee. It may work for a while and may even look cute in the eye of the gun sponsor. However once it fires… > I've been profiling some sample code around this implementation > comparing it between compilers, and it approximates the code size and > speed of using VLAIS in gcc (especially with -O2). It actually > performs better than the previously proposed macros. I'm not concerned about speed here. This is an one time setup. > But certainly if anyone has a solution which works for everyone, then > I'm more than happy to adopt it. The LLVM community has made quite a > few changes in order to help get Linux working with clang. However, That is nice to hear. Besides gcc there is the icc. > VLAIS is not something they are willing to accept (for various > reasons). There are other patches to LLVM that are still working Is this not described in C99 6.7.2.1p16? > their way upstream that are required to be able to compile Linux as > well. I hope the other are "simple" to get in :) > > Behan > Sebastian -- To unsubscribe from this list: send the line "unsubscribe netfilter-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
